Nucleation in complex multi-component and multi-phase systems: general discussion

Kenneth Harris opened a general discussion of the paper by Herma Cuppen: It is well established from solid-state NMR and other techniques that the NH3+ group in crystalline amino acids undergoes rapid rotation about the C–NH3+ bond. Is the phase transition between the β and α phases of DL-norleucine associated with any significant discontinuity in the rate of this motion, and/or the temperature dependence of the rate of this motion? Furthermore, is there any evidence for disorder (dynamic or static) of the alkyl chain of the norleucine molecules in the α and β phases, and if so, does the nature of this disorder change significantly at the phase transition? Herma Cuppen answered: Rotation barriers about the C–NH3+ bond depend on the motive of the hydrogen bonding network in the crystalline phases. For DL-norleucine this motive stays intact during the transition, and I therefore do not expect a large difference for the two phases. The changes occur where the aliphatic chains interact. In a previous study,1 we determined the rotation barrier of the methyl group by means of molecular dynamics simulations for both phases. We found them to be identical, and to be in agreement with experimental values for DL-norvaline2 and DL-norleucine.3 We did not find any correlation between internal movement of the molecules and the onset of the transition.