Betaine 0.77-perhydrate 0.23-hydrate and common structural motifs in crystals of amino acid perhydrates

The title compound, betaine 0.77-perhydrate 0.23-hydrate, (CH₃)₃N⁺CH₂COO^-·0.77H₂O₂·0.23H₂O, crystallizes in the ortho­rhom­bic noncentrosymmetric space group Pca2₁. Chiral mol­ecules of hydrogen peroxide are positionally disordered with water mol­ecules in a ratio of 0.77:0.23. Betaine, 2-(tri­methyl­azaniumyl)­acetate, preserves its zwitterionic state, with a positively charged ammonium group and a negatively charged carboxyl­ate group. The mol­ecular conformation of betaine here differs from the conformations of both anhydrous betaine and its hydrate, mainly in the orientation of the carboxyl­ate group with respect to the C-C-N skeleton. Hydrogen peroxide is linked via two hydrogen bonds to carboxyl­ate groups, forming infinite chains along the crystallographic a axis, which are very similar to those in the crystal structure of betaine hydrate. The present work contributes to the understanding of the structure-forming factors for amino acid perhydrates, which are presently attracting much attention. A correlation is suggested between the ratio of amino acid zwitterions and hydrogen peroxide in the unit cell and the structural motifs present in the crystal structures of all currently known amino acids perhydrates. This can help to classify the crystal structures of amino acid perhydrates and to design new crystal structures.