Crystals of glycylglycine phosphate monohydrate were obtained by evaporating an aqueous solution containing equimolar quantities of phosphoric acid and the dipeptide, glycylglycine. The crystals are orthorhombic, space group Pna2t, with a=17879 (6), b=12-130 (4) and c=4-621 (1)A. Threedimensional intensity data were collected using a Picker FACS-1 diffractometer. The structure was solved by the heavy-atom method and was refined by full-matrix least squares to R = 0.024. The crystal structure features a short hydrogen bond (2.503 +_ 0-003 A) between the phosphate moiety and the carboxyl group of the dipeptide. Although the least-squares refinement suggested that the hydrogen atom involved in this bond might be positioned symmetrically between the two oxygen atoms, difference Fourier maps and the observed C-O and P-O bond lengths indicate that it is bonded covalently to the carboxyl oxygen atom. Thus, the compound is properly formulated as (glycylglycine.H)+(H2PO4) -. The crystal structure also involves a short hydrogen bond of 2.571 (3) A between phosphate moieties related by the e translation. The peptide conformation is not significantly different from that found in other crystal structures.
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Freeman et al. (1972) studied this question.