79] Calorimetric measurements of light-induced processes

Abstract

This chapter discusses the calorimetric measurements of light-induced processes. Photocalorimetry utilizes the principle of isothermal heat flow microcalorimetry used widely in other areas of biological thermochemistry. Unlike classic calorimetry—in which thermal energy changes are measured directly as changes in temperature—heat energy changes in the sample are allowed to dissipate rapidly across sensitive thermopiles, producing an output voltage that is proportional to the rate of energy dissipation in the sample. Integration over time yields the total heat energy change in the sample during reaction. In the absence of any photoreaction, the absorbed light energy is rapidly converted into heat and produces a measurable heat flux that constitutes the baseline for any measurement. If physical or chemical changes are induced in the sample by illumination, then some of this light energy may be diverted and not appear as heat (endothermic reaction). Alternatively, photon absorption might stimulate exothermic processes in the sample and give rise to an enhanced heat flux. Measurement of the molar enthalpy change for a photon-induced reaction, therefore, requires the determination of two quantities: (1) the change in heat energy content of the sample brought about by the isothermal photochemical transformation, and (2) the amount of material, in moles, transformed in the reaction.