[RASMB] Fwd: negative concentrations? from Heather Peto

[John Philo]

Heather,

To expand a bit on Walter's earlier response, it's important to realize that
with interference data the zero offsets are arbitrary and usually can be
anything from +/- 1 fringe. In your case, with such a low mass peptide,
unless you are running at very high speeds the peptide concentration near
the meniscus may well still be quite high (a substantial fraction of that
near the cell base), so potentially the offset could even be more than one
fringe if the loading concentration was high (i.e. the true concentration
reading near the meniscus could be more than one fringe).

Another thing that could easily cause apparent negative concentrations is
miss-matched salt and buffer concentrations---at the high rotor speeds one
needs for a small peptide there is quite significant sedimentation of the
salts etc. and for interference data you really need to use a dialysate as
the reference.

John Philo
Alliance Protein Laboratories

-----Original Message-----
From: rasmb-admin at rasmb-email.bbri.org
[mailto:rasmb-admin at rasmb-email.bbri.org]On Behalf Of Walter Stafford
Sent: Monday, April 15, 2002 10:46 AM
To: rasmb at server1.bbri.org
Subject: [RASMB] Fwd: negative concentrations? from Heather Peto


----------------------------------------------------------------------------
------
The older archived RASMB emails can be found at:
http://rasmb-email.bbri.org/rasmb_archives
and current archives at
http://rasmb-email.bbri.org/pipermail/rasmb/
Search All the Archives at:
http://rasmb-email.bbri.org/rasmb_search.html
----------------------------------------------------------------------------
------

>Resent-Date: Mon, 15 Apr 2002 11:05:01 -0400
>Resent-From: rasmb-admin at rasmb-email.bbri.org
>Date: Mon, 15 Apr 2002 15:50:29 +0100
>From: Heather Peto <hp217 at mole.bio.cam.ac.uk>
>To: <rasmb at server1.bbri.org>
>Subject: negative concentrations?
>Resent-To: rasmb-owner at rasmb-email.bbri.org
>X-Ack: no
>X-BeenThere: rasmb at rasmb-email.bbri.org
>X-Mailman-Version: 2.0.6
>Sender: rasmb-owner at rasmb-email.bbri.org
>X-BeenThere: rasmb at rasmb-email.bbri.org
>List-Help: <mailto:rasmb-request at rasmb-email.bbri.org?subject=help>
>List-Post: <mailto:rasmb at rasmb-email.bbri.org>
>List-Subscribe: <http://rasmb-email.bbri.org/mailman/listinfo/rasmb>,
>	<mailto:rasmb-request at rasmb-email.bbri.org?subject=subscribe>
>List-Id: <rasmb.rasmb-email.bbri.org>
>List-Unsubscribe: <http://rasmb-email.bbri.org/mailman/listinfo/rasmb>,
>	<mailto:rasmb-request at rasmb-email.bbri.org?subject=unsubscribe>
>List-Archive: <http://rasmb-email.bbri.org/pipermail/rasmb/>
>
>
>Hi I am new to AUC.  I have used the XL1 Beckman with interference optics
>to test the oligomerisation state of a small peptide (28AA's monomer 3kDa)
>for technical reasons we can't use absorbance.
>I get in my data file the first values of concentrations as negative
>numbers! Which means that I can not take the natural log of them to plot
>against r2.  I wonder what do the negative numbers means? Is this an
>artifact of interference optics giving ratios of concentrations rather
>than absolute concentrations.  What can I do with the data to make it
>available to interpretation?
>
>many thanks
>
>heather :)
>
>
>
>Heather Peto			   Department of Biochemistry
>hp217 at mole.bio.cam.ac.uk	   New Building
>Tel: (lab)    +44 (0)1223 333343,  80 Tennis Court Road
>Tel: (office) +44 (0)1223 766022,  Old Addenbrookes Site
>Fax:          +44 (0)1223 766002,  Cambridge, CB2 1GA
>				   UK

--
Walter Stafford
mailto:stafford at bbri.org
617-658-7808
_______________________________________________
RASMB mailing list
RASMB at rasmb-email.bbri.org
http://rasmb-email.bbri.org/mailman/listinfo/rasmb