BioPHYSICAL chemistry

9-fold symmetry around a central symmetry

axis. Associated with each pair of subunits are

three bacteriochlorophylls and one carote-

noid that are located between the inner and

outer rings of protein subunits.

Although all of the bacteriochlorophyll a

molecules are chemically identical there are

two spectrally distinct sets of bacteriochloro-

phyllain the protein complex, with one set

contributing to an absorption band at 800 nm

and the other at 850 nm (Figure 20.3). The

structure provides a clear explanation for this

spectral division of the bacteriochlorophylls.

The bacteriochlorophylls that absorb at

800 nm form a ring with the planes of the

macrocycles perpendicular to the central

symmetry axis. These pigments lie parallel

to the plane of the membrane and are

quite well separated from each other, with

a distance of approximately 20 Å. Due to the

large distance between neighboring pig-

ments, the molecules interact very weakly

and behave as independent molecules. The

bacteriochlorophylls that absorb at 850 nm

are arranged as closely interacting dimers

with their planes approximately parallel to

the symmetry axis. The dimers form a band

of 18 (or 16) bacteriochlorophylls that are

all highly interacting.

Energy transfer among the bacterio-

chlorophylls can occur by the Forster

mechanism (Chapter 17). In this case, the

bacteriochlorophylls serve as both energy

donor and acceptor so the energies are

well matched and the energy transfer has

been measured to be very efficient. When

the bacteriochlorophyll molecules are close

then energy transfer can occur extremely

rapidly with times of less than 100 fs.

Energy transfer along the rings of bacterio-

chlorophylls has been measured using

femtosecond spectroscopy, yielding the rates of energy transfer (Figure 20.4).

Energy transfer from the 800-nm bacteriochlorophylls to the 850-nm

bacteriochlorophylls occurs in 1 ps. The 850-nm bacteriochlorophylls are

so strongly interacting that the energy is effectively delocalized among

424 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY

Figure 20.2The three-dimensional structure of the
light-harvesting complex II.

300 400 500 600 700 800 900 1000

Absorbance

Wavelength (nm)

Soret

region

Qx

region

Light-harvesting

complex II

Reaction center

and light-harvesting

complex I

Qy

region

Figure 20.3Optical absorption spectra of the
light-harvesting complexes I and II.