Structure and conformation of peptides containing the sulphonamide junction

N-acetyl-tauryl-L-phenylalanine methyl ester 1 has been synthesized. The crystal structure and molecular conformation of 1 have been determined. Crystals are monoclinic, space group P2(1) with a = 5.088(2), b = 17.112(17), c = 9.581(6) A, beta = 92.34(4) degrees, Z = 2. The structure has been solved by direct methods and refined to R = 0.043 for 2279 reflections with I greater than 1.5 sigma(I). The sulphonamide junction maintains the peptide backbone folded with Tau and Phe C alpha atoms in a cisoidal arrangement, the torsion angle around the S-N bond being 65.4 degrees. In this conformation the p-orbital of the sulphonamide nitrogen lies in the region of the plane bisecting the O-S-O angle, thus favouring d pi-p pi interactions between nitrogen and sulphur atoms. The S-N bond with a length of 1.618 A has significant pi-bond character. The CO-NH is planar and adopts trans conformation. The Tau residue is extended with the Tau-C1 alpha-Ca beta bond anti-periplanar to the S-N bond. The Phe side chain conformation corresponds to the statistically most favoured g- rotamer and exhibits a chi 1 torsion angle of -67.5 degrees. The packing is characterized by intermolecular H-bonds which the Tau and Phe NH groups form with the acetyl carbonyl and one of the two sulphonamide oxygens, respectively.