β‐Helical structures of Boc‐( L ‐Val‐ D ‐Val) 6 ‐OMe in the crystalline state and in chloroform

A conformational study was carried out on Boc‐( L ‐Val‐ D ‐Val) 6 ‐OMe in the crystalline state by X‐ray and IR. and by 1 H‐NMR. in chloroform. The dodecapeptide crystallizes from CHCl 3 /EtOAc with a left‐handed helical structure of the type ↑↓ β 5.6 , and from CHCl 3 /MeOH (or MeOH) with a different structure. In chloroform it forms three slowly interconverting species: one is a ↑↓ β 5.6 left‐handed helical species, and the other two are most likely single‐stranded β 4.4 helical species of opposite handedness. The double‐stranded helical species is predominant in fresh solutions of samples obtained from CHCl 3 /EtOAc. Because of the slow conversion or formation of this species some hours are needed to reach the conformational equilibrium in chloroform at 25°.