L-lysine amino acid is cocrystallized with L-mandelic acid by the slow
evaporation method. 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. Intermolecular interactions formed between
lysinium, mandelate and lattice water molecules are qualitatively
analyzed using Hirshfeld surfaces and 2D-fingerprint plots. The
energetics of different dimeric 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 the title complex.