Experimental conformational study of two peptides containing α‐aminoisobutyric acid. Crystal structure of <i>N</i>‐acetyl‐α‐aminoisobutyric acid methylamide

Abstract

Abstract Some theoretical studies have predicted that the conformational freedom of the α‐aminoisobutyric acid (H‐Aib‐OH) residue is restricted to the α‐helical region of the Ramachandran map. In order to obtain conformational experimental data, two model peptide derivatives, MeCO‐Aib‐NHMe 1 and Bu t CO‐LPro‐Aib‐NHMe 2 , have been investigated. The Aib dipeptide 1 crystallizes in the monoclinic system ( a = 12.71 Å, b = 10.19 Å, c = 7.29 Å, β = 110.02°, Cc space group) and its crystal structure was elucidated by x‐ray diffraction analysis. The azimuthal angles depicting the molecular conformation (ϕ = −55.5°, ψ = −39.3°) fall in the α‐helical region of the Ramachandran map and molecules are hydrogen‐bonded in a three‐dimensional network. In CCl 4 solution, ir spectroscopy provides evidence for the occurrence of the so‐called 5 and C 7 conformers stabilized by the intramolecular i → i and i + 2 → i hydrogen bonds, respectively. The tripeptide 2 was studied in various solvents [CCl 4 , CD 2 Cl 2 , CDCl 3 , (CD 3 ) 2 SO, and D 2 O] by ir and pmr spectroscopies. It was shown to accommodate predominantly the βII folded state stabilized by the i + 3 → i hydrogen bond. All these experimental findings indicate that the Aib residue displays the same conformational behavior as the other natural chiral amino acid residues.