[RASMB] fringe deviation

[Borries Demeler]

> 
> Hello!
> 
> I am trying to determine the association constant for a monomer-dimer equilibrium. I know the association occurs because I've seen it by velocity experiments.
> 
> I run equilibrium at 3 concentrations and 3 speeds, my conc. are high and therefore I used interference optics. I have a fringe displacement at equilibrium of up to 50 fringes. I fit my data with WinNonlin and at the best I can get a SQUARE ROOT OF VARIANCE=9.3317E-02.  Since I have such a big fringe displacement when I plot deviation vs. indipendent variable, in the worst case I have a deviation of -0.25 to 0.375 fringes, is it reasonable?
> 
> thanks 
> barbara

Hi Barbara!

Whenever you get very big interference gradients (50 fringes over a 2-3
mm equilibrium column is a lot!) you need to be aware of systematic
errors introduced into the equilibrium experiment. In my experience
there are two sources: 1. Wiener skewing and aperture related problems,
which can be significant with such large concentration gradients, and
as John Philo mentioned, concentration dependent nonideality. There was
a good explanation about Wiener skewing posted to this forum by Arthur
Rowe, you can find it at:

http://server1.bbri.org/rasmb_archives/1997/113.html

If you have a good idea of your Kd, my recommendation would be to measure
1 order of magnitude above and below of this concentration, so you get a good
signal for both the monomer and the dimer.

Good luck, -Borries