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[Xaal = H or DPhe; Xaa3 = Phe or Tyr; Xaa7 = Cys, Me2Cys or M e 2~C y s ;Xaa' = OH, Thr' (OH) or Thr8NH2], were examined in aqueous solution by 'H-NMR spectroscopy and circular dichroism.The influence of the helical nature of the disulfide bridge and the presence of exocyclic residues on biological activity were investigated with particular care.

The crystal structure of [ClSn(CH2CH2CH2)2NCH3]2 (1) has been determined by an X-ray study. The crystal data are as follows: triclinic space group PI, a = 10.757 ( 6), b = 7.673 (3), c = 12.662 (7) Á; a = 83.49 (4), ß = 90.26 (4), y = 72.48 (4)°; V = 989.4 Á3 4; Z = 2. The structure was refined to R = 0.045. The two five-coordinate tin moieties exhibit approximately trigonal-bipyramidal geometries with the chlorine and nitrogen atoms occupying apical positions and the tin-tin and tin-carbon bonds occupying equatorial positions and are twisted with respect to one another by about 120°a long the tin-tin bond. The , 13C, and 119Sn NMR data indicate that in solution 1 exists as only one isomer having the same structure as in the solid state. At low temperature, the 13C spectra reveal restricted internal rotation about the tin-tin bond with an activation barrier of about 11.5 ± 0.5 kcal/mol. In contrast, [CH3Sn(CH2CH2CH2)2NCH3]2 (2) exists in solution as a mixture of at least three isomers, A, B, and C. NMR data on the major isomer A of 2 are explained in terms of a structure with the two methyl groups and the two intramolecularly coordinating nitrogen atoms in apical positions at each tin and the two ring carbon chains and the tin-tin bond in equatorial positions, in agreement with both the cyclic ligand structure and the polarity rule. For the medium and minor isomers of 2, B and C, respectively, no unambiguous structure can be proposed: B and C either have respectively only one and no methyl group in the apical position, with one and two tin atoms in the apical position, or have one and two four-coordinate tin centers, respectively. An analysis of the cross peaks of a two-dimensional EXSY U9Sn NMR spectrum of 2 reveals that the isomers A and B interconvert through an uncorrelated rearrangement of one tin center at a time. No evidence could be found that the internal rotation about the tin-tin bond is restricted in 2 on the NMR time scale at temperatures at which it is in 1. The differences between the dynamic stereochemical behaviors of 1 and 2 are explained in terms of the apicophilicity differences between a methyl group and a chlorine atom.

The possible conformations of SMS 201–995, an active analogue of somastostatin, have been studied in dimethylsulfoxide solution by 500 MHz proton n.m.r. spectroscopy. The assignments have been made by use of 2D‐correlated methods to detect long‐range coupling connectivities in aromatic residues and between the α protons of consecutive residues. NOESY experiments enabled us to correlate amide and α protons of neighbouring amino acid residues, which indicate a less flexible situation than in water. Measurements of temperature coefficients of the amide protons, of NH‐CαH coupling constants and NOE effects are in favour of one predominant conformation with a β turn, of type II', involving amino acids Phe3to Thr6.