ENVIRONMENTAL CONSTRAINTS IN THE STUDY OF FLEXIBLE SEGMENTS OF PROTEINS

The structural problem posed by ill-defined segments in protein struct ures is similar to those encountered in the study of most peptide horm ones, with terminal tracts resembling linear peptides and loops resemb ling cyclic peptides, The conformational preferences of short linear p eptides in solution can be influenced by the use of solvent mixtures o f viscosity higher than that of pure water but comparable to that of c ytoplasm. In order to check whether it is possible to use these media in the structural study of proteins, we undertook an exploratory study on BPTI in a mixture of dimethylsulfoxide and water. The complete ass ignment of BPTI in an 80:20 (by volume) DMSO-d(6)/water cryomixture at two temperatures showed that all resonances parallel those in water, hinting at the persistence of the correct protein architecture, which is also confirmed by NOESY experiments. In addition to the NOEs presen t in the aqueous solution it was possible to detect numerous new cross peaks, in particular from residues belonging to the less-defined regi ons. The new cross peaks do not originate from spin diffusion and are consistent with the best NMR structure and with the X-ray structures o f BPTI.