A non-planar peptide bond in L-seryl-L-valine.

The molecular structure of (I) is shown in Fig. 1. Bond lengths and angles are normal, but the unusual non-planarity of the peptide bond is quite evident. The associated torsion angle C1ÐC3ÐN2ÐC4 (!) is 157.37 (15) , a deviation from 180 that is superceded among small chiral peptides only by the 156.6 ! angle in N-(tert-butoxycarbonyl)-l-Pro-l-Leu benzyl ester (Sugino et al., 1978). Fig. 2(a) shows the molecular packing arrangement of (I). The crystal structure is divided into hydrophobic and hydrophilic layers in very much the same manner as seen for l-Ser-lLeu, (II) (Fig. 2b; Søowikowska & Lipkowski, 2001), despite a substantial shift in the angle, which is 98.623 (6) for (I) but just 84.19 for (II), after transformation of the originally reported unit cell to match the packing observed for (I). The length of the c axis increases from 15.588 (10) AÊ for (I) to 18.1263 (9) AÊ for (II), as the thickness of the hydrophobic layer increases to accommodate the bulkier Leu side chain. Changes in the other two axes, however, are very modest [for (I), a = 5.383 (4) AÊ and b = 6.315 (4) AÊ , and for (II), a = 5.3288 (3) AÊ and b = 6.3696 (6) AÊ ]. The peptide bond twist recurs for (II), which has ! = 157.99 (12) . The origin of the low ! values for (I) and (II) is indicated by the detailed view of the hydrogen-bonding interactions of (I) shown in Fig. 3. The C-terminal carboxylate group is clearly rotated away from the planar con®guration in order to form good hydrogen bonds with four nearby donors, three amino groups and one Ser hydroxyl group (see also Table 2). The latter is twisted into an unusual eclipsed conformation, with a value of 131 (3) for the C1ÐC2ÐO1ÐH5 torsion angle (Table 1). Notably, any modi®cation of the main-chain conformation (to bring ! closer to 180 ) would break the hydrogen bonds donated by the amide NH group, the sidechain Ser-OH group, or both. The crystal packing arrangement of l-Ser-l-Ala (GoÈ rbitz, 2000), with a three-dimensional hydrogen-bonding pattern, is different from those of (I) and (II). The speci®c types of intermolecular interactions are nevertheless quite similar. The only modi®cation concerns the amide H atom, which is organic compounds