[RASMB] downward sloping baselines or "i no longer have much hair left to pull out"

[Jack Kornblatt]

I have checked the RASMB archives and there does not seem to be anything 
on this particular subject.
The problem is not the XL-I; the problem is an overly complex sample and 
reference.

The sample contains a protein that absorbs at 290 nm as well as a ligand 
that absorbs at 290 nm.
The reference contains just the  ligand. Both are in matching buffers.

The first scan: The baseline absorbance starts below zero and becomes 
more negative at higher radius.
Scan 100: The sample has almost sedimented completely giving the 
baseline absorbance what appears to be a "bow".
Scan 200: The sample has sedimented completely and the absorbance is 
well below zero. It is more negative at high radius than low radius.

The problem is clearly the sample. I have not run the "control" in which 
the sample compartment contains no protein but does contain ligand; 
reference compartment contains no ligand.

This problem must have come up in the past. Needless to say, all 
analyses are feasible but they are not likely to be correct. I'm at 
wits' end.

thanks for any help you can give. i am attaching an xlgraph of the three 
scans.
jack kornblatt
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://list.rasmb.org/pipermail/rasmb-rasmb.org/attachments/20070917/2ca311d1/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: JACK.GRF
Type: application/octet-stream
Size: 16440 bytes
Desc: not available
URL: <http://list.rasmb.org/pipermail/rasmb-rasmb.org/attachments/20070917/2ca311d1/attachment.obj>