Femtosecond absorption spectroscopy of photosynthetic reaction centers.

Photophysical and photochemical reactions, beginning with the absorption of light by photosynthetic pigment-protein complexes, are among the fastest in biology. They take place on timescales of tens of femtoseconds to a few nanoseconds. The advent of ultrafast laser systems capable of producing pulses of femtosecond duration opened a new area of research and enabled investigation of these photophysical and photochemical reactions in real time.

In transient absorption spectroscopy, a fraction of the molecules is promoted to an electronically excited state by means of an ultrashort excitation pump pulse. Then a weak probe pulse is sent through the sample with a delay τ with respect to the pump pulse. A difference absorption spectrum is then calculated, i.e., the absorption spectrum of the excited sample minus the absorption spectrum of the sample in the ground state (ΔA). By changing the time delay τ between the pump and the probe and recording a ΔA spectrum at each time delay, a ΔA profile as a function of τ and wavelength λ, i.e., a ΔA(τ,λ) is obtained. ΔA(τ,λ) contains information on the dynamic processes that occur in the photosynthetic system under study, such as excited state energy migration and electron transfer.

During the workshop operation of laser system and femtosecond pump-probe spectrometer will be demonstrated. Antennaless reaction centers of purple bacteria Rhodobacter sphaeroides will serve us as an object of investigation. Obtaining photoinduced ΔA spectra with 30 femtosecond resolution in the range of 0 to 3.5 nanoseconds, students will be able to observe a very interesting process of primary charge separation and electron transfer in reaction center.

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