Plants and some bacteria have a unique property of fixing solar energy
by photosynthesis. We investigated some photosynthetic bacteria's
light-harvesting antennas (LH1, LH2) and reaction centres (RC), the
atomic structures of which are known. The use of spectroscopic methods
and, particularly, that of short light pulses enable us to investigate
all the stages of a photosynthesis e. g. the energy transfer of an
absorbed photon in a photosynthetic pigment-protein complex, the
primary electron transfer (ET) in a RC, etc.. In our investigations we
combined optical measurements with the high-pressure technique.
The high-pressure cell we used was that of the cylinder-piston type.
For optical measurement the cell was equipped with three optical
ports. The spectral characteristics (absorption and luminescence
spectra, decay of spontaneous (ps) and stimulated (fs) emission,
Fourier transform Raman spectra) of the investigated samples were
measured at room temperatures and at pressures up to 8 kbar.
Results and discussion.
Our measurements of the absorption spectra of the photosynthetic
proteins of some bacteria show that the pressure induces a red shift
and a broadening of their absorption bands. The absorption spectra of
LH1 and LH2 antennas have a long-wavelength absorption Qy band, caused
by the transition S0-S1 in a bacteriochlorophyll (BChl) molecule.
Along with an increase in the pressure, the Qy band at 800 nm and 850
nm of LH2 complex from Rb. sphaeroides 2.4.1 shifts with the rate -4
cm-1/kbar and -97.5 cm-1/kbar respectively. The Qy band at 880 nm of
LH1 complex from Rsp. rubrum G9+ shifts with the rate -91.6 cm-1/kbar.
These different shift rates were qualitatively analysed by means of
some theoretical equations, which were developed for a small molecule
dissolved in a solvent. However ,a BChl molecule, embedded into a
protein matrix, experiences some additional interactions such as
hydrogen bond and exciton interaction. To understand the effect of
these interactions, Fourier Transform (FT) Raman measurements of the
Rsp. rubrum G9+ LH1 antenna were performed. From these spectra we can
conclude that at these pressures the BChl macrocycle within the
protein matrix experiences little if any distortion, and the
hydrogen-bonding network involving the C2 and C9 carbonyl groups of
BChl molecules is undisturbed. Thus, our FT Raman and absorption
measurements show that the pressure-induced red shift of the Qy
absorption band may be safely attributed to solvatochromic effects, in
particular, to the modulation of the pigment-pigment interaction by
the pressure. We examined the influence of high pressure on the decay
kinetics of the excited state of the special pair P on wild type (WT)
and mutant RCs. In order to describe quantitatively the dynamics of ET
in the RCs, the decay curves have been fitted to the multiexponential
function. For example, a fit result (amplitude A and time constant
k(ps)) is given for the WT complex at 1atm and 6kbar, respectively.
1atm k1=3.9 A1=0.72; k2=14.4 A2=0.2; k3=2000 A3=0.08
6kbar k1=1.8 A1=0.81; k2=10.0 A2=0.04; k3=2000 A3=0.15
Our analyses show that the classical Marcus equation more or less
realistically explains our experimental data on WT and YM210H mutant
RCs, but does not describe the data on YM210L and YM210F mutants RCs.
The results presented above were obtained in cooperation with my
colleagues from Tartu (Estonia), Lund (Sweden), Saclay (France).
1. A.Freiberg, A.Ellervee, P.Kukk, A.Laisaar, M.Tars and
K.Timpmann, Chem. Phys. Lett., 214, (1993), pp.10-16.
2. J.N.Sturgis, A.Gall, A.Ellervee, A.Freiberg, and
B.Robert, Biochemistry, 37, (1998), pp.14875-14880.