Crystal structure and NMR investigation of the serine proteinase inhibitor MR889, a cyclic thiolic compound

The serine proteinase inhibitor N-(2-oxo-2,3,4,5-tetrahydro-3-thienyl)-2- (2-thenoylthio)propionamide (MR889) has been crystallised, and its three-dimensional structure studied by X-ray diffraction and by NMR techniques in solution and in the solid state. MR889 crystallises in the monoclinic space group P2₁/n with cell constants a = 10.539(2), b = 9.647(5), c= 14.654(1) Å, β = 101.62(1)°; the asymmetric unit contains one molecule of MR889. The weighted crystallographic R factor for the refined structure is 0.044. The solution structure of MR889 has been elucidated through the measurement of proton-proton and proton-carbon nuclear Overhauser enhancements. Measurement of the ¹³C longitudinal relaxation times yields information about the dynamics of MR889 in dimethylsulfoxide solution. The comparison between solid and solution state ¹³C NMR spectra allows an understanding of the quite different structures in the two states.