Structures of L-valyl-L-glutamine and L-glutamyl-L-valine

L-Val-L-Gln crystallizes in the orthorhombic space group P2(1)2(1)2 with a=16.419 (3), b=15.309 (3) and c=4.708(1) Angstrom. The final wR(F-0(2)) is 0.100 for 2044 independent reflections, R(F-0)=0.050 for 1475 reflections with I>2.0 sigma(1). L-Glu-L-Val crystallizes in the monoclinic space group P2(1) with a=6.487 (2), b=5.505 (2), c=16.741 (4) Angstrom and beta=97.22(2)degrees. The final wR(F-0(2)) is 0.111 for 1920 independent reflections, R(F-0)=0.047 for 1576 reflections with I>2.0 sigma(I). Molecular geometries are normal, except for a unique eclipsed orientation of the charged amino group of L-Glu-L-Val. Dipeptides with a N-terminal hydrophobic residue and C-terminal hydrophilic residue are shown to have crystal packing patterns fundamentally different from those of dipeptides with the same types of residues in reversed order. Accordingly, the structure of L-Val-L-Gln [Eggleston (1984). Acta Cryst. C40, 1250-1252] is rather similar to L-Val-L-Gln, but different from its retroanalogue L-Glu-L-Val. Nevertheless, the pairing of hydrogen-bond donors and accepters is the same 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 the formation of an additional hydrogen bond from the extra donor in the L-Gln side chain.