Assessment of the conformational implications of chemical modification is an important aspect of analogue design. A new procedure, the assessment of conformational mimicry, which determines the percentage of sterically accessible conformations for the parent compound also available to the analogue, is used to show that 88% of the conformers allowed for the cis amide bond are also available to peptides in which the amide bond is replaced by a 1,5-disubstituted tetrazole ring that locks the amide bond in the cis conformer. This analysis was made possible by the crystal structure of a cyclic dipeptide, cyclo[l-Phe-i/'(CN4)-L-Ala], determined in this paper. The crystals of the diketopiperazine analogue are monoclinic, space group P2\/c, with cell parameters a = 11.677 (1), b = 7.742 (1), c = 13.086 (1) A; /? = 93.39 (1)°; Z = 4; and = 1.368 g cm-3. The tetrazole ring system is planar with all five torsional angles equal to 0°. The diketopiperazine ring system is nearly planar, and the phenylalanine ring adopts the flagpole orientation over the cyclic dipeptide. A procedure for the preparation of this class of peptide analogues by synthetic routes avoiding racemization of the amino acids of the starting dipeptide is demonstrated. The tetrazole ring provides, therefore, a synthetic probe for the role of cis-trans isomerism of A'-alkylamide bonds, such as that of proline, in molecular recognition.Replacement of the amide bond by surrogates to enhance metabolic stability and/or probe receptor specificity has become an increasingly important topic of research1 as the central biological role of peptides as chemical effectors becomes more understood. Proline occupies a special role among those amino acids

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The crystal structure of the bismuth(111) complex with (+)-tartaric acid, ammonium aquabis[(+)tartrato(2-)]bismuthate(111) hydrate, has been determined by X-ray methods and refined to a residual R 0.020 for 1288 observed reflections. Crystals are orthorhombic, space group P 212121 with Z 4 in a cell of dimensions a 7.4712 (4), b 10.856(1), c 17.609(3) A. Each nine-coordinate MOQ bismuth centre comprises three bidentate a-hydroxy carboxy residues from two tartrato(2-) ligands (with one bridging), an asymmetric bidentate carboxylato(0,O ') group, and a water. The Bi-0 range is 2.372(7)-2.738(6) .k [mean 2.509(6) A]. The resultant structure is a linear polymer which is stabilized by extensive hydrogen-bonding interactions. Mlcks, R. H., (Ed.) 'Essentials of Materia Medica Pharmacology and Therapeutics' 4th Edn (