title: Effect of pressure on the crystal structure of L-serine-I and the crystal structure of L-serine-II at 5.4 GPa.
abstract: 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 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) A 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) A 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) A. 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.
authors.affiliation: School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JJ, Scotland.
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Moggach
authors.forename: David R
authors.initials: DR
authors.lastname: Allan
authors.forename: Carole A
authors.initials: CA
authors.lastname: Morrison
authors.forename: Simon
authors.initials: S
authors.lastname: Parsons
authors.forename: Lindsay
authors.initials: L
authors.lastname: Sawyer
mesh_terms: D003461:Crystallography / Q000379:methods; D008958:Models, Molecular; D015394:Molecular Structure; D011312:Pressure; D012694:Serine / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D012694:Serine