JOURNAL O F T H E AMERICAN CHEMICAL SOCIETY CCopyright 1986 by the American Chemical Society
VOLUME108, NUMBER 23
NOVEMBER 12, 1986
Conformational Analysis of Charged Flexible Molecules in Water by Application of a New Karplus Equation Combined with MM2 Computations: Conformations of Carnitine and Acetylcarnitine William J. Colucci, Richard D. Gandour,* and Edward A. Mooberry Contribution f r o m the Department of Chemistry, Louisiana State University. Baton Rouge. Louisiana 70803- 1804. Received April 18, 1986
Abstract: The solution conformations of carnitine and acetylcarnitine in D 2 0 are estimated from high-resolution 'H NMR coupling data. Conformations and populations of conformers are calculated from vicinal coupling constants by employing a new Karplus relationship with empirically derived substituent constants (Colucci, W. J.; Jungk, S. J.; Gandour, R. D. Magn. Reson. Chem. 1985, 23, 335-343). For this study, substituent constants are assigned for the monosubstituted ethanes: XCH2CH3, where X = -COOH, -OAc, -OH, -N+Me3, -CH2N+Me3,-CH,COOH. Solvent effects are accounted for by measuring the coupling constants of the monosubstituted ethanes under the same conditions employed in the measurement of the carnitine and acetylcarnitine spectra. The assignment of diasteriotopic protons of carnitine and acetylcarnitine is made by analogy with previous experimental work and comparison with the lowest energy conformation as determined by molecular mechanics (MM2) calculations parameterized with atomic charges from ab initio (3-21G) calculations. The vicinal coupling constants are then used to calculate populations of conformers, arising from rotation about the C2
