Intermolecular hydrogen-bonding effect on carbon-13 NMR ...

H-Bonding Effect on I3C Chemical Shifts of Solid PeptidesCO is closely related to the manner of the <strong>hydrogen</strong> bond whichis formed between amide groups. In addition, the 13C isotropicchemical shift and u22 component of the carbonyl carbon in polyglycineform I1 ((Gly), 11; 31-helical form) were displaceddownfield by 3.3 and 5 ppm, respectively, relative to those of formI ((Gly), I; @-antiparallel form). In practice, a strong <strong>hydrogen</strong>bond is formed in form 11, as is evident from the fact that the N-0<strong>hydrogen</strong> bond length is as long as 2.73 A in (Gly), 11233 comparedwith 2.95 A in (Gly), I.4 From these, it is expected that the GlyCO 13C chemical shift displacement is related to the <strong>hydrogen</strong>bond length.It is well-known that the <strong>hydrogen</strong> bond plays an importantrole in forming the stable conformations of polypeptides andproteins. Thus, the nature of the <strong>hydrogen</strong> bond has been widelystudied by various spectroscopic methods. Also, NMR has beenused as one of the most powerful tools for obtaining useful informationabout the details of the <strong>hydrogen</strong> bond. The I3C NMRchemical shift of the carbonyl carbon is thought to be the mostsensitive to the spatial arrangement of the nuclei comprising the<strong>hydrogen</strong> bond, since the electronic structure of the carbonylcarbon is greatly affected by the <strong>hydrogen</strong> bond. In fact, it wasreported that the formation of the <strong>hydrogen</strong> bond causes adownfield shift on the carbonyl carbon of peptides in the solutionIt is, however, difficult to estimate the <strong>hydrogen</strong> bond<strong>effect</strong> on the 13C chemical shift because the observed chemicalshifts of the peptides are often the averaged values for all therotational isomers owing to an interconversion by rapid rotationin the solution state. Therefore, chemical shifts in the solid stateprovide useful information about the <strong>hydrogen</strong> bond of the peptideswith a fixed conformation. Nevertheless, no attempts have beenmade to correlate the manner of the hdyrogen bond and 13Cchemical shifts of peptide carbonyl carbons in the solid state.Therefore, the main purpose of the present work is to measure13C chemical shifts of the glycine carbonyl carbons in peptidescontaining the glycine residues in the solid state, of which the<strong>hydrogen</strong> bond lengths and the conformations are determined fromX-ray studies. On the basis of these results, the relationshipbetween the I3C chemical shift and the <strong>hydrogen</strong> bond length willbe discussed. In this study, we chose peptides whose Gly CO inthe amide group is involved in the intermolecular >C-O-H-N

3382 J. Am. Chem. SOC., Vol. 110, No. 11, 1988 Ando et al.Table I. I3C NMR Chemical Shifts of Glycine Residue Carbonyl Carbons for Oligopeptides Containing Glycine Residues As Determined by I3CCP-MAS NMR (k0.2 ppm from TMS) and Their Geometrical Parametersgeometrical parametersH bond"C chemical length/A H bond angle/deg dihedral angled/degsample" typeb shift, b/ppm Ns.0 Hs.0 C=O..N N-He.0 4 $ ref(G~Y*), 11 1 172R 2.73 1.84 158.6 146.2 -78.0 145.8 3Gly-Gly*-L-Val 1 171.1 2.78 1.72 138.3 173.7 -77.1 -22.3 19ClAc-Gly*-Gly 1 170.5 2.82 2.13 160.2 147.4 -74.5 154.1 20L-Pro-Gly *-Gly 1 170.0 2.84 2.21 157.0 151.9 -71.3 167.1 21L-Ala-Gly*-Gly 1 170.2 2.93 2.09 152.3 165.8 -83.2 169.4 22(GlY*)" 1 1 169.5c 2.95 2.16 149.2 132.8 146.5 -149.9 4Gly*-Gly 1 168.5 2.97 157.2 152.8 23Gly *-L-Phe-Gly 1 168.1 3.00 2.21 161.8 163.7 127.6 24Gly*-DL-Thr 1 167.8 3.02 2.11 161.2 157.8 -175.0 25Gly*-~-Thr*2H~O 1 168.3 3.05 147.4 -168.7 26L-Val-Gly*-Gly 1 169.1 3.05 2.19 156.6 152.4 -155.1 154.7 27Sar-Gly *-Gly 1 168.6 3.06 2.23 135.0 167.1 72.1 -17.4 28Gly*-GlyHN03 1 166.5 3.12 2.38 161.8 164.6 148.9 29DL-Leu-Gly*-Gly 2 167.0 2.74 170.5 -167.5 -172.3 30Gly*-m-Phe 2 167.5 2.83 2.28 172.8 115.3 -160.3 31Gly*-L-Phe.HC1.H20 2 168.4 2.83 1.93 161.7 152.8 -162.2 32Gly*-Ph*TsOH 2 169.8 2.92 139.5 -179.0 33Gly *-Gly-Val 2 170.9 2.99 2.62 109.9 100.9 -155.8 19"The asterisk indicates the glycine residue whose carbonyl carbon was measured. bThe manner of <strong>hydrogen</strong> bond. See text. 'Reference 1.Converted to the present tetramethylsilane reference by adding 1 ppm. dFor glycine residue.X',,c,N0IIH>fl;C:H I H A--$E-* 5553L . , . l . . . l . . . l . . I . . I 1 . 1 1 1 . I . I . I , ) . . . II80 168 140 120 100 80 60 48 20(ppm trom TMS)Figure 1. I3C CP-MAS (100.63 MHz) NMR spectrum of L-valylglycylglycine.used. The calculation was performed as a function of <strong>hydrogen</strong> bondlength (the distance between nitrogen and oxygen atoms, abbreviated asRN4) with the same dihedral angles for the skeletal bonds as those of(Gly), I and (Gly), 11.A HITAC M280H computer at the Computer Center of the TokyoInstitute of Technology and a HITAC M200H computer at the ComputerCenter of the Institute for Molecular Science, Okazaki, were usedfor the calculation.Results and Discussion13C NMR Chemical Shifts of the Glycine Carbonyl Carbons inPeptides. A 100.63 MHz 13C CP-MAS NMR spectrum of valylglycylglycine(Val-Gly-Gly) using the TOSS pulse techniqueis shown as a typical example in Figure 1. 13C CP-MAS spectraof the other remaining samples were also obtained with similarresolutions (figures not shown). Each Gly CO signal can bestraightforwardly assigned, because it is resonated upperfieldcompared with those of the carbonyl carbons of other amino acidresidues.'* This upperfield shift arises from the absence of the(18) Howarth, 0. W. Prog. NMR Spectrosc. 1978, 12, 1.a. 1-type b. 2-typeFigure 2. Schematic representations for the two types of <strong>hydrogen</strong> bonds:(a) 1-type and (b) 2-type.C, carbon in the Gly residue. The carbonyl carbon of the C-terminal carboxylic group is resonated downfield by several ppmas a rather sharp peak relative to the internal amide carbonylcarbon. The isotropic "C chemical shifts of Gly CO carbonsdetermined for all the peptides measured from "C CP-MASspectra are listed in Table I, together with the geometrical parametersobtained by X-ray diffraction studies. 13C chemical shiftsof the Gly COS in (Gly), I and I1 reported previously' are alsolisted therein. Some of the geometrical parameters were calculatedby using the unit cell parameters and fractional coordinates inthe literature.(19) Lalitha, V.; Subramanian, E.; Border, J. In?. J. Peptide Protein Res.1984. 24. 437.(20) Rao, S. T. Cryst. Struct. Commun. 1973, 2, 257.(21) Lathitha, V.; Subramanian, E.; Parthasarathy, R. Int. J. PeptideProtein Res. 1986, 27, 223.(22) Lalitha, V.; Subramanian, E. Indian J. Pure Appl. Phys. 1985, 23,506.(23) Biswas, A. B.; Hughes, E. D.; Sharma, B. D.; Wilson, J. N. ActaCrystallogr. 1968, 824, 40.(24) Marsh, R. E.; Glusker, J. P. Acta Crystallogr. 1961, 14, 1110.(25) Swaminathan, P. Acta Crystallogr. 1975, 831, 1608.(26) Yadava, V. S.; Padmanabhan, V. M. Acta Crystallogr. 1973, 829,854.(27) Lalitha, V.; Murali, R.; Subramanian, E. In?. J. Peptide Protein Res.1986, 27, 472.(28) Glusker, J. P.; Carrel, H. L.; Berman, M.; Gallen, B.; Peck, R. M.J. Am. Chem. SOC. 1977, 99, 595.(29) Rao, S. N.; Parthasarathy, R. Acta Crystallogr. 1973, 829, 2379.(30) Goswami, K. N.; Yadava, V. S.; Padmanabhan, V. M. Acta Crystallogr.1977, 833, 1280.(31) Marsh, R. E. Acta Crystallogr. 1976, 832, 66.(32) Cotrait, P. M.; Barrans, Y. Acta Crystallogr. 1974, 830, 1018.(33) VanDerVeen, J. M.; Low, B. W. Acta Crystallogr. 1972,828, 3548.

H-Bonding Effect on "C Chemical Shifts of Solid Peptides169-=2 170-u 4171 -=; 172-173-0030AAA %0027 28 29 30, 31 27 28 29 30 31N 0 LENGTH (A)N 0 LENGTH (ilFigure 3. Plots of the observed I3C chemical shifts in the solid stateagainst the N-0 <strong>hydrogen</strong> bond length (&a) for the 1-type (a) andthe 2-type (b) <strong>hydrogen</strong> bonds. Triangles, open circles, and full circlesdenote Gly CO of the N-terminal glycyl residues, Gly CO of the secondglycyl residues, and Gly CO of polyglycine, respectively.Figure 2, a and b, shows schematic representations of the twotypes of <strong>hydrogen</strong> bonds which exist in peptides considered hereas classified by the nature of the proton-donating nitrogen atom.One is denoted as the 1-type <strong>hydrogen</strong> bond which is formedbetween amide >cIo and amide >N-H. The other is denotedas the 2-type <strong>hydrogen</strong> bond which is formed between amide>C-O and N-terminal -NH3+. The 2-type <strong>hydrogen</strong> bond existsonly in oligopeptides with free end-groups, since such a peptideusually exists as a zwitterion in the crystalline state, where theN-terminal is protonated as NH3+ and the C-terminal is deprotonatedas COO-.Figure 3, a and b, shows the plot of I3C chemical shifts of GlyCO against the N.-0 <strong>hydrogen</strong> bond length (RN-0) in the 1-typeand 2-type <strong>hydrogen</strong> bonds, respectively. The <strong>hydrogen</strong> bond<strong>effect</strong> on the I3C isotropic chemical shift is entirely differentbetween the two types of <strong>hydrogen</strong> bonds.In the 1-type, a decrease of RN.-O causes a downfield shift.There exists approximately a linear relationship between RN.-oand the I3C chemical shifts. It is noted that not only in oligopeptides(dimer or trimer) but also in polypeptides ((Gly), I and11) the Gly CO chemical shifts give a similar <strong>hydrogen</strong> bonddependence. This suggests that the I3C chemical shift of any GlyCO forming the 1-type <strong>hydrogen</strong> bond is predominantly determinedby the manner of <strong>hydrogen</strong> bond. As is evident fromCIO-N and N-H--O angles, most of the 1-type <strong>hydrogen</strong>bonds are neither so bent nor entirely straight, where a preferreddirection of the <strong>hydrogen</strong> bond approaches the direction of theoxygen atom s$ lone pair. The mean value of the C=%N anglesis 153'. This value is slightly larger than that of 120-130Oobtained by averaging over the angles for N-H.-O=C type<strong>hydrogen</strong> bonds in 1357 peptides,I0 but the angles of peptidesconsidered here distribute with small discrepancy. Therefore, itcan be expected that the 'length <strong>effect</strong>" on the I3C chemical shiftin the 1-type <strong>hydrogen</strong> bond is larger than the 'direction <strong>effect</strong>".Further, the carbonyl carbons of N-terminal glycine residues(indicated by triangles) are resonated upperfield from those ofthe inner gycine residues (the second residue and polypeptide areindicated by open and full circles, respectively). This may be dueto the difference of <strong>hydrogen</strong> bond length rather than the sequence<strong>effect</strong> of amino acids.This result is similar to that by Imashiro et al.34 that the formationof intra- and intermolecular >C=O-H-O- type <strong>hydrogen</strong>bonds of hydroxybenzaldehydes in the solid state resultsin a downfield displacement of I3C chemical hifts of the carbonylcarbon with varying the 0-0 <strong>hydrogen</strong> bond length. Similarly,Berglund and Va~ghan~~ pointed out a similar correlation betweenthe isotropic 'H chemical shift and 0-0 <strong>hydrogen</strong> bond distance.(34) Imashiro, F.; Maeda, S.; Takegoshi, K.; Terao, T.; Saika, A. Chem.Phys. Lerr. 1983, 99, 189.(35) Bergund, B.; Vaughan, R. W. J. Chem. Phys. 1980, 73, 2037.P.bAAJ. Am. Chem. SOC., Vol. 110, No. 11, 1988 3383H? (b) IH /cyH"-77h 0 icn91h RNIFipe 4. Molecular structure of N-acetyl-N'-methylglycine amide: (a)model A (taking the 1-type <strong>hydrogen</strong> bond with two formamide molecules)and (b) model B (taking the 2-type <strong>hydrogen</strong> bond with a protonatedmethylamine molecule). The calculation was made for thecarbon marked by asterisks.On the other hand, I3C chemical shifts of the Gly CO arelinearly displaced to upperfield with decreasing RN4 in the 2-type<strong>hydrogen</strong> bond. The direction of the chemical shift change isopposite to that in the 1-type. Such a considerable differencebetween the two types of <strong>hydrogen</strong> bonds should be responsiblefor the electronic structure of the groups participating in the<strong>hydrogen</strong> bond. Detailed discussion about the electronic structuredrawn on the basis of quantum chemical calculation will be givenin the following section. A similar phenomenon was also reportedfor the <strong>hydrogen</strong> bond <strong>effect</strong> on nitrogen chemical shift that thedirection and magnitude of the chemical shift change dependcritically on the electronic environment around nucleus.36As is seen in Figure 3a, the plots are slightly scattered fromthe dotted straight line. Such a scatter may come from theconformational <strong>effect</strong> of the skeletal bonds and the experimentalerrors for the determination of the <strong>hydrogen</strong> bond length. In spiteof the dispersity in C=O--N angles, the 2-type <strong>hydrogen</strong> bondgave a clear linearity between the I3C chemical shift and RN ...Ocompared with the 1-type. This can be attributed to the fact thatthe distribution of distortion angles + about the glycyl residueC,-CO bond in the 2-type is much less than that in the 1-type.The direction <strong>effect</strong> of the <strong>hydrogen</strong> bond is thought to be smallin the 2-type.I3C Shielding Constant Calculation. Figures Sa-d and 6a-dshow the calculated isotropic shielding constants (ah) and theirparamagnetic terms of tensor components (ullr uZ2, and u33) ofGly CO using the model compounds A and B (Figure 4, a andb) which correspond to the 1-type and 2-type <strong>hydrogen</strong> bonds,respectively. Calculated values are all expressed in parts permillion (ppm) with an opposite sign to that of Table I. Note thatthe negative sign for the calculated shielding constant denotesdeshielding, in contrast to the positive sign of the experimentalchemical shift values. A shielding constant or tensor componentis usually represented as a sum of the diamagnetic and theparamagnetic terms. However, the behavior of the shielding tensorcan be explained by the paramagnetic term, since the diamagneticterm is isotropic.Figure 5a shows the RNa dependence of the calculated isotropicI3C shielding constant ( u~) of Gly CO in the 1-ty e <strong>hydrogen</strong>bond. In the region that RN,..o is larger than 2.6 1, uiso valuesindicate no significant RN-0 dependence in both form I and form11. This means that there exists no <strong>hydrogen</strong> bond <strong>effect</strong> at RN-0> 2.6 A. As shown in Figure Sa, there is significant differencein uiso between form I and form I1 in this region. This may comefrom the conformation <strong>effect</strong> (conformation change) in going fromform I to form 11. On the other hand, in the region that RN .ois shorter than 2.6 A, uiso values largely depend on the <strong>hydrogen</strong>bond length RN-0, that is, the <strong>hydrogen</strong> bond <strong>effect</strong> is predominant.Therefore, the experimental data that the I3C chemicalshift values largely depend on RNq should be compared with thecalculated results at RN-.o < 2.6 A.Here, we must be concerned with the critical RN.4 value being2.6 A. It seems that this value of 2.6 A is somewhat short comparedwith the experimental data as shown in Figure 3. As the(36) Webb, G. A.; Witanowski, M. Proc. Indian Acad. Sci. 1985,94,241.

3384 J. Am. Chem. SOC., Vol. 110, No. 11, 1988Ando et al.-'87--EB -188 -(a)Form II5 -189-83. -190-s5x -191 --192--220 IN o LENGTU (AII 8 IN 0 LENGTHIL)F i e 5. Variation of the calculated 13C shielding constant and its tensorcomponents with the Ne-0 <strong>hydrogen</strong> bond length (RN4) for the 1-type<strong>hydrogen</strong> bond: (a) isotropic shielding constant and (b) ullr (e) u22, and(dl 033.-180 -- t8-!81 - -2 -182-e"t -183--184--185-- -190-- 9-200-i2 -210-805 -220-yI -230-3;O-- E. !-320-0-. 5 330-$8 YO-. -350--360-L ' I I2.25 25 215 30 2.25 2.5 235 3.0N 0 .EWGlH IA:N 0 LENGTHIII2.25 2.5 2.75 3.0 2.25 2.5 275 10h 0 LENGTrlii N C .ENGT*IiiFigure 6. Variation of the calculated I3C shielding constant and its tensorcomponents with the N-0 <strong>hydrogen</strong> bond length (RNa) for the 2-typehydroge bond: (a) isotropic shielding constant and (b) uI,, (e) u22, and(4 u33.adopted semiempirical INDO MO approximation neglects sometwo-center electron integrals, the intermolecular interactions areconsidered to be reproduced reasonably in the short RNa region3'The variation of the total energy also supports this view as shown(37) Morokuma, K. Chem. Phys. Lett. 1971, 19, 129.20 23 25 275 3C 35N o I ~ ~ ~ . ~ ~ dF i e 7. Variation of the calculated total energy for the 1-type <strong>hydrogen</strong>bond with model A.in Figure 7, where the total energy minimum appears around theRN...o value of 2.3-2.5 8,. Taking into consideration that the<strong>hydrogen</strong> bond length in a stable conformation is longer by 0.228, for (Gly), I than for (Gly), II,z4 the "C shielding constantat the RNa of 2.3 and 2.5 A, as shown by the dotted line, shouldbe taken for form I1 and form I, respectively. As a result, weobtain an appropriate uiso value for form I1 resonated at about1.5 ppm downfield relative to form I. This reproduces the experimentalresult qualitatively. Although u,,~ shows a slightlycomplicated change with RNa, each shielding component changesrather monotonously. As shown in Figure 5b-d, the particularchange of ulso arises from the fact that uII increases with decreasingRN...O, while u2* decreases with a comparable degree of changefor ull. The difference of u, between form I and form I1 at theRN.* > 2.6 A region is due to the difference of ull component.This indicates that the above-mentioned conformation <strong>effect</strong> onuw can be contributed by uI1. However, the experimental findingthat the ull is almost independent of conformational change, asreported previously,' is approximately reproduced in the shorterRN-.0 region with regard for the difference of <strong>hydrogen</strong> bondlength. On the other hand, the <strong>hydrogen</strong> bond <strong>effect</strong> is evidentin the uZ2 component, which is the dominant factor for thedownfield shift in the shorter RN.-o region. Accordingly, it canbe said that the ull component and the uz2 component reflectrelatively the conformation <strong>effect</strong> and the <strong>hydrogen</strong> <strong>effect</strong>, respectively,in the 1-type. The calculated u33 component is notsensitive to the changes of RNa while the observed u33 componentshows a relatively large displacement by changes of the RN..,O asreported previously.' If one judges from the large experimentalerror which arises from the overlapping of the C, signal on theregion of uj3 of the powder spectrum, it can be said that the u33component may reflect neither the conformation <strong>effect</strong> nor the<strong>hydrogen</strong> bond <strong>effect</strong>.As shown in Figure 6a, the variation of u,,, in the 2-type <strong>hydrogen</strong>bond gives a satisfactory coincidence with the experimentalresult. The magnitude of change for uI1 is much larger than thatfor u2z and u33, and uI1 changes monotonically to upperfield withdecreasing R N ~ It . is entirely a dominant factor in determiningthe <strong>hydrogen</strong> bond dependence of the 2-type. Although the uz2component represents the contribution of deshielding in the samemanner as in the 1-type, this component does not contribute tothe whole change of a,. The <strong>hydrogen</strong> bond <strong>effect</strong> which appearsin fhe uli component dominates the relative I3C chemical shiftof Gly CO in the 2-type, even in the RN.-O > 2.6 8, region wherethe conformation <strong>effect</strong> is larger than the <strong>hydrogen</strong> bond <strong>effect</strong>in the 1-type. From the result of calculations, the origin of thedifference in the experimental results between two types of <strong>hydrogen</strong>bonds is explicitly accounted for in terms of the ull component.Figure 8a shows the directions of the principal axes of Gly COI3C chemical shift tensor components determined by the NMRstudy of [ l-13C]glycyl['5N]glycine.HCI.H20 single crystal,3s and~ ~(38) Stark, R. E.; Jelinski, L. W.; Rubin, D. J.; Torchia, D. A.; Griffin,R. G. J. Magn. Reson. 1983, 55, 266.

H-Bonding Effect on I3C Chemical Shifts of Solid PeptidesFigure 8. Orientation of the principal axes of the observed and calculated"C chemical shift tensors of glycyl residue carbonyl carbon in (a) gly-~ylglycine.HCl~H~O,~* (b) model A (I-type <strong>hydrogen</strong> bond), and (c)model B (2-type <strong>hydrogen</strong> bond), respectively. The direction of uj3 (notshown here) is perpendicular to the directions of uII and u2*.parts b and c of Figure 8 show those determined by theoreticalcalculations performed in this study. The conformation of themodel compounds indicated here is (Gly), I and the RN-.O is 2.5A It is shown that a good agreement was obtained between theexperimental and theoretical results. The uz2 component liesapproximately along the amide CO bond, and the ull is in theamide sp2 plane and lies along the direction normal to the C=Obond. The uj3 component is aligned in the direction perpendicularto the amide sp2 plane. Kempf et al.39 have pointed out that ulland uZ2 components are dominated by singlet-singlet u-** andnq* excitations, respectively. From a comparison of the behaviorof the calculated ull and uz2 components between the two typesof <strong>hydrogen</strong> bond, the nature of the proton-donating nitrogen atomis suggested to be very influencial to the electronic configurationof the u bond near the carbonyl group, but it does not change theproperty of the non<strong>bonding</strong> orbital located on the carbonyl oxygen.The uZ2 component indicates a similar variation between the 1-typeand 2-type <strong>hydrogen</strong> bonds, but ull show a considerable differencein the two types.In the FPT-INDO calculation, since a shielding constant isexpressed as a gradient of density matrix elements against themagnetic field," the calculated shielding constant cannot bedivided into contributions from certain one-electron excitations.However, such an <strong>effect</strong>ive contribution from a particular excitationcan be attributed to the specific shielding components.Therefore, we can differentiate the <strong>hydrogen</strong> bond and conformational<strong>effect</strong> by elucidating the changes of each I3C shieldingcomponent as described above.The Conformational Dependence of the Gly CO Cbemical Shift.In order to give a theoretical background to the conformationdependence of the I3C chemical shift, TonelliM attempted toexplain the behavior of the chemical shift of polypeptides in termsof the empirical rule of the "gauche y-<strong>effect</strong>". It has been revealedthat this rule is very useful to interpret the I3C chemical shiftsof paraffinic hydr~arbons.~' However, this attempt cannot beapplied for elucidating the chemical shift behavior of C, carbonsof peptides. As pointed out by Sait6?, an argument of chemicalshift displacement in terms of the gauche y-<strong>effect</strong> can apply onlyto carbonyl and C, carbons, not to the C, carbon, because C,carbons are located via trans (or rarely cis) arrangement acrossthe amide bond from each other. Nevertheless, chemical shiftsof the C, carbon vary appreciably with a conformational changeexcept for the case of polygly~ine.~~~ In addition, the chemical(39) Kempf, J.; Spiess, H. W.; Haeberlen, U.; Zimmermann, H. Chem.Phys. 1974, 4, 269.(40) !a) Tonelli, A. E. J. Am. Chem. SOC. 1980,102,7635. (b) Tonelli,A. E. Biopolymers 1984, 23, 819.(41) Grant, D. M.; Paul, E. G. J. Am. Chem. SOC. 1964, 86, 2984.(42) SaitB, H. Mugn. Reson. Chem. 1986, 24, 835.(43) Taki, T.; Yamashita, S.; Satoh, M.; Shibata, A,; Yamashita, T.;Tabeta, R.; SaitB, H. Chem. Left. 1981, 1803.(44) SaitB, H.; Tabeta, R.; Shoji, A.; Ozaki, T.; Ando, I. Mucromolecules1983, 16, 1050.(45) SaitB, H.; Tabeta, R.; Ando, I.; Ozaki, T.; Shoji, A. Chem. Lett. 1983,1437.(46) SaitB, H.; Iwanaga, Y.; Tabeta, R.; Asakura, T. Chem. Letr. 1983,427.(47) SaitB, H.; Tabeta, R.; Asakura, T.; Iwanaga, Y.; Shoji, A.; Ozaki, T.;Ando, I. Macromolecules 1984, 17, 1405.(48) Shoji, A.; Ozaki, T.; SaitB, H.; Tabeta, R.; Ando, I. Macromolecules1984, 17, 1412.(49) SaitB, H.; Tabeta, R.; Shoji, A.; Ozaki, T.; Ando, I.; Miyata, T.Biopolymers 1984, 23, 2279.J. Am. Chem. SOC., Vol. 110, No. 11, 1988 3385shift of the carbonyl carbon is thought to be characterized by thetorsion angle 4 and to be displaced upfield by several ppm whenthe angl I) takes the value of about 60'. However, the carbonylcarbons of a-helical polypeptides are resonated downfield by 4-7ppm relative to that for @-sheet Similarly, the Gly COchemical shift of (Gly), I1 is resonated downfield by 3.5 ppm fromthat for (Gly), I.] It is noted that the 4 values of the a-helicaland 31-helical form of -58O and -78O,, respectively, are in theallowable angle for the gauche conformation, while that of the@-sheet form is about -150°,4 which is clearly different from thegauche angle.As was mentioned above, (Gly), I1 forms the shorter intermolecular<strong>hydrogen</strong> bond compared with (Gly), I, and the a-helixform is generally stabilized by the strong intramolecular <strong>hydrogen</strong>bond. Apparently, the <strong>hydrogen</strong> bond plays an important rolein determining the I3C chemical shift of carbonyl carbons. Further,Pease et reported the solid-state I3C CP-MAS spectrum ofa cyclic peptide, (Gly-Phe-Gly),, where a carbonyl carbon signalappears at the upfield region compared with that of the solution-stateNMR. It was found that this molecule takes a symmetricform stabilized by two intramolecular <strong>hydrogen</strong> bonds inthe solution state,', but this symmetry is lost in the crystallinestate accompanying a cleavage of one of the two intramolecular<strong>hydrogen</strong> bondses3 The new signal appearing in the solid statecould arise from the breakdown of one of the two <strong>hydrogen</strong> bondswhich occurs by crystallization, because the angles of 4 whichare thought to affect to the chemical shifts of both Gly COS inthe solid state are almost identical. On the other hand, SaitS etal.49 reported that the Gly CO peaks of (Pro-Gly-Pro), and(Pro-Ala-Gly), taking collagen-like triple helices are displacedupfield by 5.1 and 4.1 ppm, respectively, compared with that of(Gly), 11. It is noted that there is no appreciable conformationaldifference between the triple helix and 3,-helical (Gly),s. GlyCO in the triple helix does not participate in the <strong>hydrogen</strong> bond,while a strong <strong>hydrogen</strong> bond is formed in (Gly), 11. They showthat the <strong>hydrogen</strong> bond causes the downfield shift in (Gly), 11.This coincides with the previous results obtained by the tightbindingMO calculation for (Gly), I and 11 which takes intoaccount the <strong>hydrogen</strong> bond between the inter chain^.^^As mentioned above, the quantum chemical calculation showsthat the <strong>hydrogen</strong> bond <strong>effect</strong> on the chemical shift is dominantin the vicinity of the <strong>hydrogen</strong> bond length at RN..o< 2.6 A, butat RN ...O > 2.6 A, the conformation <strong>effect</strong> becomes dominant asshown in Figure 5 because of the rapid reduction of the <strong>hydrogen</strong>bond strength owing to the increase of the <strong>hydrogen</strong> bond length.For polypeptides with no <strong>hydrogen</strong> bonds such as the abovementioned(Pro-Gly-Pro), and (Pro-Ala-Gly),, the chemical shiftdifference of 1 ppm between Gly COS of the two polypeptides isattributed only to their conformational change.It can be said, therefore, that the quantum chemical calculationcan separate successfully the conformational <strong>effect</strong> and the <strong>hydrogen</strong>bond <strong>effect</strong> on the chemical shift of Gly CO of peptides,the former being dominant in the region of long <strong>hydrogen</strong> bondlength (or in no <strong>hydrogen</strong> bond) and the latter in the region ofshort <strong>hydrogen</strong> bond length. In other words, the Gly CO chemicalshift in the solid state can be interpreted in terms of a change inelectronic structure caused by the formation of <strong>hydrogen</strong> bondas well as conformational change.Finally we can conclude as follows. The observed 13C chemicalshift in the amide type <strong>hydrogen</strong> bond moves linearly downfieldwith a decrease of the <strong>hydrogen</strong> bond length. This could bejustified by quantum chemical calculation. These results may beapplicable to the biopolymer field as a means for obtaining in-(50) Ramachandran, G. N.; Sasisekharan, V. In Aduances in ProteinChemistry; Academic: New York, 1968.(51) Pease, L. G.; Frey, M. H.; Opella, S. J. J. Am. Chem. Soc. 1981,103,467.(52) Pease, L. G.; Deber, C. M.; Blout, E. R. J. Am. Chem. Soc. 1973,95, 258.(53) Kostansek, E. C.; Thiessen, W. E.; Schomburg, D.; Lipscomb, W. N.J. Am. Chem. SOC. 1979, 101, 5811.(54) Yamanobe, T.; Ando, I.; SaitB, H.; Tabeta, R.; Shoji, A,; Ozaki, T.Bull. Chem. Soc. Jpn. 1985, 58, 23.

3386J. Am. Chem. SOC. 1988, 110, 3386-3392formation about the <strong>hydrogen</strong> bond length or <strong>hydrogen</strong> bondstrength in the solid state.Registry No. H-Gly-Gly-Val-OH, 20274-89-9; CIAc-Gly-Gly-OH,15474-96-1; H-Pro-Gly-Gly-OH, 7561-25-3; H-Ala-Gly-Gly-OH, 3 146-40-5; H-Gly-Gly-OH, 556-50-3; H-Gly-Phe-Gly-OH, 14656-09-8; H-Gly-or-Thr-OH, 27 174-1 5-8; H-Gly-Thr-OH, 7093-70- 1; H-Val-Gly-Gly-OH, 21835-35-8; H-Sar-Gly-Gly-OH, 18479-98-6; H-Gly-Gly-OH.HN03, 50998-1 2-4; H-or-Leu-Gly-Gly-OH, 4337-37-5; H-GIY-DL-Phe-OH, 721-66-4; H-Gly-Phe-OH-HC1, 78410-67-0; H-Gly-Phe-OH.TsOH, 40301 -89-1; H-Gly-OH, 56-40-6; glycine homopolymer, 257 18-94-9; polyglycine, SRU, 25734-27-4.Low-Temperature 3C Magnetic Resonance. 8. ChemicalShielding Anisotropy of Olefinic CarbonslaAnita M. Orendt, Julio C. Facelli, Alvin J. Beeler, Karl Reuter, W. J. Horton,Peter Cutts, David M. Grant,* and Josef Michl*lbContribution from the Department of Chemistry, University of Utah,Salt Lake City, Utah 84112. Received August 14, 1987Abstract: The principal values of the "C NMR shielding tensor were measured at cryogenic temperatures for a series of olefiniccarbons, including methyl-substituted ethylenes, 1 -methyl- and 1,2-dimethylcycloalkenes, methylenecycloalkanes, and bicyclo[n.m.O]alkenes.Information on the orientation of the principal axes was obtained from ab initio calculations of the chemicalshielding tensors using the IGLO (individual gauge for localized orbitals) method. The results for several compounds withunusual principal values of the shielding tensor were analyzed in terms of the bond contributions in the principal axis system.Over the last several years the combination of low-temperatureI3C NMR spectroscopy with the quantum mechanical calculationof the chemical shielding tensor has been shown to be very valuablein the interpretation of chemical shielding data in small organicmolecules.2 Studies on carbon atoms in a wide range of <strong>bonding</strong>situations have been completed, including methyl groups? methinecarbons: and methylene carbon^.^ Other studies have lookedat carbons in linear molecules6 and in the series cyclopropane,bicyclo[ 1.1 .O] butane, and [ l.l.l]pr~pellane.~ The calculationswere used to determine the orientation of the principal axis systemof the chemical shielding tensor in the molecular frame, informationthat is not determined experimentally from naturalabundance powder samples.Previously the experimental chemical shielding values for thecycloalkenes from cyclopropene to cyclooctene, ethylene, cis-2-butene, and trans-2-butene were reported.8 For the sake ofcompleteness these experimental results are also included herealong with their calculated shielding tensors. A previous experimenton ethylene-1 ,2-I3C2 provided the orientation of the principalaxis system in the molecular frame. The findings were thatthe downfield component, ull, was perpendicular to the doublebond and lay in the plane of the molecule, u22 was along the double(1) (a) For part 7 in this series, see ref 3. (b) Present address: Departmentof Chemistry, University of Texas at Austin, Austin, Texas 78712-1167.(2) Facelli, J. C.; Grant, D. M.; Michl, J. Acc. Chem. Res. 1987, 20, 152.(3) Solum, M. S.; Facelli, J. C.; Michl, J.; Grant, D. M. J. Am. Chem. Soc.1986, 108, 6464.(4) Facelli, J. C.; Orendt, A. M.; Solum, M. S.; Depke, G.; Grant, D. M.;Michl, J. J. Am. Chem. Soc., 1986, 108. 4268.(5) Facelli, J. C.; Orendt, A. M.; Beeler, A. J.; Solum, M. S.; Depke, G.;Malsch, K. D.; Downing, J. W.; Murthy, P. S.; Grant, D. M.; Michl, J. J. Am.Chem. SOC. 1985, 107, 6749.(6) Beeler, A. J.; Orendt, A. M.; Grant, D. M.; Cutts, P. W.; Michl, J.;Zilm, K. W.; Downing, J. W.; Facelli, J. C.; Schindler, M.; Kutzelnigg, W.J. Am. Chem. Soc. 1984, 106, 7672.(7) Orendt, A. M.; Facelli, J. C.; Grant, D. M.; Michl, J.; Walker, F. H.;Dailey, W. P.; Waddell, S. T.; Wiberg, K. B.; Schindler, M.; Kutzelnigg, W.Theor. Chim. Acta 1985, 68, 421.(8) Zilm, K. W.; Conlin, R. T.; Grant, D. M.; Michl, J. J. Am. Chem. SOC.1980, 102, 6672.(9) Zilm, K. W.; Grant, D. M. J. Am. Chem. SOC. 1981, 103, 2913.bond, and u33 was perpendicular to the double bond and to theplane of the molecule. Similar orientations have been found inseveral single-crystal studies,I0J1 and in another dipolar study12on more complicated alkene derivatives. Early theoretical studieshave also shown the same results for the orientation of the shieldingtensors.I3-l6In this paper, the experimental principal values of the shieldingtensor as well as the calculated results are presented for a widerange of olefinic carbons. The types of olefinic carbons that arestudied can be divided into five groups: methyl-substitutedethylenes, cycloalkenes, 1 -methyl- and 1,2-dimethylcycloalkenes,methylenecycloalkanes, and bicyclo[n.m.O]alkene. The structuresof the compounds studied are shown in Figure 1.Experimental and Computational MethodsMaterials. Commercial samples of tetramethylethylene, methylenecyclobutane,methylenecyclopentane, propene, 1 methylcyclopentene, andisobutene were used without any further purification. A sample of 1,2-dimethylcyclobutene was provided by Professor D. Aue (University ofCalifornia at Santa Barbara). The precursor for the synthesis of bicycl0[2.2.0]hex-l(4)-eneas well as a sample of bicyclo[3.2.0]hept-l(4)-enewas provided by Professor K. B. Wiberg (Yale University). Drs. P. J.Okarma and J. J. Caringi (Yale University) provided a sample of bicyclo[3.3.0]oct-l(5)-ene.The remaining compounds were synthesized according to publishedliterature procedures: methylenecyclopropane,*' 1,2-bismethylenecyclobutane,181,2-dimethylcy~lohexene,'~ bicycl0[3.2.0]hept-l(5)-ene,~~ bi-(10) Wolff, E. K.; Griffin, R. G.; Waugh, J. S. J. Chem. Phys. 1977, 67,2387.(11) Igner, D.; Fiat, D. J. Magn. Reson. 1982, 46, 233.(12) Manenschijn, A.; Duijvestijn, M. J.; Smidt, J.; Wind, R. A,; Yannoni,C. S.; Clarke, T. C. Chem. Phys. Lett. 1984, 112, 99.(13) Ditchfield, R.; Ellis, P. D. In Topics in '.'C NMR Spectroscopy;Wiley: New York, 1974; Vol. 1.(14) Ditchfield, R.; Miller, D. P.; Pople, J. A. J. Chem. Phys. 1971, 54,4186.(15) Strong, A. B.; Ikenberry, D.; Grant, D. M. J. Mugn. Reson. 1973,9,145; errata: 1976, 21, 157.(16) Ebraheem, K. A. K.; Webb, G. A. In Progress in NMR Spectroscopy;Pergamon Press: Oxford, 1977; Vol. 11.(17) Salaun, J. R.; Champion, J.; Conia, J. M. Org. Synrh. 1977, 57, 36.(18) Borden, W. T.; Reich, I. L.; Sharpe, L. A,; Weinberg, R. B.; Reich,H. J. J. Org. Chem. 1975, 40, 2438.0002-7863/88/1510-3386$01.50/0 0 1988 American Chemical Society