'pH' in a small volume?

tivol at tethys.ph.albany.edu tivol at tethys.ph.albany.edu
Tue Sep 20 15:25:46 EST 1994


Dear Peter,
	The small-volume/small-#-of-protons problem is one which concerned me
a lot some time ago.  The short answer is that the pH is less important than
the chemical potential and/or free energy.  The reaction(s) in the small vol-
umes occur as though the pH were well-defined, but the usual definition does
not apply.  This is analogous to the "temperature" of an ensemble o few sys-
tems--which turns out to be determined from the time average of the energies
of the systems.
	If you have reactants, an enzyme, and products, the time-average of the
numbers of molecules of reactant 
                                s and products will conform to that expected
in a larger volume, so the biochemistry is not messed up.  BTW, the active
site of an enzyme is a good example of small-volume conditions--the site can
behave as though it has a pH much different from that of the surrounding med-
ium, thus the catalysis.
				Yours,
				Bill Tivol



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