The time-resolved thermodynamics of the chromophore-protein interactions in biological photosensors as derived from photothermal measurements

Laser-induced optoacoustic spectroscopy, LIOAS, and photothermal beam deflection, PBD, were applied to the determination of the time ( nanosecond to millisecond) resolved enthalpy and structural volume changes photoinduced in various biological photosensors and in the phototriggered bacterial ion pump halorhodopsin. A brief section introduces the various photosensors, several of them recently discovered. The photosensors studied with photothermal methods ( LIOAS and PBD) have either a chromophore with an isomerizable double bond, such as phytochrome A of etiolated plants, sensory rhodopsins of Archaea, and photoactive yellow protein of Eubacteria, or an FMN (flavin mononucleotide) chromophore, such as the LOV ( light, oxygen, or voltage) sensing domains of phototropins and related proteins. In particular cases the data allow the evaluation of the entropy changes produced in some of the photoinduced steps, a quantity not available by other methods. In some cases UV-vis spectroscopically silent transient species could be observed. The molecular origin of the structural volume changes is analysed. The problems involved in the application of these techniques to several photosensors are addressed, such as their time limits and the determination of the absorption-determined refractive index changes in PBD.