serine-I and the crystal structure of L-serine-II at 5.4 GPa

# 2005 International Union of Crystallography Printed in Great Britain ± all rights reserved The crystal structure of l-serine has been determined at room temperature at pressures between 0.3 and 4.8 GPa. The structure of this phase (hereafter termed l-serine-I), which consists of the molecules in their zwitterionic tautomer, is orthorhombic, space group P212121. The least compressible cell dimension (c), corresponds to chains of head-to-tail NH carboxylate hydrogen bonds. The most compressible direction is along b, and the pressure-induced distortion in this direction takes the form of closing up voids in the middle of Rtype hydrogen-bonded ring motifs. This occurs by a change in the geometry of hydrogen-bonded chains connecting the hydroxyl groups of the ÐCH2OH side chains. These hydrogen bonds are the longest conventional hydrogen bonds in the system at ambient pressure, having an O O separation of 2.918 (4) AÊ and an O O O angle of 148.5 (2) ; at 4.8 GPa these parameters are 2.781 (11) and 158.5 (7) . Elsewhere in the structure one NH O interaction reaches an N O separation of 2.691 (13) AÊ at 4.8 GPa. This is amongst the shortest of this type of interaction to have been observed in an amino acid crystal structure. Above 4.8 GPa the structure undergoes a single-crystal-to-single-crystal phase transition to a hitherto uncharacterized polymorph, which we designate lserine-II. The OH OH hydrogen-bonded chains of l-serineI are replaced in l-serine-II by shorter OH carboxyl interactions, which have an O O separation of 2.62 (2) AÊ . This phase transition occurs via a change from a gauche to an anti conformation of the OH group, and a change in the NC CO torsion angle from ÿ178.1 (2) at 4.8 GPa to ÿ156.3 (10) at 5.4 GPa. Thus, the same topology appears in both crystal forms, which explains why it occurs from one single-crystal form to another. The transition to l-serine-II is also characterized by the closing-up of voids which occur in the centres of other R-type motifs elsewhere in the structure. There is a marked increase in CH O hydrogen bonding in both phases relative to l-serine-I at ambient pressure. Received 2 August 2004 Accepted 30 November 2004