Thermal fluctuation-induced selective CO2 uptake of seemingly nonporous N,C-protected dipeptide crystals as elucidated by in situ X-ray crystallographic analysis

Abstract

Described are two experimental findings: (1) selective CO₂ uptake by dipeptide-based molecular crystals of Boc-L-methionyl-L-alanine methyl ester 1 with seemingly nonporous crystal architecture and (2) its CO₂ sorption state successfully visualized at the atomic level by means of in situ X-ray crystallographic analysis. More important was the latter, from which intermolecular interactions between CO₂ and the surrounding molecules of 1 in the crystal were analyzed by ab initio calculations, indicating that attraction between them was mainly composed of a dispersion force. In addition, a comparison study of the crystal structure of 1 with those of analogous dipeptide crystals (isomorphous with 1) demonstrated that the thermal fluctuations of the dipeptide molecules in the solid state triggered CO₂ permeation into the isolated voids present in the crystal lattice of 1.