Effect of Dipyridamole on Membrane Energization and Energy Transfer in Chromatophores of Rba. sphaeroides

Abstract

Effect of dipyridamole (DIP) at concentrations up to 1 mM on fluorescent characteristics of light-harvesting complexes LH2 and LH1, as well as on conditions of photosynthetic electron transport chain in the bacterial chromatophores of Rba. sphaeroides was investigated. DIP was found to affect efficiency of energy transfer from the light-harvesting complex LH2 to the LH1-reaction center core complex and to produce the long-wavelength ("red") shift of the absorption band of light-harvesting bacteriochlorophyll molecules in the IR spectral region at 840-900 nm. This shift is associated with the membrane transition to the energized state. It was shown that DIP is able to reduce the photooxidized bacteriochlorophyll of the reaction center, which accelerated electron flow along the electron transport chain, thereby stimulating generation of the transmembrane potential on the chromatophore membrane. The results are important for clarifying possible mechanisms of DIP influence on the activity of membrane-bound functional proteins. In particular, they might be significant for interpreting numerous therapeutic effects of DIP.

Keywords: chromatophores; dipyridamole; energy transfer; membrane energization.

Conflict of interest statement

The authors declare no conflict of interests in financial or any other sphere. This article does not contain any studies involving humans or animals performed by any of the authors.

Figures

The chemical structural formula of dipyridamole

Fig. 1.

Fluorescence spectra of a suspension of Rba. sphaeroides chromatophores (a) under control conditions and (b) in the presence of 1 mM DIP. Original spectra (dotted line 1) were approximated by the sum of two Gaussian components (solid black curve 4). The component with maximum near 855 nm (solid gray curve 2) represents fluorescence of the LH2 complex, and the band with maximum at 890 nm (solid gray line 3) displays fluorescence of the LH1 complex.

Fig. 2.

Differential (light-minus-dark) spectra of Rba. sphaeroides chromatophores. Curves: 1) under control conditions, 2) in the presence of 10 mM of electron transfer inhibitor orthophenanthroline, and 3) in the presence of 1 mM DIP. Differential “spectrum 3 minus spectrum 2” after normalization of both spectra to absorption at 790 nm is shown in inset.

Fig. 3.

Amplitudes of photoinduced absorbance changes in the chromatophores of Rba. sphaeroides in the presence of 1 mM DIP as a function of light intensity. Absorbance changes at 790 nm (curve 1) reflect redox transformations of the photoactive P and absorbance changes at 860 nm (curve 2) represent superposition of the redox conversions and energy-dependent red shift of the 850 nm band.

Fig. 4.

Kinetics of P+ dark reduction in the chromatophores of Rba. sphaeroides after activation with a single laser flash (532 nm, 7 ns) under control conditions (1), in the presence of 1 mM DIP (2), and in the presence of TMPD-H2 at concentrations of 1 µM (3) and 1 mM (4).

Fig. 5.

Rate constants of P+ dark reduction in the chromatophores of Rba. sphaeroides (black circles) after illuminating the sample with a laser flash (532 nm, 7 ns) in the presence of TMPD-H2 at various concentrations. Calculations are based on the time during which the light-induced absorbance change decreased 2.7-fold. The graph also shows effect of TMPD-H2 concentration on the decay time τ of the fast BChl fluorescence component (open circles) with amplitude of approximately 95%.

Fig. 6.

Fluorescence decay kinetics in the chromatophores of Rba. sphaeroides under control conditions (1), in the presence of 1 mM DIP (2), and in the presence of TMPD-H2 at concentrations of 1 µM (3) and 1 mM (4). Curve 5 is the instrumental function with width ~16 ps. Dots are experimental data; solid lines show the result of two-exponential approximation.