Conformational Analysis of Charged Flexible Molecules in Water by Application of a New Karplus Equation Combined with MM2 Computations: Conformations of Carnitine and Acetylcarnitine

Article abstract of DOI:10.1021/ja00283a001

The solution conformations of carnitine and acetylcarnitine in D2O are estimated from high-resolution 1H 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, -CH2COOH.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-C3 and C3-C4 bonds, in solution.Both compounds adopt a highly preferred g^- conformation about the N1-C4-C3-O3 torsion angle.In contrast, the C1-C2-C3-C4 torsion angle exhibits substantial rotational freedom between g^- and a.In carnitine, the anti conformer dominates, regardless of the solution pD, whereas in acetylcarnitine the g^- conformer is most prevalent.In either compound, ionization from cation to zwitterion results in a net population increase of g^-, g^- (folded) conformer over the a,g^- (extended) conformer.The relative energetics of extended and folded conformers suggest that binding of either carnitine or acetylcarnitine to the enzyme carnitine acetyltransferase occurs with the folded form.