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<TITLE>Re: [RASMB] fringe deviation</TITLE>
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<BLOCKQUOTE><FONT COLOR="#800000">Dear Allen<BR>
<BR>
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. <BR>
<BR>
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.<BR>
<BR>
Arthur<BR>
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<BLOCKQUOTE><TT>Responding to Arthur's query: Published treatments of nonideal SE by Wills <BR>
& Winzor, and by Behlke, correctly treat first order deviations from <BR>
ideality (2-body interactions), but become calculationally intractable <BR>
beyond the two-body interaction limit (ca 20-30 g/l total solute <BR>
concentration). We have very recently published a rigorous treatment valid <BR>
for an arbitrary number of solute species over the entire range of <BR>
experimentally accessible concentration (up to several hundred g/l total <BR>
solute concentration): Zorrilla et al, Biophys. Chem. 108, 89-100: 2004. I <BR>
will send a PDF to anyone requesting it.<BR>
<BR>
Responding to Tom's comment: He is correct in principle that Wiener <BR>
skewing can influence absorbance gradients, but we found empirically (see <BR>
my previous message for reference) that you need a huge RI gradient for <BR>
this to be manifested and such effects are HIGHLY non-linear in the <BR>
magnitude of the RI gradient: in general, if you don't see the dreaded <BR>
'black band' in an absorbance scan, you are ok, and even in scans <BR>
exhibiting a black band, absorbance data at radial positions clearly <BR>
outside of the 'black band' are ok. Tom correctly emphasizes that the best <BR>
way to avoid these complexities is to keep solute gradients <BR>
modest. Another plus for SE as against SV for studies of association <BR>
equilibria.<BR>
<BR>
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