BioPHYSICAL chemistry

oxidized state. For example, the iron in heme has two axial ligands

(Chapters 15 and 16). A more basic axial ligand is a better electron donor

with a greater attraction for Fe^3 +. Thus a more basic ligand will stabilize

the oxidized state and lower the midpoint potential. Cytochrome hemes

with two axial ligands usually have a more negative midpoint potential

than hemes with one methionine ligand and one histidine since the

imidazole side chain of histidine is a better electron donor than the side

chain of methonine.

Hydrogen bonding and other electrostatic interactions will also system-

atically alter the midpoint potential of a cofactor. For bacteriochlorophylls,

there are two carbonyl oxygens that are part of the conjugated ring and

serve as acceptors of hydrogen bonds from the surrounding protein

(Figure 6.5). As proton donors were introduced in the hydrogen-bonding

position, the midpoint potential was found to increase (Figure 6.6). By

performing electron nuclear double resonance (ENDOR) measurements

(Chapter 16), the distribution of electrons over macrocycles was determined

and the increase in midpoint potential could be explained by a Hückel

model (Chapter 13), with changes in midpoint as a result of stabilization

of the reduced state due to the hydrogen-bonding interactions.

Gibbs energy of formation and activity

When a salt such as sodium chloride is present in solution, the salt dissolves

and the sodium and chloride ions separate due to the resulting increase

120 PARTI THERMODYNAMICS AND KINETICS

1.0

0.2

0.4

0.6

0.8

0.0

0.4 0.6 0.8

Fraction reduced

Potential (V)

H Increasing H bonds

H H

H

C O

O

CH 3

O

CH (^3) O
CH 2
CH 3
CH 2
H 3 C
H 2 C
COOR
CH 3
N
III
III
V
IV
Mg
N
NN
CH 3
H
H
H
H
H 3 C
2a
1a^212
13 3
14
15
5
5a
6
17 16
9
7
8
18
10
4
α
δ β
1
11
2b
Figure 6.5The structure of
bacteriochlorophyll afound in the
bacterial reaction center.
Figure 6.6Redox titrations of the bacterial reaction
center showing systematic increases in the midpoint
potential due to the addition of hydrogen bonds.