Peptides containing the sulfonamide junction. 2. Structure and conformation of Z-Tau-Pro-D-Phe-NHiPr

The taurine (Tau) containing N-protected pseudotripeptide isopropylamide Z- Tau-Pro-D-Phe-NHiPr (1) has been specifically designed and synthesized as s uitable model to test the ability of the sulfonamido group to participate a s II-bond acceptor to a type II beta -turn and to get information on the pr eferred rotameric conformation around the S-N bond and the hybridization st ale of the nitrogen atom. The present structural investigation reveals that , although the sulfonamide junction is invariably folded in a gauche mode, the beta -turn structure, stabilized by the 4 --> 1 hydrogen bond, is not f ound in the crystal and the sulfonamido oxygen atoms are not involved in an y intra- or intermolecular hydrogen-bond interaction. More than one conform er populates the CDCl3 solution with only a minor contribution by the expec ted beta -turn. The Pro nitrogen is significantly pyramidalized and the nit rogen lone pair points in opposite direction to that of the Pro (CH)-H-alph a bond thus adopting R chirality, in an arrangement practically identical t o that found in the previously studied homochiral analogue Z-Tau-Pro-Phe-NH iPr. (C) 2000 John Wiley & Sons, Inc.