Chemical exchange and the effect on NMR spectra
For the effect of chemical exchange one has to consider various situations:
Exchange of protons in D2O solutions:
R- NH + D2O R- ND +HOD (D2O in huge excess)
Although this is an equilibrium process, the back reaction doesn’t occur due the D2Os excees. Thus, the exchange behaves as a quasi-unimolecular decay, a first order reaction. All exchangeable signals will be (eventually) lost in the NMR-spectra. Depending on the exchange kinetics, we can follow the exchange by NMR. If kex= ɭ/min and signal will be completely exchanged within the dead-time of dissolving and data acquisition. “Fast and medium exchange”
However, solvent exchange can be followed for signals/labile protons in “slow chemical exchange”, i.e.with exchange rates ɘ.5/min.
Examples are backbone amide protons in proteins, if in structured areas = restricted water accessibility (steric or electronic reasons/ H-bonds == important structural information!!
Exchange of labile protons in H2O solutions:
In principle, this is the same situation as above. The exchange has several effects depending on the kinetics and experiment conditions. If we first any exchange with water in NMR-experiments using supression methods for water. then the exchange will lead to:
R- NH + H*2O R- NH* +H*OH (H*2O = saturated/non excited water in excess).
The difference to the above/D2O situation is: that the R-NH* can relax, i.e. return to R-NH if sufficient time given and appropriately treated. This depends on how we supress the water, and at what time during the pulse sequence.
Distinguish in water supressed spectra:
1) Exchange during the fid = acquisition of the excited, observable signal
2) Exchange before acquisition
3) Exchange during the preparation/relaxation delay