[RASMB] fringe deviation

[Allen Minton]

Responding to Arthur's query:  Published treatments of nonideal SE by Wills 
& Winzor, and by Behlke, correctly treat first order deviations from 
ideality (2-body interactions), but become calculationally intractable 
beyond the two-body interaction limit (ca 20-30 g/l total solute 
concentration).  We have very recently published a rigorous treatment valid 
for an arbitrary number of solute species over the entire range of 
experimentally accessible concentration (up to several hundred g/l total 
solute concentration): Zorrilla et al, Biophys. Chem. 108, 89-100: 2004.  I 
will send a PDF to anyone requesting it.

Responding to Tom's comment:  He is correct in principle that Wiener 
skewing can influence absorbance gradients, but we found empirically (see 
my previous message for reference) that you need a huge RI gradient for 
this to be manifested and such effects are HIGHLY non-linear in the 
magnitude of the RI gradient: in general, if you don't see the dreaded 
'black band' in an absorbance scan, you are ok, and even in scans 
exhibiting a black band, absorbance data at radial positions clearly 
outside of the 'black band' are ok.  Tom correctly emphasizes that the best 
way to avoid these complexities is to keep solute gradients 
modest.  Another plus for SE as against SV for studies of association 
equilibria.

Best regards,
Allen


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