pD Dependence of Hydrogen Exchange
A sensitive method of determining if an exchange is following
EX1 or EX2 kinetics is to determine the pD dependence of the exchange rate.
The magnitude of kint is a function of
the concentration of OD- ions in solution
and as a result shows a exponential dependence on pD. Therefore EX2 kinetics
show an exponential dependence on pD whereas EX1 kinetics are independent
of pD (as long as kop is pD independent).
We detected no pD dependence for the exchange kinetics for WT [Figure 4,
below] and therefore conclude that the exchange is not an EX2 process.
This is consistent with exchange of a deeply buried residue or residues.
Figure 4: Measuring the kinetics of the hydrogen exchange
for WT-AP at 60 °C at pD 6.76 and pD 7.76.
How Many Residues Must Exchange To Affect The Lifetime?