title: Homotropic cooperative binding of organic solvent vapors by solid trypsin.
abstract: Homotropic cooperative binding was observed at vapor sorption of organic solvents (acetonitrile, propionitrile, ethanol, 1-propanol, 2-propanol, nitroethane) by dried solid trypsin from porcine pancreas (0.05 g H2O/g protein). The vapor sorption isotherms were obtained by the static method of gas chromatographic headspace analysis at 298 K for 'vapor solvent+solid trypsin' systems in the absence of the liquid phase. All isotherms have a sigmoidal shape with significant sorbate uptake only above the threshold of sorbate thermodynamic activity. On the sorption isotherms of non-hydroxylic sorbates the saturation of trypsin by organic solvent was observed above the sorbate threshold activity. The formation of inclusion compounds with phase transition between solvent-free and solvent-saturated trypsin is supposed. Approximation of obtained isotherms by the Hill equation gives the inclusion stoichiometry S, inclusion free energy, and the Hill constant N of clathrates. The inclusion stoichiometry S depends significantly on the size and shape of sorbate molecules and changes from S=31 mol of sorbate per mol of trypsin for ethanol to S=6 for nitroethane. The inclusion free energies determined for the standard states of pure liquid sorbate and infinitely dilute solution in toluene are in the range from -0.5 to -1.2 kJ/mol and from -3.1 to -8.1 kJ/mol, respectively, per 1 mol of sorbate. The Hill constants are relatively high: from N=5.6 for 1-propanol to N approximately equal to 10(3) for nitroethane. The implication of the obtained results for the interpretation of solvent effects on the enzyme activity and stability in low-water medium is discussed.
authors.affiliation: Department of Chemistry, Kazan State University, Kremlevskaya 18, 420008, Kazan, Russia. valery.gorbatchuk@ksu.ru
authors.forename: V V
authors.initials: VV
authors.lastname: Gorbatchuk
authors.forename: M A
authors.initials: MA
authors.lastname: Ziganshin
authors.forename: N A
authors.initials: NA
authors.lastname: Mironov
authors.forename: B N
authors.initials: BN
authors.lastname: Solomonov
mesh_terms: D000433:1-Propanol / Q000378:metabolism; D000097:Acetonitriles / Q000378:metabolism; D000818:Animals; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002849:Chromatography, Gas; D004980:Ethane / Q000031:analogs & derivatives / Q000378:metabolism; D000431:Ethanol / Q000378:metabolism; D009570:Nitriles / Q000378:metabolism; D009595:Nitroparaffins / Q000378:metabolism; D010179:Pancreas / Q000201:enzymology; D011485:Protein Binding; D012995:Solubility; D012997:Solvents / Q000378:metabolism*; D013552:Swine; D013816:Thermodynamics; D014357:Trypsin / Q000378:metabolism*; D014835:Volatilization
chemical_list: D000097:Acetonitriles; D009570:Nitriles; D009595:Nitroparaffins; D012997:Solvents; D000431:Ethanol; C014705:nitroethane; D000433:1-Propanol; C005557:propionitrile; D014357:Trypsin; D004980:Ethane; C032159:acetonitrile