Follow-up Comment #4, sr #3155 (project relax):
Hi Edward,
Took a while to get back to this:
Dimitri's function 'peak correct' is exactly what you need. From what you
describe below, Dimitri implemented Nikolai's formula for the exchange induced
fit. This is good approximation from the 2002 reconstructing invisible states
paper and will be right almost always.
The exchange induced fit is the factor that the ground state moves by when
experiencing exchange. It's zero when slow, PgDeltaOmega when fast, and
somewhere between when somewhere in between. The exact value is the imaginary
part of the ground state eigenvalue from the 2x2 (f00i in my paper), though
Nikolai's formula is good.
Again, in a real spectrum, you observe a peak. If it's in intermediate
exchange, it's not actually sitting at the ground state position - it'll be
shifted more or less depending on the above. So when you set an offset in an
experiment in an R1rho experiment, pragmatically you do this from its observed
position, not the ground state position.
The limiting case is fast exchange. In this case, your peak is sitting of
course at the population average. This is going to maximally far from the
ground state in terms of chemical shift, and when things are going to be
maximally wrong.
Have a read of this:
J. Biol. NMR (2012) 53(1): 1
It's not a controversial thing. Think about it. I'm happy to send some code
that incorporates this implicitly if this is helpful. Literally just take the
experimental offsets and add or subtract the exchange induced fit (depending
on how exactly you the offset frequencies have been defined).
The devil is in the details.
By the way, the manual you have written for relax is very impressive indeed.
That's probably the largest collection of modern NMR relaxation theory and
references anywhere. At least as far as I've seen. There's not much more to
add I think to take you from cutting edge to bleeding edge.
Best,
Andy.
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[sr #3155] An R1rho expression for a spin in chemical exchange between two sites with unequal transverse relaxation rates