Utilizing the Dynamic Stark Shift as a Probe for Dielectric
Relaxation in Photosynthetic Reaction Centers During Charge Separation
- Publication date
- Publisher
Abstract
In photosynthetic reaction centers,
the electric field generated
by light-induced charge separation produces electrochromic shifts
in the transitions of reaction center pigments. The extent of this
Stark shift indirectly reflects the effective field strength at a
particular cofactor in the complex. The dynamics of the effective
field strength near the two monomeric bacteriochlorophylls (B<sub>A</sub> and B<sub>B</sub>) in purple photosynthetic bacterial reaction
centers has been explored near physiological temperature by monitoring
the time-dependent Stark shift during charge separation (dynamic Stark
shift). This dynamic Stark shift was determined through analysis of
femtosecond time-resolved absorbance change spectra recorded in wild
type reaction centers and in four mutants at position M210. In both
wild type and the mutants, the kinetics of the dynamic Stark shift
differ from those of electron transfer, though not in the same way.
In wild type, the initial electron transfer and the increase in the
effective field strength near the active-side monomer bacteriochlorophyll
(B<sub>A</sub>) occur in synchrony, but the two signals diverge on
the time scale of electron transfer to the quinone. In contrast, when
tyrosine is replaced by aspartic acid at M210, the kinetics of the
B<sub>A</sub> Stark shift and the initial electron transfer differ,
but transfer to the quinone coincides with the decay of the Stark
shift. This is interpreted in terms of differences in the dynamics
of the local dielectric environment between the mutants and the wild
type. In wild type, comparison of the Stark shifts associated with
B<sub>A</sub> and B<sub>B</sub> on the two quasi-symmetric halves
of the reaction center structure confirm that the effective dielectric
constants near these cofactors are quite different when the reaction
center is in the state P<sup>+</sup>Q<sub>A</sub><sup>β</sup>, as previously determined by Steffen et al. at 1.5 K (Steffen, M. A.; et al. Science 1994, 264, 810β816). However, it is not possible to determine from static,
low-temperature measurments if the difference in the effective dielectric
constant between the two sides of the reaction center is manifest
on the time scale of initial electron transfer. By comparing directly
the Stark shift dynamics of the ground-state spectra of the two monomer
bacteriochlorophylls, it is evident that there is, in fact, a large
dielectric difference between protein environments of the two quasi-symmetric
electron-transfer branches on the time scale of initial electron transfer
and that the effective dielectric constant in the region continues
to evolve on a time scale of hundreds of picoseconds