Heat Activation of Phycomyces blakesleeanus Spores: Thermodynamics and Effect of Alcohols, Furfural, and High Pressure

Abstract

The thermodynamic parameters for the heat activation of the sporangiospores of Phycomyces blakesleeanus were determined. For the apparent activation enthalpy (Δ H # ) a value of 1,151 kJ/mol was found, whereas a value of 3,644 J./°K·mol was calculated for the apparent activation entropy (Δ S # ). n -Alcohols (from methanol to octanol), phenethyl alcohol, and furfural lowered the activation temperature of P. blakesleeanus spores. The heat resistance of the spores was lowered concomitantly. The effect of the alcohols was a linear function of the concentration in the range that could be applied. When the log of the concentration needed to produce an equal shift of the activation temperature was plotted for each alochol against the log of the octanol/water partition coefficient, a straight line was obtained. The free energy of adsorption of the n -alcohols to their active sites was calculated to be −2,487 J/mol of CH 2 groups. Although still inconclusive, this points toward an involvement of protein in the activation process. The effect of phenethyl alcohol was similar to the effect of n -alcohols, but furfural produced a greater shift than would be expected from the value of its partition coefficient. When the heat activation of the spores was performed under high pressure, the activation temperature was raised by 2 to 4°K/1,000 atm. However, with pressures higher than 1,000 atm (1.013 × 10 5 kPa) the activation temperature was lowered until the pressure became lethal (more than 2,500 atm). It is known that membrane phase transition temperatures are shifted upward by about 20°K/1,000 atm and that protein conformational changes are shifted upward by 2 to 6°K/1,000 atm. Consequently, heat activation of fungal spores seems to be triggered by a protein conformational change and not by a membrane phase transition. Activation volumes of −54.1 cm 3 /mol at 38°C and −79.3 cm 2 /mol at 40°C were found for the lowering effect of high pressure on the heat activation temperature.