Linear oligopeptides: 65′. Conformational analysis of the N-protected aromatic α-amino acid [formula omitted] by X-ray diffraction and infrared absorption

Abstract

The X-ray diffraction and i.r. absorption conformational analysis of N -tert-butyloxycarbonyl- l-phenylalanine has showed the absence of intramolecularly hydrogen-bonded peptide conformations in the solid state. The molecules are held together in rows of ‘cyclic dimer’ motifs through intermolecular N H … O C (acid) and O H … O C {urethane} hydrogen bonds, the secondary amide-like group of the urethane moiety being in the unusual cis conformation, whereas the carboxylic acid group in the common syn conformation. The two molecules in the unit cell present a centrosymmetric set of ϕ, ψ 1, and ψ 2 values. In polar solvents solvated species largely predominate. In saturated hydrocarbon solution non-associated and associated (mostly involving the carboxylic acid CO as the proton acceptor) species simultaneously occur. The extent of association decreases with dilution. The amount of intramolecularly hydrogen-bonded oxy- C 7 and C 5 forms if any, should be extremely small. The type of association at saturation seems to differ from that found in the crystalline compound obtained by precipitation with saturated aliphatic hydrocarbons (from a diethyl ether solution).