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6.2.6. Calculations of M
w
, A
2
and R
G
from
light
scattering
measurements.
A classical light scattering (‘batch mode’) experiment provides R
θ
values for
typically 15 angles (30-150
°
) and 3-8 concentrations. Here is an example
(Kc/R
θ
values from 4 concentrations, 5 angles):
q
sin
2
(q/2) c
1
(g/ml) c
2
(g/ml) c
3
(g/ml) c
4
(g/ml)
0.00023 0.0007
0.00097 0.0014
q1 30 0.07
6.76E-06 9.72E-06 1.14E-05 1.41E-05
q2 60 0.25
7.65E-06 1.10E-05 1.29E-05 1.60E-05
q3 90 0.50
8.88E-06 1.28E-05 1.50E-05 1.86E-05
q4 120 0.75
1.01E-05 1.45E-05 1.71E-05 2.11E-05
q5 150 0.93
1.10E-05 1.58E-05 1.86E-05 2.30E-05
How to proceed? The literature often presents data as the famous Zimm
diagram, which is very elegant, but relatively complicated and less intuitive to
most biochemists. The software of course produces it automatically. However,
we will employ the underlying 4 steps (Zimm diagram is explained in a later
section).
Step. 1.
We note from the main equation that Kc/R
θ
is a linear function of both sin
2
(
θ
/2)
and c. If we for each concentration (c
i
) perform a linear regression and
extrapolate the Kc/R data to zero angle (sin
2
(
θ
/2)
→
0) we obtain new values,
Kc/R
θ
=0,
one for each concentration.