Studies on the Conformation of Boc‐Protected (S)‐(+)‐Isovaline Homopeptide Methyl Esters in the Solid State and in Solution

X‐Ray diffraction analyses of the fully protected peptides Boc‐[( S )‐Iva] n ‐OMe ( n = 3, 4, 6) reveal two independent molecules in the asymmetric unit. The structures of these can be described as β‐turns or 3 10 helices (depending on the length of the oligopeptide) of alternating screw sense ( M and P ) in a head to tail alignment. This structure is stabilized by hydrogen bonds between the NH(1) of the ( M )‐helix and the OC(ω‐1) of the ( P )‐helix and the NH(2) ( M ) and the ester carbonyl group ( P ). Low temperature 1 H‐NMR spectra of the hexamer in CD 2 Cl 2 solution show two interchanging species in a ratio of 4:1; NOESY experiments prove that these are the two helical conformers found in the crystal ( P : M , 4:1). The NOESY spectrum at −90°C indicates the pairing of ( P ) and ( M ) helices. Thermodynamic and kinetic parameters for the helix transformation P ⇌ M (unfolding/folding) are presented.