th  - 1988 - 51st ENC Conference

-- 154 I
DISCRIMINATION BETWEEN SYMMETRIC AND ASY~9~ETRIC HYDROGEN BONDS BY
ISOTOPIC PERTURBATION OF EQUILIBRIUM. Charles L. Perrin and John
D. Thoburn*, Department of Chemistry, University of California,
San Diego, La Jolla, California 92093.
The method of isotopic perturbation of equilibrium was used to
distinguish symmetric and asymmetric hydrogen bonds in monoanions
of two carboxylic acids. Oxygen-18 substituted maleic and succinic
acids were prepared by hydrolyzing their respective anhydrides with
H2180. The 13C resonances of these acids as well as their dianions
in aqueous solution show an intrinsic isotope shift of 0.027 ppm.
Titration with KOH to the monoanion of succinic acid produces an
increase in chemical-shift difference to 0.048 ppm, which is at-
tributed to an asymmetric hydrogen bond. The mmleic acid monoanion
shows no such increase in chemical-shift difference, demonstrating
that its proton lies in a symmetric single-well potential. This is
a general method which can be used to~stinguish between symmetric
and asymmetric hydrogen bonds in a wide variety of systems.
155 I
NMR STLIDIES OF PHOSPHATIDYLCHOLINES AND THIOPHOSPHATIDYLCHOL]NES
Mufeed H. Basti* and Laurine A. LaPlanche
Department of Chemistry
Northern ]11inois University
DeKalb, II. 60115
The substitution of a sulfur atom for oxygen in the polar head
group of dioctanoylphosphatidylcholine causes siqnificant
differences in the NHR spectrum of the two phospSolipids.
Three-bond proton-proton, phosphorus-carbon and phosphorus-
proton coupling constants for groups near to the sulfur (oxygen)
atom indicate conformational differences not only in the polar head
group region but also in the glycerol portion of the two molecules.
Chemical shifts of nearby proton and phosphorus nuclei are also
affected by the substitution. The methylene protons adjacent to
the sulfur atom in the thio-analogue exhibit magnetic nonequivalence
at both 200 and 500 HHz, while those of the oxygen analogue are
equivalent.
Results are analyzed in terms of population differences betweer,
preferred conformations of phosphatidylcholine and thiophospha-
t idylchol ine.
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