198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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Directional Aspects of Intermolecular Interactions 27 group may bind up to six cations and that it may, in certain circumstances, share the metal cation between both oxygen atoms of the carboxylate group in a bidentate fashion, as has been found for calcium ions [54]. The lone-pair electrons in a carboxylate group are designated syn or anti,as shown in Fig. 18. The proton on another molecule can approach in either of these directions. In the syn conformation (Z-form) the proton is on the same side of the C-O bond as the other C-O bond; this is the conformation found when carboxyl groups dimerize by forming two hydrogen bonds. On the other hand, in the anti conformation (E-form) the proton is on the opposite side of the C-O bond from the other C-O bond. Ab initio quantum chemical studies of formic acid indicate that the syn (Z) conformation is more stable than the anti (E) conformation by about 4.5 kcal mol –1 , implying that the syn lone pairs are more basic (and therefore bind metal ions more readily) than do the anti lone pairs [55]. Carboxylates in active sites of enzymes generally seem to employ the more basic syn lone pairs for metal chelation [56], and it has been estimated that syn protonation is 10 4 -fold more favorable than anti protonation (since 1.4 kcal mol –1 corresponds approximately to a tenfold increase in rate). The carboxylate ion is therefore a weaker base when constrained to accept a proton in the anti (E) direction. An analysis of the directions in which metal ions approach a carboxyl group in crystal structures [57] showed that alkali metal and some alkaline-earth metal ions, which form strong bases, have less specific directions of binding, and show extensive out-of-plane interactions. By contrast, most other metal cations bind in or near the plane of the carboxylate group. Direct or bidentate binding, in which the metal ion is equidistant from the two oxygen atoms of the carboxylate ion, appears to be preferred when the metal-oxygen distance lies in the range 2.3–2.6 Å; otherwise syn binding is generally preferred. These results are a function of the carboxylate “bite” size (2.2 Å) and of a need to keep O◊◊◊M n+ ◊◊◊O angles larger than approximately 60°. These results are also found in protein crystal structures [58]. Here carboxylate groups are found in aspartate and glutamate side chains. When ligands Fig. 18. Geometry of a carboxylate group showing that it has four lone pairs of electrons on its oxygen atoms [57]. These lie in the plane of the carboxylate group and provide optimal directions for hydrogen bonding and the binding of metal ions
28 J.P. Glusker bind to a metal ion there are steric interactions between them. As a result, calcium ions can only bind near the end of an a helix. Metal ions with a smaller coordination number, such as zinc ions, do not have this constraint. When the carboxyl group is not ionized it is found, from detailed neutron diffraction studies, that the hydroxyl portion of the carboxyl group, while a powerful hydrogen-bond donor, does not accept hydrogen bonds [59]. This information can be used to assign ionization states in protein crystal structures in which hydrogen atoms are not located. The results for lysozyme, shown in Fig. 19, were in line with those deduced by other methods. a b Fig. 19 a, b. Different surroundings of carboxylate and carboxylic acid groups [59]: a ionized carboxylate group which can form several hydrogen bonds (a maximum of six in general); b carboxylic acid group which only forms two hydrogen bonds. This information can be used to differentiate between ionization states when a protein structure determination does not have hydrogen atoms located
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Page 3 and 4: Design of Organic Solids Volume Edi
Page 5 and 6: Volume Editors Prof. Dr. Edwin Webe
Page 7 and 8: VIII preface tion at an amazing lev
Page 9 and 10: Contents of Volume 196 Carbon Rich
Page 11 and 12: 2 J.P. Glusker 5 Interactions of Pl
Page 13 and 14: 4 J.P. Glusker Perturbations to the
Page 15 and 16: 6 J.P. Glusker 2.1 Sources of Cryst
Page 17 and 18: 8 J.P. Glusker other induced, will
Page 19 and 20: 10 J.P. Glusker The forces here hav
Page 21 and 22: 12 J.P. Glusker lographic structure
Page 23 and 24: 14 J.P. Glusker a c Fig. 6 a - c. D
Page 25 and 26: 16 J.P. Glusker hydrogen bond H◊
Page 27 and 28: 18 J.P. Glusker Fig. 9. Citric acid
Page 29 and 30: 20 J.P. Glusker peptide plane and a
Page 31 and 32: 22 J.P. Glusker 4.5 F◊◊◊H Int
Page 33 and 34: 24 J.P. Glusker rine-containing com
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Page 41 and 42: 32 J.P. Glusker Fig. 24. The magnes
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Page 59 and 60: 50 J.P. Glusker Fig. 39. Hydrogen-b
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92 A. Nangia · G.R. Desiraju all a
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94 A. Nangia · G.R. Desiraju 42. J
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Hydrogen-Bonded Ribbons, Tapes and
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132 Y. Aoyama 4 Catalysis . . . . .
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134 Y. Aoyama 2.2 Symmetry-Controll
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136 Y. Aoyama A trigonal centre is
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138 Y. Aoyama 18 19 Fig. 5. 2D net
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140 Y. Aoyama 24 ded (O-H◊◊◊O
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142 Y. Aoyama 2.4 Interaction-Suppo
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144 Y. Aoyama In the presence of a
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146 Y. Aoyama clusion compounds (se
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148 Y. Aoyama are intriguing guests
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150 Y. Aoyama The tuning or enginee
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152 Y. Aoyama Fig. 16. Binding isot
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154 Y. Aoyama The desorption of the
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156 Y. Aoyama Fig. 17. Suggested me
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158 Y. Aoyama 5 Concluding Remarks
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160 Y. Aoyama 4. Hölderich W, Hess
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Crystalline Polymorphism of Organic
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Author Index Volumes 151-198 Author
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Author Index Volumes 151 - 198 2 1
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Author Index Volumes 151 - 198 213
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Author Index Volumes 151 - 198 2 ]
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Springer and the environment At Spr
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