[Circular dichroism of heterodetic cyclic cystinyl tripeptides (author's transl)]

The circular dichroism of two series of heterodetic cyclic tripeptides, t-butoxycarbonyl-L-cysteinyl-X-cysteine disulfide methyl esters [X=Ala, Val, Leu, Phe, Glu(OBut), Lys(Z)] and L-cysteinyl-X-cysteine disulfide methyl ester formates, and a corresponding series of t-butoxycarbonyl-L-cysteinyl-(glycyl)n-cysteine disulfide methyl esters (n=1-5) was investigated. The interaction between the disulfide group and the amide bonds results in a long-wavelength Cotton effect at 300 nm, which exhibits a strong solvent dependence. In contrast to the long-wavelength disulfide band and to the n-pi amide Cotton effect a bathochromic shift is observed from less polar to strongly polar solvents. Within the series of N-terminal protected tripeptide esters the influence of the central chiral amino acid X is small, but it increases in the non-protected peptide esters. The circular dichroism is dominated by the influence of the conformation of the disulfide ring, which is essentially the same in both series. Due to interactions of the disulfide group with the amide chromophores, the sign of the long-wavelength disulfide Cotton effect cannot be used for the determination of the helicity of the CSSC group. Infrared spectra of the protected compounds indicate trans-peptide bonds in the solid state, which may also be true for the solutions. A pronounced time-dependence is observed for the circular dichroism of t-butoxycarbonyl-L-cysteinyl-glycyl-cysteine disulfide methyl ester after dissolving in ethanol. This is due to a uniform conformational change of the ring structure as shown by spectroscopic analysis using extinction and ellipticity diagrams. In the series of the cyclic cystinyl peptides containing glycyl residues a strong electronic disulfide-amide interaction is observed is observed for the tripeptide, being still observable for the tetra-peptide. The comparison of the glycyl tripeptide with the tripeptides containing chiral central amino acids reveals a negative contribution to the circular dichroism at 260 nm due to these L-residues.