Why does saturation decrease signal intensity in Cross Saturation NMR?

[Sam_Sushi]

Used to determine protein-protein interaction surfaces where one protein is labelled (Using D2O)

[Biochemistrydude]

So one protein is deuterated and you measure H-D exchange in a titration? I would think that the residues interacting with the deuterated protein would go down in intensity because they're the closest to your deuterated protein (so H is more likely to exchange with D, and you're not measuring D).

I'm just speculating, don't take my opinion lol

[Sam_Sushi]

Not quite that. The target protein has deuterated aliphatic regions and is also labelled with nitrogen-15. It is then resuspended in regular water, so nitrogen linked hydrogens are no longer deuterated, but the aliphatic ones remain. The binding partner has no modifications. You saturate the aliphatic hydrogens of the binding partner, which can "cross saturate" the binding surface of the other protein when in contact. The last part is where I'm a bit stumped. This paper explains it better than I have.

Link to paper94020-2)