FEMTOBIOLOGICAL STUDIES ON BACTERIORHODOPSIN

Femtobiology is a novel branch of science, focusing on ultrafast processes in biological systems, occurring on a femtosecond (10^-15 s) timescale. Although common biological reactions are much slower, elementary molecular events, such as chemical bonding and unbonding, as well as vibrational and rotational motions take place in this time range. In this context all chemistry is femtochemistry and all biology is femtobiology. By the methods of classical spectroscopy the above processes could be investigated only indirectly in the frequency domain. The advent of ultrafast lasers made possible studying these events directly in time domain, yielding detailed, previously unobtainable information. (The spectrum can be calculated from the time-evolution, but not vice versa.)

Our recent femtobiological research – carried out in national and international collaborations – mainly focused on the light-induced primary charge separation processes taking place in bacteriorhodopsin (bR), a protein utilizing light energy. Our calculations indicated that these ultrafast charge motions – as sources of the familiar Hertz dipole radiation – could emit electromagnetic radiation at an experimentally detectable level. Based on this finding, we started spectroscopic experiments with femtosecond time-resolution to study the components of this radiation falling to the far infrared and terahertz regime. The main results of this study are the following:

The ultrafast charge redistribution in the active center of
bacteriorhodopsin induces electromagnetic radiation.

By coherent infrared emission of 10 fs resolution we demonstrated for the first time the phenomenon of optical rectification on a biological sample, originating from the excited-state electron polarisation in the retinal chromophore of bR. For exact interpretation of the effect we elaborated the theory of resonant optical rectification, derived from the fundamentals of quantum electrodynamics. Following the excitation we also observed the existence of time-domain synchronized (coherent) vibrations. It was found that these vibrations partially originate from modes characteristic of the excited state, reporting among others about the isomerisation of retinal, a functionally important process in the energy conversion.

Terahertz radiation of bacteriorhodopsin and its modelling by electron and proton translocation.

Also for the first time we detected coherent terahertz radiation from a biological sample by a measuring apparatus built directly for this purpose and ensuring the investigation of somewhat slower (200 fs – 5 ps) charge motions. This system made feasible to follow the complete process of the excited-state charge redistribution in retinal. In addition, in the emission signal we observed an additional component of several ps life-time, attributable to the appearance of the primary functional proton motions.

For the analysis of the above and further experiments, we also studied the ultrafast absorption kinetics of native and chemically modified bR samples. Furthermore, we experimentally determined the optical refractive index of bR in a wide spectral range, and calculated the corresponding Sellmeier coefficients, describing the dispersion properties of the protein.

Further research plans, possible utilization of the results

In our laboratory the construction of an ultrafast optical pump-probe unit is in progress, with the capability of measuring absorption kinetics as well as fluorescence up-conversion in the time range of 100 fs – 1 ns. Our aim is to operate this laboratory as a national service for solving appropriate problems in the area of both research and development.