Mixtures of trifluoroethanol or hexafluoroisopropanol and dimethylformamide are not of general applicability for peptide condensations catalyzed by trypsin

Mixtures of a good hydrogen bond donor, 2,2,2-trifluoroethanol (TFE) or 1,1,1,3,3,3-hexafluroisopropanol, and an acceptor, dimethylformamide (DMF) (1:1,v/v), containing 4% buffer have been described as adequate solvent systems for trypsin-catalyzed peptide fragment condensations Mihara et al. (1993) Int. J. Pept. Protein Res. 41 , 405]. Thus, we decided to study the behaviour of trypsin in such solvent systems. We investigated whether this protease would efficiently catalyze condensations between fragments derived from an analogue of the gp-41 capsid protein of HIV virus or from cholecystokinin-22. None of the reactions carried out yielded the desired condensation products. However, when Fmoc-NLQNLDPSHR-OH and cholecystokinin-12 (H-ISDRDYMGWMDF-NH₂) were used as substrates, the last had its R-D peptide bond hydrolyzed producing cholecystokinin-8. The proteolytic activity of this enzyme measured against a fluorogenic peptide derivative was 50 times lower in DMF/TFE containing 5% of aqueous phase than in buffer. Steady-state fluorescence studies in DMF/TFE buffer were performed to examine the structure of this protease in these media. Steady-state spectra obtained with increasing proportions of these two organic solvents in buffer showed that the emission intensities built up. Quenching studies with iodide revealed that the I_o/I ratio (where I_o and I are the fluorescence emission intensities in the absence and presence of quencher, respectively) changed from 1.2 in aqueous media to 2.2 in DMF/TFE (1:1, v/v) containing 11% 0.2 m Tris-HCI buffer, pH 8.0, for 0.5 m iodide. The complete data indicated a higher exposure of tryptophan residues to the quencher in organic media, probably because of the partial unfolding of the enzyme.