L-VALYL-L-GLUTAMINE AND L-GLUTAMYL-L-VALINE. Carl Henrik Gorbitz and Paul Hoff Backe, Department of Chemistry, University of Oslo, PO.Box 1033 Blindern, N-0315 Oslo, Norway.

The structure of L-Val-L-Glu [Eggleston, (1984). Acta Cryst. C40, 1250-1252] has a crystal packing arrangement and H-bond pattern rather similar to L-Val-L-Gln, but radically different from L-Glu-L-Val, which contains the same amino acid residues in reversed order. Nevertheless, the specific hydrogen bond interactions are almost identical for L-Val-L-Glu and L-Glu-L-Val, indicating very distinct hydrogen bonding preferences. This is the first demonstration of such a coincidence among dipeptide structures.

The differences between L-Val-L-Glu and L-Val-L-Gln structures stem from modifications of the molecular geometry and cell parameters due to formation of an additional hydrogen bond from the extra donor in the L-Gln side chain.

Both molecular geometries are normal, except for a unique eclipsed orientation of the main chain amino group of L-Glu-L-Val.

Crystal data and refinement Both data sets were collected on a Nicolet P3 diffractometer at 120 K.

L-Val-L-Gln: Space group P2₁2₁2, a= 16.419(3), b = 15.309(3) and c = 4.708(1)Å, final wR(F_o²) = 0.100 for 2044 independent reflections, R(F_o) = 0.050 for 1475 refl. with I > 2.0[[sigma]](I).

L-Glu-L-Val: Space group P2₁, a = 6.487(2), b = 5.505(2), c = 16.741(4) Å and [[beta]]= 97.22(2)deg., final wR(F_o²) = 0.111 for 1920 independent reflections, R(F_o) = 0.047 for 1576 refl. with I > 2.0s(I).