From: Richard M Thomas <rthomas@ifp.mat.ethz.ch>
  To  : rasmb@bbri.harvard.edu
  Date: Mon, 30 Jul 2001 12:56:42 +0200

Small peptides

Good Afternoon,
Been following the discussion on small peptides with interest. As has been 
mentioned, we've looked at lots of these things over the years so I thought 
I'd add a couple of things of a practical nature that haven't been 
mentioned yet. Briefly, we use the sort of approach discussed by John Philo 
and Jack Correia and, in general, we've looked at associating systems using 
equilibrium methods.
Often in these cases, depending on the size and composition, charge effects 
can be come important and need to be considered in the analysis. There is 
often also a need to add components to the system to force the system to 
associate (salts) or dissociate (denaturants) or to 'become structured' 
(TFE). This considerably increases the complexity of the analysis and in 
all cases ( those we've looked at anyway) these additives decrease data 
curvature and the apparent mass. Often these factors appear together and, 
when the system itself is reversibly associating at the same time, things 
get entertaining - we had a lot of fun trying to analyse a 'halophilic' 
peptide (Biochem. 37:7539, 1998).
Also bear in mind that, depending on how the peptide has been designed or 
selected and its overall composition, small peptides have a tendency to 
aggregate. They are also much more susceptible to proteolysis than proteins 
and it's not uncommon for the mass simply to disappear during the 
experiment, particularly if data are gathered on a single sample at a 
variety of rotor speeds.
Hope this helps, regards to all
Richard

Dr Richard M Thomas
Inst. für Polymere
ETH-Zentrum
CH-8092 Zürich
Switzerland
(tel) +41 1 632 5540
(fax) +41 1 632 1073