[phth]

Is there a way to tell if a reaction is endothermic or exothermic through isotope effects?

[clarkstill]

By 'isotope effects' do you mean by comparing the rates of a reaction with two isotopologues of the substrate? If so, then no, I don't think so. Kinetic isotope effects relate only to activation energy of the reaction (ie kinetics), and aren't causally linked to the thermodynamics of the reaction (although there may be some correlation due to Curtin-Hammett-type considerations).

But I could be wrong!

[Babcock_Hall]

I don't think so.  Are you thinking about uncatalyzed reactions or enzyme-catalyzed reactions?

[phth]

Thanks for the responses.  I meant equilibrium isotope effects in the context of enzymatic mechanisms.  The curtain hammett postulation is what I was getting at.  I am just not sure how great of a magnitude the effects may be. 

[Babcock_Hall]

I know just enough to be dangerous.  Let me list out some of the names in the field of enzymatic isotope effects that come to mind:  W. Wallace Cleland, Dexter Northrop, Richard Schowen, Judith Klinman, and Paul F. Cook.  Cook and Cleland wrote a book, and Northrop has written at least one review article. 

Much of the work on enzymes focuses on kinetic isotope effects.  The only equilibrium isotope effect that I have heard much about is the fractionation factor.

[phth]

I know just enough to be dangerous.  Let me list out some of the names in the field of enzymatic isotope effects that come to mind:  W. Wallace Cleland, Dexter Northrop, Richard Schowen, Judith Klinman, and Paul F. Cook.  Cook and Cleland wrote a book, and Northrop has written at least one review article. 

Much of the work on enzymes focuses on kinetic isotope effects.  The only equilibrium isotope effect that I have heard much about is the fractionation factor.

Thanks.

[Babcock_Hall]

There are also some interesting effects when an isotope is used to perturb an established equilibrium.  I seem to recall a Chapter in Methods in Enzymology coauthored by Cleland on this subject.  I cannot recall the details.