Oscillations of chiral preference in proline clusters.

Ion mobility/mass spectrometry techniques are used to study the chiral preferences of small proline clusters (containing 2 to 23 proline monomers) produced by electrospray ionization. By varying the composition of the electrospray solution from enantiomerically pure (100% L or 100% D) to racemic (50:50 L:D), it is possible to delineate which cluster sizes prefer homochiral (resolved) or heterochiral (antiresolved) compositions. The results show a remarkable oscillation in chiral preference. Singly protonated clusters, [xPro+H](+) (where x corresponds to the number of prolines), favor homochiral assemblies (for x = 4, 6, 11 and 12); heterochiral structures are preferred (although the preferences are not as strong) for x = 5 and 7. Larger, doubly protonated clusters [xPro+2H](2+) favor homochiral assemblies for x = 18, 19, and 23 and heterochiral structures for x = 14, 16, 17, 20, 21, and 22. Some of the variations that are observed can be rationalized through simple structures that would lead to especially stable geometries. It is suggested that some antiresolved clusters, such as [22Pro+2H](2+), may be comprised of resolved D- and L-proline domains.