Evaluation of charge assisted hydrogen bonds in L-(S)-lysinium L-(S)-mandelate dihydrate and L-(S)-alanine L-(S)-mandelic acid complexes: Inputs from Hirshfeld surface, PIXEL energy and QTAIM analysis

Abstract

Single crystals of two of the l-amino acids (lysine and alanine) complexed with L-mandelic acids are prepared by slow evaporation technique and X-ray diffraction analysis has been carried out. Single crystal X-ray analysis reveals that lysine-mandelic acid crystallized as a dihydrate form. In the crystalline state, the lysine molecule exists in the cationic form in which the backbone and side chain amino groups are protonated and the carboxylic acid is deprotonated. The carboxylic acid proton of the mandelic acid is transferred to the lysine side chain and thus carries a negatively charged ion. The lattice water molecules are located near the amino groups of the lysine. In contrast, alanine molecule exists in the zwitterionic form in which amino group is protonated and carboxylic acid is deprotonated and the mandelic acid is in the neutral form in the re-determined alanine-mandelic acid complex. Crystal packing of lysine-mandelate and alanine-mandelic acid complexes is qualitatively analyzed using Hirshfeld surfaces and 2D-fingerprint plots. The energetics of different dimeric pairs observed in complexes is quantitatively analyzed using PIXEL energy analysis. Topological parameters derived from QTAIM framework are used to delineate the nature of different intermolecular interactions formed in lysine/alanine-mandelic acid complexes. In this work, we used IQA scheme to evaluate the strength of charged assisted hydrogen bonds in the title complexes.