Structure and conformation of linear peptides

The dipeptide, (DL)-alanyl-(DL)-norvaline, crystallizes in the monoclinic space group P2₁/c, with a = 12.559(2)A, b = 5.265(1), c = 16.003(3), β = 103.53(2)°, Z = 4. The structure was solved by direct methods and refined to an R-value of 0.054 for 871 reflections with I > 2. The molecule exists as a zwitterion in the crystal. The peptide unit is trans and shows significant deviations from planarity (Δω = 12.4°). The peptide backbone adopts an extended conformation. The unit cell contains D-Ala-L-norval and its enantiomer. The molecular conformation and packing features show a striking resemblance to those for D-Ala-L-Met (1), and leads to the speculation that norvaline might act as an analog of methionine.