Lanthanoids and actinoids: shift and relaxation

Paramagnetic Chemical Probes for Studying Biological Macromolecules

“…126−129 Other lanthanoid ions are much less frequently used for paramagnetic NMR of proteins because their Δχ tensors are smaller. 86…”

“…Curie spin relaxation is driven by rotational tumbling, with the rotational correlation time τ r characterizing the time-dependent modulation. Because of a quadratic dependence on the nuclear Larmor frequency, Curie spin relaxation can outweigh SBM relaxation for high-molecular weight systems in a strong magnetic field , (note that the calculations of Table 2 in ref were performed for τ r = 10 ns instead of τ r = 1 ns as stated in the original) while decreasing noticeably with increasing temperature . A quantitative description of PREs by Curie spin relaxation is given by eqs and for longitudinal ( R 1 Curie ) and transverse ( R 2 Curie ) relaxation rates. …”

“… a Electronic configuration of the ions. b Total angular momentum quantum number. c Electronic relaxation time (in seconds) reported in refs , , , and . d Range of paramagnetic effect (in Å) reported in ref , , and . e + indicates the intensity of the effect based on refs , , and . No entry indicates not applicable or small effect. f Nitr., nitroxide. …”

“… e + indicates the intensity of the effect based on refs , , and . No entry indicates not applicable or small effect. …”

“…Gd­(III) is unique because its seven unpaired electrons are distributed equally among the f-orbitals, producing a magnetic dipole moment that is independent of molecular orientation in an external magnetic field. As its electronic relaxation time is long (>10 –8 s) at field strengths >3 T, , Gd­(III) generates large PREs and enjoys increasing popularity in EPR as spin label. − Although the remaining paramagnetic lanthanoid ions generate anisotropic χ tensors and produce all the paramagnetic effects discussed above, it has recently been shown that PRE measurements with Er­(III), which generates large PREs associated with relatively small PCSs, can yield more accurate distance measurements than commonly used paramagnetic agents with long electronic relaxation times . In general, Tb­(III) and Dy­(III) generate the largest Δχ tensors and, hence, cause the largest PCSs, while Tm­(III) and Ho­(III) produce medium-sized PCSs.…”