HYDROGEN-BONDING EFFECTS ON AMINE ROTATION RATES IN CRYSTALLINE AMINO-ACIDS

Rates of rotation for amines in a variety of crystalline environments are reported, and the trends are explained in terms of the strengths o f local hydrogen bonding interactions. Proton spin-lattice relaxation times (T-1) and deuterium broad-line NMR spectra have been measured fo r D-, DL- and L- aspartic acid, two polymorphs of glycine, alanine, an d leucine in the temperature range from -40 to 110 degrees C. The ener gy barriers for amine rotation are 27 +/- 2 kJ mol(-1) for D- or L-asp artic acid and 22 +/- 2 kJ mol(-1) for DL-aspartic acid; these energie s are slightly lower than the previously reported value for the L form based on direct proton T-1 measurements at 60 MHz. The values for the alpha and gamma forms of glycine were 24 +/- 2 and 30 +/- 2 kJ mol(-1 ) respectively, that for L-alanine was 40 +/- 2 and that for L-leucine was 49 +/- 3 kJ mol(-1). These are all in rough agreement with previo usly reported values (although the differences for the polymorphs of g lycine and for L- vs. DL-aspartic acid were not reported). Crystal str uctures of these amino acids indicate differences in hydrogen bonding environments around the R-NH3+ groups that are probably responsible fo r the different activation barriers. A molecular mechanics calculation of the rotation energy barriers for L- and DL-aspartic acid based on the crystal structures gave satisfactory agreement with experimental r esults if a uniform (and arbitrarily chosen) dielectric constant of 2. 5 was assumed. Differences between L- and DL-aspartic acids and betwee n two polymorphs of glycine were well represented qualitatively. Inclu ding additional neighboring molecules not involved in the hydrogen bon ding or including periodic boundary conditions to describe the crystal packing did not significantly affect these results. If vacuum dielect ric constants are used, the barriers are uniformly overestimated, and if the experimental macroscopic dielectric constant values are used, t he barriers are generally underestimated. Dielectric constants differ substantially from one amino acid to another and significantly affect the estimated barriers; in fact, the bulk dielectric constants appear to be the major difference between the highest and the lowest values. The difficulty of accurately including dielectric relaxation into mole cular mechanics calculations resulted in the disagreement between expe rimental measurements and theoretical calculations.