Ch 777 Spring 03, page 33
16. Measuring kinetics of dynamic processes by NMR
What is the "NMR timescale"?
What dynamic processes can be monitored?
a) motion which enhances relaxation or averages an NMR parameter
examples:
1. Methyl group rotation at right frequency promotes T1 relaxation
2. Motion of lipid in different phases (bilayer in gel vs liquid crystal vs
vesicles…) averages lipid headgroup 31P spectrum in diagnostic ways
when motional frequency is faster than frequency range of parameter
being averaged
b) chemical exchange between 2 states which differ in or J or T1 or T2
Compare the rate k=1/τc of the dynamic process to
the NMR parameter being averaged: ∆δ or ∆J or ∆T1 or ∆T2
timescale: slow exchange intermediate exchange fast exchange
δ>> 1
A−B
≈1
A−B
<< 1
A−B
k<< A−B k≈A−B k>> A−B
J>> 1
JA−JB
≈1
JA−JB
<< 1
A−B
T1 or T2>> 1
1T1A−1T
1B
etc etc
