Marc Roussel wrote:>>>>>>>>>>>>>>>>>>>>>>>>>>>
Pentcho Valev wrote:
> Guy Brown wrote:>>>>>>>>>>>>>>
> Why is it that if a reaction has a second order rate constant of
> anything from 10^7 to 10^12 (s^-1) it is described as
> "diffusion controlled"? Surely if collisions can occur at 10^12
> then a rate constant of 10^7 can not be diffusion controlled.
> Presumably steric factors are invoked: the molecules collide
> without reacting. But how can we distinguish steric factors
> from the "binding" of the substrate by the enzyme?
> How low could a rate constant go and still be diffusion
> controlled?<<<<<<<<<<<<<<<<<<
>> I am not competent, but here is a suggestion: not steric factors are
> essential but rather the effectiveness of conversion of ES into
> E + Product. In the limit it is absolutely effective so the
> concentration of ES is zero and the Michaelis-Menten curve is linear
> everywhere.
How large the diffusion limit is depends on what the interacting
molecules are. The diffusion-limited rate constant depends on the
diffusion constants of the two reactants. For very small reactants with
large diffusion constants, we might get rate constants of up to 10^{12}
M^{-1}s^{-1}. For reactions involving enzymes or other large molecules,
10^7 M^{-1}s^{-1} is a closer guesstimate of the diffusion-limited rate
constant.<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
You are right. However the term "diffusion-controlled" contains the additional
idea that the next step (after the two molecules have "found" one another)
is fast enough so that "founding" is relatively slow and determines the rate
of the reaction.
In fact, the same diffusion factor is responsible for the unpleasant habit of
catalysts to shift chemical equilibrium. True, a catalyst can by no means
change the ratio of the forward and reverse process at the microscopic level;
however adding a catalyst can change dramatically and independently the
diffusion control of the two reactions - e.g. a monomolecular reaction (C -> )
which is not diffusion-dependent at all can become diffusion-controlled on
adding a catalyst. The reverse reaction ( -> C) may undergo no such change,
so there is no reason to expect the the catalyst would increase the two
rates in the same proportion.
Best regards,
Pentcho
