Apolar Peptide Models for Conformational Heterogeneity, Hydration, and Packing of Polypeptide Helices: Crystal Structure of Hepta- and Octapeptides Containing \alpha-Aminoisobutyric Acid

The crystal structures of two helical peptides Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe (VALU-7) and Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Aib-OMe (VALU-8) have been determined to a resolution of 1.0 and 0.9 A, respectively. Both the seven and eight residue peptides crystallize with two conformers per asymmetric unit. The VALUS conformers are completely helical and differ only at the C-terminus by a sign reversal of the \phi, \psi angles of the last residue. One of the VALU-7 conformers occurs as a normal \alpha-helix, whereas in the other, the N(7)=O(3) \alpha-type hydrogen bond is ruptured by the entry of a water molecule (W) into the helix, which in turn makes hydrogen bonds N(7)…W = 2.97 A and W…O(3) = 2.77 A. The other side of the water molecule is surrounded by a hydrophobic pocket. These two conformers give a static representation of a step in a possible helix unwinding or folding process. In the VALU-8 crystal the helices aggregate in a parallel mode, whereas the aggregation is anti-parallel in the VALU-7 crystal. The crystal parameters are VALU-7, $P2_1$, a = 10.203 (3) A, b = 19.744 (6) A, c = 22.561 (6) A, \beta = 96.76 deg, Z = 4, $C_{38}H_{69}N_7O_{10}$.0.5 $H_2O$, R = 6.65% for 3674 reflections observed >3\sigma(F); and VALU-8, $P2_1$ a = 10.593 (4) A, b = 27.57 (6) A, c = 17.745 (5) A, \beta = 95.76 (3) deg, Z = 4, $C_{42}H_{76}N_8O_{11}$.0.25 $CH_3OH$, R = 6.63% for 4701 reflections observed >3\sigma(F).