On the possibility of high-precision photothermal microeffects and the measurement of fast thermal denaturation of proteins

The precision of laser-induced effects is often limited by thermal and thermomechanical collateral damage. Adjusting the pulsewidth of the laser to the size of the absorbing structure can at least avoid thermal side effects and facilitates a selective treatment of vessels or pigmented cells. Further extending the precision of thermal effects below cellular dimensions by using nanometer sized particles could open up new fields of applications for lasers in medicine and biology. Calculations show that under irradiation with nano- or picosecond laser pulses gold particles of submicrometer size can easily be heated by several hundred K. High temperatures have to be used for subcellular thermal effects, because heat confinement to such small structures requires the thermal damage to occur in extremely short times. Estimating the denaturation temperature by extrapolating the Arrhenius equation from a time range of minutes and seconds into a time range of nano- and picoseconds leads to temperatures between 370 K-470 K. There is evidence that in aqueous media, due to the surface tension, these temperatures can be generated at the surface of nanometer sized particles without vaporization of the surrounding water. In order to show whether or not an extrapolation of the damage rates over six to nine orders of magnitude gives correct data, a temperature-jump experiment was designed and tested which allows to measure denaturation rates of proteins in the millisecond time range. Denaturation of chymotrypsin was observed within 300 ps at temperatures below 380 K. The rate constants for the unfolding of chymotrypsin followed the Arrhenius equation up to rates of 3000 s/sup -1/.