BioPHYSICAL chemistry

(which have the structure of bacteriochlorophyll without the central Mg),

two quinones, and one nonheme iron. Spectroscopic studies have revealed

many aspects of the roles of the many different cofactors. In response to

light the electron donor, D, becomes excited and transfers an electron to

an initial electron acceptor, A:

(20.1)

The reaction center has no electron para-

magnetic resonance (EPR) signal in the dark

but upon illumination a signal is observed

near g=2.0026 (Figure 20.5). The signal

was identified as arising from an oxidized

bacteriochlorophyll based upon the gvalue

that matches the g value observed for

oxidized bacteriochlorophyll in solution but

the linewidth of the EPR signal was much

more narrow. It was soon realized that the

width of the signal reflected the delocal-

ization of the unpaired electron over two

highly interacting bacteriochlorophylls.

The linewidth ΔHis proportional to the

number of nuclei, N, and the strength of

the interaction between the nuclei and

inpaired electron, A, due to heterogeneous

broadening. Since the nuclei are equiva-

lent, their contribution is a function of the

square root of the number:

ΔH∝AN1/2 (20.2)

A dimer has twice the number of nuclei but each electron spends on

average half the time, which reduces the interaction strength by half. The

ratio of the linewidths for the monomer and dimer is predicted to be:

(20.3)

Thus, EPR experiments showed that the primary electron donor was

a bacteriochlorophyll dimer. In response to light, one of the absorption

bands of the steady-state optical spectrum disappears and several band

shift (Figure 20.6). The band that disappears, or bleaches, with the same

kinetics observed for the EPR signals showing that the pigment associated

Δ

Δ

H

H

AN

AN

monomer

dimer

monomer

dimer

==

1

2

2

2

2

= 2

DA→→D*A D A

hν +−

426 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY

 2.0026

Oxidized

bacteriochlorophyll

EPR signal

Magnetic field (G)

Dark

P865

5 G

0

Figure 20.5Light and dark EPR spectra and
bacteriochlorophyll spectra. Light produces an
oxidized donor, identified as P865+, in reaction
centers. Modified from Feher (1998).