Chirality and packing in small proline clusters.

The chiral composition of amino acid clusters may be related to the origin of chirality in biological systems. Here, we use ion mobility/mass spectrometry techniques to investigate the gas phase structures of singly charged proline clusters containing two to six monomers. Using deuterated L-proline (L(D7)) and different electrospray solution compositions varying from enantiopure (50:50 L:L(D7)) to racemic (50:50 L(D7):D), it is possible to study collision cross sections of L-, D-, and mixed xL:xD-proline clusters (where x refers to the number of monomers). These results show that [2Pro+H](+) and [3Pro+H](+) clusters, previously shown (Holliday et al. J. Phys. Chem. A 2012, DOI: 10.1021/jp302677n) to have a very small heterochiral preference, have similar collision cross sections for homochiral and heterochiral proline assemblies. The [4Pro+H](+) and [6Pro+H](+) clusters that exhibit homochiral preference have smaller collision cross sections for homochiral clusters and larger collision cross sections for heterochiral clusters. The [5Pro+H](+) cluster with heterochiral preference has a smaller collision cross section for its heterochiral compositions than for its homochiral compositions. These results suggest that the packing efficiency of subunits within each cluster influences the stability and prevalence of proline multimers as either homochiral or mixed L- and D- clusters.