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

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 P2(1)2(1)2(1). 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 R-type hydrogen-bonded ring motifs. This occurs by a change in the geometry of hydrogen-bonded chains connecting the hydroxyl groups of the - CH2OH 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) Angstrom and an O ... O ... O angle of 148.5 (2)degrees; at 4.8 GPa these parameters are 2.781 (11) and 158.5 (7) degrees. Elsewhere in the structure one NH ... O interaction reaches an N ... O separation of 2.691 (13) Angstrom 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 L-serine-II. The OH ... OH hydrogen-bonded chains of L-serine-I are replaced in L-serine-II by shorter OH ... carboxyl interactions, which have an O ... O separation of 2.62 (2) Angstrom. This phase transition occurs via a change from a gauche to an anti conformation of the OH group, and a change in the NCalphaCO torsion angle from -178.1 (2)degrees at 4.8 GPa to -156.3 (10)degrees 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.