Chem 156
Spring 2005
Prof. Hubbell
Summary of Lecture 16
1. Proteins with non-uniform charge distribution: implications for function Highly non-
uniform charge distributions in some proteins have apparently evolved for specific
functional purposes. Superoxide dismutase is an example where a positive electrostatic
potential around the active site serves to bias the diffusion of the negative substrate
super oxide to enhance the catalytic rate.
2. Electrostatics of planar surfaces (membranes): the Gouy-Chapman model (see
handout of the same title for details).
The solution of the Poisson-Boltzmann equation for a planar surface is given in two
parts, the potential at x = 0 (the surface potential, Φo) and the distance dependence:
valid for T = 298K, water as a solvent, 1:1 electrolyte.
=Φ
σ
sinh
0
C
Z
−
o
6.1360514. 1
Φo is in volts, σ is the surface charge density in charges/A2 (with correct sign), C is the
molar concentration of electrolyte, and Z is the valence of the electrolyte (absolute
value). The inverse hyperbolic sine is defined as
]
lnsinh ++ x
()
[
1
21 =
−xx
The distance dependence from the surface is
()
x
oex
κ
−
Φ=
Φ
3. The Hydrophobic effect (see handout of the same title for details). For the phase
partition equilibrium of a non-polar solute S between a non-polar solvent and water:
()
X=KRT
X
RT
np
w
o
np
o
wlnln −−=−
µµ
or, if the solute is a liquid and the equilibrium is between pure liquid S and water
(
)
KRT
oo lnln −=Χ−=−
µµ
RT
wnpw
When the Henry’s law SS is chosen,
(
)
o
np
o
w
µµ
−, the “free energy of transfer” , is just the
difference in free energy of the solute S with the two solvents. This is the quantity
needed to understand the insolubility of non-polar solutes in water.
