Possibility of measuring thermal protein denaturation by an optoacoustic method

The coupling of proteins to a chromophore allows in principle to achieve a high temperature for a short time in a highly confined microvolume. On the one hand this can be used for the study of very rapid thermal denaturation, which may happen in microseconds. On the other hand it. is possible to produce very precise damage to special cellular and subcellular structures by heating a microscopic volume. Calculations show that extremely photostable absorbers with picosecond relaxation time have to be used, a condition which can not be fulfilled by dye molecules. Nevertheless submicron absorbing solid-state particles such as melanin granules can be used due to their higher absorption cross-section and better photostability. Preliminary optoacoustic experiments were conducted to explore the possibility of detecting denaturation of proteins in such conjugates by their volume change, which is associated with the change of the tertiaiy structure. As expected, the melanin granules proved to be efficient photon-to-heat converters. Nevertheless, the volume change due to protein denaturation may be too small compared to the thermal expansion ofwater to allow quantitative evaluations.