Stark effect ( electroabsorption ) spectroscopy of photosynthetic reaction centers at 1 . 5 K : evidence that the special pair has a large excited - state polarizability

The Stark effect or electroabsorption spectrum of the Qr transition of the special pair, P, of Rb. sphaeroides reaction centers has been measured at 1.5 K in frozen glycerol/buffer glasses. Under these conditions, inhomogeneous broadening of the absorption band is minimized, and some structure is resolved in the absorption and Stark effect spectra. As a result it is possible to greatly refine earlier analyses of Stark spectra which were obtained at higher temperatures in poly(vinyl alcohol) films. In addition to the substantial change in dipole moment which has been documented previously, the new data provide evidence for a substantial change in polarizability associated with the lowest electronic transition of P. It has generally been assumed that the dipolar character of * P results from mixing with charge-transfer states. It is shown that as a consequence of this same interaction, * P should be highly polarizable, consistent with the Stark data. This leads to the hypothesis that the observed substantial change in dipole moment is induced by the matrix electrostatic field in the vicinity of P. In this sense, the properties of * P are quite similar to those of carotenoid excited states in ordered protein matrices (Gottfried et al., Science (1991) 251,662-665; Biochim. Biophys. Acta (1991) 1059, 76-90). The specific geometric arrangement of bacteriochlorophylls in the bacterial special pair results in a highly anisotropic excited-state polarizability which is very sensitive to environmental perturbation. The observed polarizability of • P can be used to estimate a likely lower limit for the matrix electrostatic field in the reaction center (> 1.2.106 V/cm). It is expected that such a large electrostatic field plays an important role in determining the kinetics and thermodynamics of the initial electron transfer steps in photosynthesis.