Characterization of polylactides with different stereoregularity using electrospray ionization ion mobility mass spectrometry

We investigated the effect of stereoregularity on the gas-phase conformations of linear and cyclic polylactides (PLA) using electrospray ionization ion mobility mass spectrometry (ESI-IM-MS) combined with molecular dynamics simulations. IM-MS analysis of PLA ions shows intriguing difference between the collision cross section (Ω_D) value of poly-L-lactide (PLLA) and poly-LD-lactide (PLDLA) ions with respect to their chain architecture and stereoregularity. In the singly sodiated linear PLA (l-PLA∗Na⁺) case, both l-PLLA and l-PLDLA up to 11mer have very similar Ω_D values, but the Ω_D values of l-PLLA are greater than that of l-PLDLA ions for larger ions. In the case of cyclic PLA (c-PLA), c-PLLA∗Na⁺ is more compact than c-PLDLA∗Na⁺ for short PLA ions. However, c-PLLA exhibits larger Ω_D value than c-PLDLA for PLA ions longer than 13mer. The origin of difference in the Ω_D values was investigated using theoretical investigation of PLAs in the gas phase. The gas-phase conformation of PLA ions is influenced by Na⁺-oxygen coordination and the weak intramolecular hydrogen bond interaction, which are more effectively formed in more flexible chains. Therefore, the less flexible PLLA has a larger Ω_D value than PLDLA. However, for short c-PLA, concomitant maximization of both Na⁺-oxygen coordination and hydrogen bond interaction is difficult due to the constricted chain freedom, which makes the Ω_D value of PLAs in this range show a different trend compared with other PLA ions. Our study facilitates the understanding of correlation between stereoregularity of PLAs and their structure, providing potential utility of IM-MS to characterize stereoisomers of polymers.