Acta Crystallographica Section B

Structural Science

Volume 61, Part 1 (February 2005)

research papers

Acta Cryst. (2005). B61, 58-68 [ doi:10.1107/S0108768104031787 ]

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

S. A. Moggach, D. R. Allan, C. A. Morrison, S. Parsons and L. Sawyer

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 NHcarboxylate 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 OO separation of 2.918 (4) Å and an OOO angle of 148.5 (2)°; at 4.8 GPa these parameters are 2.781 (11) and 158.5 (7)°. Elsewhere in the structure one NHO interaction reaches an NO separation of 2.691 (13) Å 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 OHOH hydrogen-bonded chains of L-serine-I are replaced in L-serine-II by shorter OHcarboxyl interactions, which have an OO separation of 2.62 (2) Å. This phase transition occurs via a change from a gauche to an anti conformation of the OH group, and a change in the NCCO torsion angle from -178.1 (2)° at 4.8 GPa to -156.3 (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 CHO hydrogen bonding in both phases relative to L-serine-I at ambient pressure.

Keywords: pressure; polymorphism; hydrogen bonding; topology.


	Structure factor file (CIF format) (8.0 kbytes) Contains datablock ser103

	Structure factor file (CIF format) (7.7 kbytes) Contains datablock ser114

	Structure factor file (CIF format) (7.5 kbytes) Contains datablock ser129

	Structure factor file (CIF format) (7.3 kbytes) Contains datablock ser141

	Structure factor file (CIF format) (7.2 kbytes) Contains datablock ser148

	Structure factor file (CIF format) (7.4 kbytes) Contains datablock ser254

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