N,N′-[Dimethyl-(2,2′-dithiobisacetyl)]ethylenediamine (1) has been synthesized in 30% overall yield from N,N′-dimethylethylenediamine and thioacetic acid by an improved procedure involving simultaneous deprotection and oxidative cyclization with iodine. This cyclic diamide disulfide exists in solution as a mixture of two Z,Z and one Z,E disulfide, and amide ring conformers and has been characterized by nuclear Overhauser effect (NOE), 1H–1H, 1H–13C shift-correlated 2D-NMR and molecular modelling studies. Among the Z,Z ring conformers Z,Z1 and Z,Z2, the former predominates and interconverts with the latter isomer by rotation about the S—S bond with an activation energy of 14.5 ± 1.3 kcal/mol. Coalescence of N-CH3 signals occurred at ca. 127 °C (500 MHz), which corresponded to an approximate barrier to amide rotation of 19.3 kcal/mol. Aromatic solvent-induced shifts in C6D6 corroborated molecular mechanics and NOE predictions of amide stereochemistry. The structure of the Z,E stereoisomer of 1 has been determined by single-crystal X-ray diffraction at 296 K. A large geminal N-CH2 inequivalence (>2 ppm in CDCl3) was observed for the Z,Z conformers. Proton chemical shifts have been calculated for the conformers of 1 and related molecular fragments with DFT/GIAO theory. Absolute chemical shifts are modelled within 0.2 ppm of experiment. The unusual nonequivalence of geminal N-CH2 and S-CH2 protons can be understood as a combination of shielding mechanisms derived from short N-methyl contacts, amide group orientation, and sulfur lone-pair disposition. An implication of these results is the possibility of using α-CH (and eventually α-CH) shifts to probe the local conformational space in cyclic peptides and other conformationally constrained rings. Keywords: amide/disulfide rotamers, conformational analysis, density functional theory, DFT/GIAO NMR shift calculations, methylene nonequivalence, molecular modelling.
Enhanced Conformational Space Sampling Improves the Prediction of Chemical Shifts in Proteins