Hydrogen-bonding features in the 1:2 adduct of 4-aminobenzoic acid and L-proline.

The title adduct, 4-aminobenzoic acid-L-proline-water (1/2/1), C(7)H(7)NO(2).2C(5)H(9)NO(2).H2O, contains two independent proline chains with a C(5) motif, each of the head-to-tail type and each held together by N-H...O hydrogen bonds, propagated parallel to the b and c axes of the unit cell. Thus, the proline residues aggregate parallel to the ac plane. 4-Aminobenzoic acid (PABA) residues are arranged on both sides of the proline aggregate and are connected through water O atoms, which act as acceptors for PABA and as hydrogen-bond donors to the amino acids. The characteristic features of PABA, viz. twisting of the carboxyl plane from the aromatic ring and the formation of a head-to-tail chain motif [C(8)] along the b axis, are observed. A distinct feature of the structure is that no proton transfer occurs between proline and PABA.