From: hans-georg.mueller.hm@bayer-ag.de
  To  : -         *rasmb@alpha.bbri.org <rasmb@alpha.bbri.org>
  Date: Tue, 15 Aug 2000 14:55:43 +0200

association behaviour of small charged organic molecules in

Dear RASMB members,

association behaviour of small organic molecules in aqueous solutions

has been studied by analytical ultracentrifugation quite extensively in the

past for example  by van Holde and his coworkers.

Results of his work refering to the self-

association of purine in water are quoted in Fujita`s famous

book "Foundations of ultracentrifugal analysis", Wiley 1975,

page 407 ff.

Van Holde also published one article on the association of the charged

adenosine-5`-phosphate in "Biochemical and Biophysical Research

Communications" 26 (1067) 717 ff taking work of Scatchard into account,

finding surprisingly a self-association of this charged molecule.



In a later work ( Annals New York Academy of Sciences

164 1969  279)  van Holde however

restricted his discussion to uncharged molecules and mentioned "difficulties

in the interpretation of data"  with charged molecules - hoping to overcome
them.


Does anyone of you know something about the results of these considerations?



I personally  feel following the work of Roark and Yphantis

(Biochemistry 10 (1971) 3241, again quoted in Fujita`s book,

that, if sufficient supporting electrolyte is added, the molecular weights

should have a slight tendency  to be too small (Donnan effect) - so one would

expect van Holde`s results for the charged adenosine-5`-phosphate to be in

principle correct, but that  the tendency for self-aggregation is in fact even
more

pronouced.


Would  you agree with me or which phenomenon could  lead to a too

steep concentration gradient (I am talking about the equilibrium method all the
time)

 in the analytical cell in the case of small charged molecules?


But why then did van Holde hesitate to trust his own results?

Has anyone got an idea?


Thank you for your answer (s) in advance


Yours
 sincerely


H.G. Mülller,

Bayer AG, Leverkusen, Germany