The crystal structure of the amino-terminal pentapeptide of suzukacillin. Occurrence of a four-fold peptide helix

Abstract

The monohydrate of the protected amino-terminal pentapeptide of suzukacillin, t-butoxycarbonyl--aminoisobutyryl-L-prolyl-L-valyl-α-aminoisobutyryl-L-valine methyl ester, C₂₉H₅₁N₅O₈, crystallizes in the orthorhombic space group P2₁2₁2₁ with a= 10.192, b= 10.440, c= 32.959 Å , and Z= 4. The structure has been solved by direct methods and refined to an R value of 0.101 for 1 827 observed reflections. The molecule exists as a four-fold helix with a pitch of 5.58 Å The helix is stabilised by N-H...O hydrogen bonds, two of the 5→1 type (corresponding to the α-helix) and the third of the 4→1 type (3₁₀ helix). The carbonyl oxygen of the amino-protecting group accepts two hydrogen bonds, one each from the amide NH groups of the third (4→1) and fourth (5→1) residues. The remaining 5→1 hydrogen bond is between the two terminal residues. The lone water molecule in the structure is hydrogen bonded to carbonyl oxygens of the prolyl residue in one molecule and the non-terminal valyl residue in a symmetry-related molecule.