Photoionization of Bacteriochlorophyll a in Solution and in Solid State

Abstract

Photoionization of becteriochlorophyll a (Bchl), the most essential pigment in bacterial photosynthesis, in organic solution and the photoconductivity of Bchl in solid state were studied for the purpose of understanding the photoelectronic processes of the pigment in vitro.
Bchl was extracted from photosynthetic bacteria, Chromatium vinosum, and was purified by chlomatography on a sucrose column. The ionic species such as Bchl^± in solution were detected by the electrical conductivity measurements.
The excitation spectrum of photoconduc tion in Bchl solution agreed well with the absorption spectrum (Fig.1). When the solution was irradiated by a visible and near infrared flash, the photocurrent rose slowly as compared with the rise of T-T absorption (Fig.2). The molar extinction coefficient 2.0× 10⁴ mol^-1⋅dm³cm⋅^-1 at 510 nm, was used to determine the concentration of triplet molecule (3Bchl). The concentration of Bchl^± was calculated from the photocurrent, by assuming the ion mobility being 5×10^-4 cm²⋅V^-1⋅s^-1. As shown in Fig.3, both the decay of 3Bchl and the rise of Bchl^± were first order reactions. The rate constant of Bchl^± rise (4.3×10⁴s^-1) was almost twice as large as that of ³Bchl decay (1.8×10⁴s^-1). Ionization proceeded from electron transfer between triplet molecules of Bchl. The second order rate constant of the electron transfer reaction was estimated to be 1×10⁸ mol^-1dm³⋅s^-1.
A rapid response of the photocurrent due to the direct ionization was obs erved on the excitation of a UV nitrogen laser (Fig.5).
Photoconduction was observed in sol id state Bchl which was composed of Bchl-water adducts. The excitation spectra of the photocurrent and an absorption spectrum are shown in Fig.6. A typical response of the transient photocurrent is shown in Fig.7. The initial value of the photocurrent was proportional to O.8 power of light intensity. The carrier generation was ascribed to the dissociation of a singlet exciton.