with various substitutents. Chlorophyll a is found in all eukaryotic photosynthetic organisms. The structural difference between chlorophyll a and
bacteriochlorophyll a lie 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 absorption spectrum of chlorophyll a shows 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 conjugate macrocycle. The two lowest energy transitions are called the Q bands,
with the extinction coefficient of chlorophyll a being 9.0 × 10
4 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
CH3OOC
Bacteriochlorophyll
O
O
H3C
CH 2CH3
H3C
H
H
H
ROOC
H
H
CH 3
N
A
B
C
E
D
Mg
N
N
N
CH 3
H3C
CH3OOC
Chlorophyll
O
CH
H2C
CH 2CH3
H3C
H
H
H
ROOC
CH3
N
A
B
C
E
D
Mg
N
N
N
CH 3
H3C
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).
9781405124362_4_020.qxd 4/29/08 14:08 Page 422
bacteriochlorophyll a lie 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 absorption spectrum of chlorophyll a shows 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 conjugate macrocycle. The two lowest energy transitions are called the Q bands,
with the extinction coefficient of chlorophyll a being 9.0 × 10
4 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
CH3OOC
Bacteriochlorophyll
O
O
H3C
CH 2CH3
H3C
H
H
H
ROOC
H
H
CH 3
N
A
B
C
E
D
Mg
N
N
N
CH 3
H3C
CH3OOC
Chlorophyll
O
CH
H2C
CH 2CH3
H3C
H
H
H
ROOC
CH3
N
A
B
C
E
D
Mg
N
N
N
CH 3
H3C
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).
9781405124362_4_020.qxd 4/29/08 14:08 Page 422