Probing the molecular interaction of chymotrypsin with organophosphorus compounds by 31P diffusion NMR in aqueous solutions

In the present study, we applied for the first time (31)P diffusion NMR to resolve different species obtained by the addition of organophosphorus compounds (OP) such as diisopropyl phosphorofluoridate (DFP) or 1-pyrenebutyl phosphorodichloridate (PBPDC) to alpha-chymotrypsin (Cht). (31)P diffusion NMR was used since the products of these reactions constitute a mixture of OP-covalent conjugates of the enzyme and OP-containing hydrolysis products that have noninformative (1)H NMR spectra. It was shown that the peak, attributed to the covalent native diisopropylphosphoryl-Cht (DIP-Cht) conjugate by chemical shift considerations, has a greater diffusion coefficient (D = (0.65 +/- 0.01) x 10(-5) cm(2) s(-1)) than expected from its molecular mass (approximately 25 kDa). This peak was therefore suggested to consist of at least two superimposed signals of diisopropyl phosphoryl (DIP) pools of high and low molecular weights that happen to have the same chemical shift. This conclusion was substantiated by the use of DMSO-d(6) that separated the overlapping signals. Diffusion measurements performed on the extensively dialyzed and unfolded DIP-Cht conjugate still resulted in a high diffusion coefficient ((0.30 +/- 0.05) x 10(-5) cm(2) s(-1)) relative to the assumed molecular mass. This observation was attributed to a dynamic dealkylation at the OP moiety (i.e., aging) that occurred during the relatively long diffusion measurements, where DIP-Cht was converted to the corresponding monoisopropyl phosphoryl Cht (MIP-Cht) conjugate. Homogeneous aged forms of OP-Cht were obtained by use of DFP and heat-induced dealkylation of DIP-Cht, and by PBPDC that provided the aged form via the hydrolysis of a P-Cl bond (PBP-Cht). The thermally stable aged conjugates enabled a reliable determination of the diffusion coefficients over several days of data acquisition, and the values found were (0.052 +/- 0.002) x 10(-5) cm(2) s(-1) and (0.054 +/-0.004) x 10(-5) cm(2) s(-1) for the MIP-Cht and the PBP-Cht adducts, respectively, values in the range expected for a species with a molecular weight of 25 kDa. The advantages and limitations of (31)P diffusion NMR in corroborating the type of species that prevail in such systems are briefly discussed.