[RASMB] fringe deviation

[Arthur Rowe]

Dear Allen

I think that the occurrence of 'schlieren' type effects in absorption
velocity analysis is machine dependent. Possibly minor differences in
geometry are involved. We have most certainly seen scans in which a 'peak'
is superimposed on the normal absorption trace, although no 'black band'
indicating total cut-out is present.

We have even on occasion estimated an s-value from the migration of said
peak! However I would not quarrel with the general assertion that the best
thing to do is to avoid such conditions.

Arthur


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.



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