Methyl dynamics in crystalline amino acids: MD and NMR

Correlation times for rotation of deuterated methyls in crystalline leucine, valine, and cyclo‐ L ‐alanyl‐ L ‐alanine are calculated with molecular dynamics and compared with NMR data. The simulations distinguish between methyls having different steric environments in the crystal, yielding correlation times differing by a factor of up to 30 for methyls within a given crystal. MD and NMR correlation times agree to within a factor of 2. However, averaging over nonequivalent methyls can yield correlation functions that, although actually multiexponential, are well fit by single exponentials. This may have significance for interpreting NMR data; previous NMR data did not distinguish between the methyls in these crystals. Adiabatic rotational barriers calculated with the X‐ray structure differ from effective barriers during simulation by up to ±1 kcal/mol; the difference indicates that dynamical effects have a significant role in determining rotational correlation times. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1052–1058, 2003