Laser-induced photoacoustic calorimetric determination of enthalpy and volume changes in photolysis of 5′-deoxyadenosylcobalamin and methylcobalamin

Photolysis of 5′-deoxy-5′-adenosylcobalamin (AdoCbl) in neutral aqueous solution and methylcobalamin (MeCbl) in neutral and acid aqueous solution has been investigated using pulsed, time-resolved photoacoustic calorimetry in the temperature range 10–30 °C. The enthalpy changes for the above cobalamins, 129 ± 17, 163 ± 21 and 176 ± 23 kJ mol–1, respectively, are consistent with the values obtained by thermolytic kinetic methods. The reaction volume changes for them, 6 ± 1, 2 ± 0.5 and 5 ± 1 ml mol–1, respectively, are probably due to the conformational changes of the corrin ring and its side chains accompanying Co–C bond cleavage.