Entropy-Enthalpy Compensation

[Petr Kuzmic]

Athel Cornish-Bowden wrote:
> 
> I should be interested to know what readers of this news group think about
> entropy-enthalpy compensation, 

A name that springs to mind is that of Otto Exner, who has diligently
debunked several myths of this sort as statistical artifacts.  I will
try to dig out a few reviews he authored, which perhaps may be not as
widely known as they deserve.  My former colleague Exner was
particularly insterested in the "iso-kinetic relationship", which, by
the way, in biochemistry almost always ends up pointing to about 310 K
as being the 'magic temperature'.  

It transpires that, aside from faulty statistics, there is another
reason why we should be suspicious about dS vs. dH plots for series of
enzyme/substrate/etc data obtained in aqueous solutions.   At one time,
I was looking at several sets of temperature-dependent kinetic data [a
series of substrates for recombinant human cyclophilin] having the
'magic' isokinetic temperature [310 K], so I read up on the phenomenon
in physical chemistry journals.   To my surprise, I found at least one
paper in J. Phys. Chem. [have to look up that reference] showing that
the "iso-kinetic temperature" of 310 K is predicted solely from the
properties of -water-.  

Apparently, at that temperature there is a certain transition in the
'water structure', causing entropy changes due to re-organization of
hydrogen bonds.  Somehow, on the macroscopic scale, this kind of solvent
effect manifests itself as the "iso-kinetic" temperature for a series of
biochemical reactants.  Obviously we never published the data, much less
[unlike some colleagues at a large pharmeceutical corporation doing the
same work] tried to interpret the data in terms of thermodynamic
properties of the enzyme.

I'll look up those references again and post them later.

	-- Petr 
_____________________________________________________________
Petr Kuzmic Ph.D. * BioKin Ltd. * Madison, WI 53708-8336, USA
pkuzmic at biokin.com * http://www.biokin.com * 608.256.1269 fax