From: Marc S. Lewis <mslewis@helix.nih.gov>
  To  : RASMB <rasmb@bbri.harvard.edu>
  Date: Fri, 6 Apr 2001 11:19:07 -0400

Re: centrepieces again

As noted by Richard Thomas, the black centerpieces with an aluminum band
surround are not epon but are actually aluminum with a Kel-F coating.
Beckman ceased making them some years ago because of problems with the
Kel-F coming loose from the aluminum with unfortunate consequences for
experimental results.  I have several of these which I keep for use for
particularly critical equilibrium experiments, particularly when there
might be the possibility of solute adsorbed onto the sector walls or when
using an "aggressive" solvent which might attack epon.  As far as strength
for sedimentation velocity experiments is concerned, the gray aluminum
powder-filled centerpieces are the best.  However, over 25 years ago, when
studying the self-association of beta lactoglobulin, E. T. Adams and I
found that this protein formed irreversible trimers in such centerpieces
and while this may not be the case for all associating systems, I recommend
against using them for the study of reversible associations.  Thus,
virtually all of our work is done using the black carbon-filled epon
centerpieces for equilibrium and velocity studies.  We have not had any
problems using these at 55000 rpm for velocity studies and occasional use
up to 60000 rpm for smaller molecules has not resulted in any failures,
although I can not recommend this as a general practice, particularly since
the Lamm equation analysis developed here by Peter Schuck works quite well
with experiments performed at lower rotor speeds.  While the carbon-filled
epon centerpieces have proved to be quite satisfactory in general, it would
be very nice to have the Kel-F coated centerpieces available again, but I
doubt that BeckmanCoulter will give this serious consideration unless they
can see a significant demand with the prospects of selling enough of them
to warrent the effort on their part.

Regards to all-
Marc S. Lewis, Ph. D., Chief
Molecular Interactions Resource
Division of Bioengineering and Physical Science
Office of Research Services
National Institutes of Health
Buiding 13  Room 3N17
13 South Drive  MSC 5766
BETHESDA MD 20892-5766

Phone: 301-496-9044;  Fax:  301-496-6608