So-called chemical exchange occurs when microsecond to millisecond motions cause a modulation in the isotropic chemical shift. Chemical exchange is relevant both in the situation where a protein undergoes internal motions (e.g. a conformational change) or where it interacts with another molecule (e.g. complex formation).
Let’s take the example of a nucleus in an amino acid residue that switches between two separate (but equally populated) conformations A and B: each conformation will give rise to a distinct isotropic chemical shift of value δA and δB. If the rate at which the conformation switches is slow compared to the duration of the NMR experiment (typically in the millisecond regime) then two separate peaks will be observed in the NMR spectrum (one from each conformation) at δA and δB ppm. Essentially, while the NMR experiment is being recorded, half the molecules are in conformation A and the other half in conformation B, and thus you see signals for both.
As the rate of exchange between the two conformations increases, the conformational change will begin to occur during the time in which the NMR experiment is being recorded (i.e. while your pulse sequence is busy running). So as you start pulsing a molecule will be in conformation A, but by the time the pulse sequence has come to an end, the molecule is in conformation B. And as the exchange rate increases, the molecule will start to swap between conformations A and B multiple times during the NMR experiment. The two original chemical shifts that were measured during the NMR experiment will, therefore, each become modulated by the other. The effect is for each peak in the spectrum to move and become broader because the nuclei in the sample no longer all have chemical shift δA any more. Instead, some now have a chemical shift of wδA + (1-w)δB where w is the proportion of time spent in conformation A. If the rate of exchange is sufficiently high, then the nucleus will simply spend half its time in each conformation and a single peak at the average chemical shift will be observed in the NMR spectrum.
If the populations between the two conformations are not equal, then the average chemical shift during fast exchange, δav, is weighted by the populations:
δav = pA δA + pB δB
Overall, three exchange regimes can be identified: slow, intermediate and fast exchange. In the intermediate regime, peaks may be so broad, that they are not observed in an NMR spectrum. It is also worth noting that if there is a large difference in the populations between the two conformations, then despite slow exchange, only the large peak may be observable (though it may be somewhat broader than you would normally expect).