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

L-Val-L-Gln crystallizes in the orthorhombic space group P2₁2₁2 with a = 16.419 (3), b = 15.309 (3) and c = 4.708 (1) Å. The final wR(F_o²) is 0.100 for 2044 independent reflections, R(F_o) = 0.050 for 1475 reflections with I > 2.0σ(I). L-Glu-L-Val crystallizes in the monoclinic space group P2₁ with a = 6.487 (2), b = 5.505 (2), c = 16.741 (4) Å and β = 97.22 (2)°. The final wR(FF_o²) is 0.111 for 1920 independent reflections, R(F_o) = 0.047 for 1576 reflections with I > 2.0σ(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-Glu [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 acceptors 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.

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