Interactions between cyclic oligo(l-lactic acid) and alkali metal ions in organic solvent

Abstract

We investigated the interaction between cyclic oligo (L-lactic acid) (c-OLLA) and alkali metal ions, including sodium, potassium, rubidium, and cesium. The analysis was conducted by electrospray ionization mass spectrometry (ESI-MS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) in the presence of a mixture of c-OLLA with 21 or fewer repeat units, which was extracted from poly(L-lactic acid), and alkali metal ions. The results suggested that an interaction between c-OLLA and alkali ions exists only in solution, not in bulk. In addition, ESI-MS/MS analysis of higher m/z analytes suggested that formation of two kinds of c-OLLA with sodium ions occurred, and thus it is assumed that c-OLLA includes alkali ions in its inner cavity. The liquid-liquid ion transformation in the presence of sodium ions and c-OLLA further supports this interaction. c-OLLA was found to exhibit higher complexation efficiency than PLLA by extracting sodium ions from the aqueous phase to the organic phase.