Sterically hindered disulfide bridges in cystine diketopiperazine, cysteinyl-cysteine disulfide and derivatives.

L-Cystine diketopiperazine (1), L-cysteinyl-cysteine disulfide -HCl (2), L-cysteinyl-cysteine disulfide methyl ester -HCl (3), and t-butyloxycarbonyl-L-cysteinyl-cysteine disulfide methyl ester (4) are investigated by CD, ultraviolet, 13C NMR, infrared and laser Raman spectroscopy. The temperature dependence of the 13C NMR signals of 1 reveals an exceptionally high energy barrier of deltaGNo. = 15.8 +/- 0.2 kcal/mol for the reversible change in helicity of the inherently dissymmetric disulfide bridge of 1. The P-helical diastereomer predominates in dimethyl-sulfoxide at 25 degrees C, with 80-85% of the molecules having this configuration. The Cotton effects of 1 are larger and show smaller temperature coefficients than the conformationally more mobile cystine compounds 2 and 3. After dissolving crystals of 1 in 95% ethanol there is a time-dependent decrease of the ellipticity of the negative Cotton effect at 225 nm, indicating a conformational change in going from crystal to solution. Besides 1, 2 and 3 are at present the only known examples of cystine derivatives with C-S-S-C torsional angles around 90 degrees, which do not exhibit optical activity in the long wavelength disulfide absorption, as is predicted for 1 from the Linderberg-Michl model. At 305 nm a new weak Cotton effect was discovered for 1. The solvent dependence of the CD spectra is discussed and the infrared and Raman spectra are assigned.