Just as an addition note, you typically measure the T2 of 1H as proton is more sensitive to paramagnetic relaxation than heavier nuclei.
There are two ways of measuring the difference in T2 for paramagnetic and diamagnetic samples. The ratio method (Baptiste and Wagner) and the difference method (Iwahara and Clore). It can be argued (and I agree) that the difference method is better. In this method you subtract R2dia from R2para to generate a gamma2 value. This value is proportional to r-6. When using MTSL it is easy to start with a paramagnetic sample (measure T2) and then quench the nitroxide label with ascorbate, generating a diamagnetic sample (measure T2 again).
answered Sep 08 '10 at 06:51
Here is a wiki page about PRE with a few literature links. Briefly, with PRE decrease of T2 relaxation time is measured introduced by a covalently attached paramagnetic label, such as a nitroxide-based radical.
Increase of relaxation rate R2 (or decrease of T2) of certain nuclear spin resonances is correlated with a distance between the label and the nucleus, and the actual distance can be determined if the label is fixed (does not move) relative to the target molecule.
The shorter is distance - the stronger is the relaxation enhancement and the enhancement will be proportional to inverse sixth power of distance.
With this method distances up to ~25A can be accessed, whereas NOE shows only short distance contacts of <5-6A.
As for how this can be applied to your cellulose studies - not so clear. Can you clarify in your question - what problem are you trying to solve? Also - what is the molecular weight of the cellulose species? Keep in mind that with flexible molecules distance restraints obtained by PRE or NOE may not be meaningful, because they will be averaged and heavily biased towards shorter ones (due to the R^-6 dependence).
Thank you for both answers. A colleague of mine is doing the 13C CP-MAS NMR measurements for me so I would like to know better about relaxation editing experiments vs PRE. I am a beginner in NMR spectroscopy and I would like to learn more on this topic. He is using as samples cellulose II powder, regenerated cellulose and milled reg cellulose. We are interested in C4 resonance of cellulose II, good resolved resonance, to better understand the supramolecular structure of cellulose II. As experiments: long relaxation experiments - PRE with aqueous CuSO4 solution of certain concentration, does the CuSO4 sol conc has any effect on the experim? What is the difference between relaxation editing experiments and PRE?
Thank you for your help. Nico
answered Oct 14 '10 at 03:51
Hi! Nico, The most often used application of Paramagnetic relaxation effect is to provide for fast relaxation to slowly relaxing Carbons and the intensity artefacts that arises due to NOE effects while acquiring Proton decoupled 13C spectra.
The terms like long relaxation experiments and the specific carbon (carbon 4) resonance being studied reminds me of the early experiments in which paramagnetic relaxing agents are used at appropriate concentration levels so that the differences in relaxation times are all even out and all carbons relax preferentially through the interaction with the paramagnetic ion. This uniformity in relaxation characteristics attained for carbons within the molecule can enable inteegrating Carbon NMR spectra and process similar to Proton lines.
answered Oct 16 '10 at 05:52