Thermal stability and thermodynamic analysis of native and methoxypolyethylene glycol modified trypsin

Four methoxypolyethylene glycols (MPEG, molecular masses 350, 750, 2000 and 5000 Da), each activated by nitrophenyl chloroformate, were used to modify trypsin. Compared with the native trypsin, the MPEG-modified trypsin was m ore stable against temperature between 30 degrees C and 70 degrees C, longe r chain of MPEG moiety corresponding to higher thermal stability. The T-50% for the native and the modified trypsin (0.4 mg ml(-1)) was increased from 47 degrees C to 66 degrees C. The stabilization effect caused by MPEG modi fication was the result of decreasing in both the autolysis rate and the th ermal denaturation rate. The thermodynamic analysis of the thermal denatura tion process showed that the activation free energy (Delta G*) of the nativ e and the modified trypsin at 60 degrees C was increased from 102.9 to 109. 3 kJ mol(-1); the activation enthalpy (Delta H*) was increased from 57.4 to 86.9 kJ mol(-1); the activation entropy (Delta S*) was increased from -136 to -67 J molK(-1). A possible explanation for the decreased thermal denatu ration rate caused by MPEG modification was also discussed.