BioPHYSICAL chemistry

with various substitutents. Chlorophyll ais found in all eukaryotic photo-

synthetic organisms. The structural difference between chlorophyll aand

bacteriochlorophyll alie in ring A on the C3 position and in ring B with

a difference in the degree of saturation. These structural differences alter

the conjugation and the electronic structure of the cofactor. The absorp-

tion spectrum of chlorophyll ashows a number of absorption bands with

the lack of bands in the green region giving rise to their characteristic

color. These absorption bands arise from π-to-π* transitions of the con-

jugate macrocycle. The two lowest energy transitions are called the Q bands,

with the extinction coefficient of chlorophyll abeing 9.0 × 104 M−^1 cm−^1.

The absorption bands at higher energy are commonly termed the Soret

bands. The spectrum of bacteriochlorophyll shows the same set of bands,

with comparable extinction coefficients, but the peaks are shifted due to

the difference in conjugation compared to chlorophyll.

422 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY

CH 3 OOC

Bacteriochlorophyll

O

H 3 C O

CH 2 CH 3

H 3 C

H

H

ROOC H

H

H

CH 3

N

AB

C

E

D

Mg

N

NN

CH 3

H 3 C

CH 3 OOC

Chlorophyll

O

CH

H 2 C

CH 2 CH 3

H 3 C

H

H

ROOC H

CH 3

N

AB

C

E

D

Mg

N

NN

CH 3

H 3 C

Soret

region

Qy

region

Bacteriochlorophyll

in solution

Chlorophyll

in solution

Qx

region

400 500 600 700 800

Absorbance

Wavelength (nm)

Figure 20.1
The structure and
optical absorption
spectra of
bacteriochlorophyll a
and chlorophyll a.
Modified from
Blankenship (2002).