Solid state and solution conformation of phenylacetyl-L-cysteinyl-D-penicillamine cyclic disulphide methyl ester; a cyclic dipeptide containing a trans amide

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Abstract
The conformation of the title compound (2a) has been determined by X-ray crystallography and by solution 1H n.m.r. studies. The compound crystallises in the triclinic space group P1 and its structure was solved and refined to an R factor of 0.051, from 1 256 observed reflections. In the solid state the molecules possess a distorted trans lactam structure (Δω≈ 30°) with a P-helical disulphide bridge. Individual molecules in the crystal are H-bonded between the side chain amide and lactam functions to form a β-pleated sheet array. Assignment of the solution conformation was carried out by comparison of coupling constants with calculated values derived from crystal data, comparison of chemical shifts, and temperature coefficients of the NH resonances in (CD3)2SO and CDCl3 and by n.o.e. difference measurements. The structure and conformation of (2a) in solution was found to be similar to that in the solid state. The sulphone (6) was shown to have a similar solution conformation.

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