[RASMB] hydrodynamic non-ideality

[mitrana at mail.utexas.edu]

Hi All

Is there anyway I can ignore the hydrodynamic non-ideality? After all  
my protein concentrations only go up to ~ 12 mg/ml! :)

Ok that was said in jest or may be half jest since ignoring the ks of  
0.009 ml/mg seems to allow me to explain my velocity data by a simple  
monomer-trimer equilibrium.

More seriously though -
1. For an isodesmic association model (either the good old N^2(/3)  
approach used by Timasheff or the more recent bead modeling used by  
Sontag et al. 2004 Biophys. Chem. both used for tubulin  
self-association) the s20,w for the oligomers are fixed a priori to  
any curve fitting (whether Delta C in SEDANAL or the isotherm)

2. Using the Rowe derivation for ks - one should be theoretically able  
to calculate the ks for a given f/f0. Since according to point 1 the  
s20,w water for the oligomers is already assumed to be known, one can  
calculate the f/f0 for each of the oligomers (I guess this only  
applies to bead modeled oligomers since the basis for N^(2/3) is a  
constant f/f0).

3. Bottomline - can one use the Rowe equation to calculate the ks for  
each of the oligomers and then use this value for fitting the weight  
average isotherm? Or am I missing something crucial and there is a  
fundamental problem with this approach? Or for the sake of simplicity  
can I just presume a ks of 0.009 ml/mg for all oligomers and fit the  
data.

If anyone wishes or has the time for it - here are the numbers -
0.03(4.57), 0.37(4.77), 0.745 (5.00), 1.4 (5.3), 3.39 (5.86), 6.65  
(6.51), 12.2 (6.99).
The first number is [P] in mg/ml and in brackets the weight-average  
s20,w. The best fit I have so far is for a modeled isodesmic  
assocation with the relation ln(sn/s1) = 0.5781 ln n - 0.0493 (ln n)^2  
. s1 (20,w) ~ 4.53S ks=0.009 mg/ml. Fitted by sedanal Kiso=1.37x10^4.  
Isotherm = 1.29x10^4.

Any and all thoughts are welcome. Have a good weekend.
Mitra