BioPHYSICAL chemistry

with the 865-nm absorption band, was the

primary electron donor, now termed P865.

The two quinones could be biochemically

removed from the reaction center and then

bound back to the protein, providing the

means for their identification as electron

acceptors (Chapter 8). EPR studies of the

nonheme iron were consistent with the

iron always remaining in the Fe^2 +state and

thus not being an active participant in the

electron-transfer process.

The optical spectrum has proved to be

a rich means of characterizing the roles of

the other cofactors of the reaction center

and determining the electron-transfer rates.

The absorption bands were seen to have

different responses after illumination with

a fast laser pulse (Figure 20.7). The band

at 865 nm quickly bleached and then was

unchanged due to rapid formation of the excited state, P865*, followed

by the oxidized state, P865+. The kinetics at other wavelengths, such as

755 nm, were more complex but could be interpreted using models of

sequential reactions (Chapter 10) as arising due to one of the bacterio-

pheophytins serving as an intermediate electron acceptor.

These experimental studies established the kinetics and energetics of

the photochemical and early secondary events (Figure 20.8). Excitation

of the primary electron donor at 865 nm corresponds to an energy

increase of 1.4 eV:

CHAPTER 20 PHOTOSYNTHESIS 427

300 400 500 600 700 800 900 1000

Absorbance

Wavelength (nm)

No light

excitation

With light

excitation

Figure 20.6Light and dark optical spectra of the

bacterial reaction center.

Absorbance change

Wavelength

Time

1.0

0.0

P865*

P865

3 ps

1 ps

BChl

BPh

200 ps

QA

QB

200 μs

10 ns

100 ms

0.5

0.5

Midpoint potential ( V )

Light excitation

Figure 20.8Energetics of the

bacterial reaction center.

Figure 20.7Transient optical spectra.
Pronounced spectral changes are evident on the
picosecond timescale. Figure courtesy of Su Luin.