High-resolution study of nuclear magnetic relaxation dispersion of purine nucleotides: Effects of spin–spin coupling

High resolution NMR study of T1 magnetic relaxation dispersion. III. Influence of spin 1/2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons

“…Such conditions are much less demanding than strong coupling of the protons with the hetero-nuclei and can be met even at high fields of the order of several Tesla. Other spin 1 2 nuclei, e.g., 13 C, 15 N, 31 P, affect the proton NMRD in the same way.…”

“…However, features in the NMRD of protons and hetero-nuclei are located at different magnetic fields because the characteristic chemical shift values are usually considerably different for the nuclei of different kinds. While for protons (δ i − δ j ) are rarely larger than a few ppm, the range of chemical shift can be several hundred ppm for instance, for 19 F or 13 C nuclei. As a consequence, the features in NMRD of hetero-nuclei are expected to be shifted to much lower fields according to Eq.…”

“…The idea of polarization transfer is as follows: 13,14,19 First, at high magnetic field the two states of protons H3 and H6, |αβ and |βα , have different populations, p αβ and p βα , due to a suitable preparation. Such a population pattern can be prepared, as in our case, by applying selective RF pulses, which affect the NMR lines of only one spin, to initially thermally polarized molecules.…”

“…They may vary the spin eigen-states with the field and cause pronounced changes in the relaxation behavior. Experimental studies of the third factor are limited by the fact that (with very few exceptions [13][14][15][16][17][18] ) the experimental methods are lacking NMR spectral resolution and therefore allow one to investigate only the relaxation of the total magnetization of solvent or solute molecules. Such studies are unable to directly identify the consequence of spin-spin interactions.…”

“…13,14,19 Although the strength of spin-spin interactions is quite small (not larger than 30 Hz for protons) as compared to the large Zeeman interactions with the magnetic field (often reaching hundreds of MHz), these couplings can strongly affect the eigen-states of the spin system and thus cause a field dependence of the R 1 rates. The features observed are caused by the fact that at low magnetic fields the spins reach the regime of strong coupling.…”