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The mobility of (15)N labeled proteins can be characterized by measuring the cross-correlation rates delta(N,NI) that govern the conversion of Zeeman order N(z) of an amide (15)N nucleus into longitudinal two-spin order 2N(z)I(z) involving the amide (15)N and (1)H nuclei. This represents an alternative to the measurement of (15)N self-relaxation rates 1/T(1) and 1/T(2) or 1/T(1rho). The rate of interconversion between N(z) and 2N(z)I(z) is due to cross-correlation between fluctuations of different interactions and is not affected by a variety of relaxation mechanisms that contribute to the self-relaxation rates 1/T(1), 1/T(2) and 1/T(1rho). Spin diffusion among protons, which affects the measurements, can be quenched by various means that are evaluated by experiments and simulations. By applying an off-resonance radio-frequency (RF) field in the vicinity of the nitrogen resonance, the spectral density function J(omega) can be determined at the frequency origin and at the nitrogen Larmor frequency. The methods are applied to the paramagnetic High-Potential Iron-Sulfur Protein iso I (HiPIP I) from E. halophila in its reduced state.

The mobility of betaCH2 moieties in oxidized and reduced cytochrome b5 was studied by analyzing the 13C relaxation of the J-split components, in terms of C-H dipole-C-H dipole cross correlation rates. A 2D 13C-1H experiment is proposed to measure these rates that provide the internal effective reorientation correlation time for each CH2 moiety. It is found that higher mobility is present in the alpha helices forming the heme pocket. On the contrary, the beta strands, which form the hydrophobic core of the molecule, have the lowest mobility. The general pattern is the same for the oxidized and reduced species, indicating that any oxidation-dependent property detected for backbone NH moieties does not affect the CH2 mobility.

The Ca2 calbindin protein in which one calcium has been substituted with Ce(III), Yb(III) and Dy(III) displays substantial alignment in high magnetic fields due to the high anisotropy of the metal magnetic susceptibility. This property has allowed the measurement of residual dipolar coupling contributions to 1J(HN) and 2J(HH) couplings of asparagine and glutamine NH2 moieties. Such data have been used to aid structural characterization of these groups. The exploitation of auto-orientation of magnetic anisotropic metalloproteins represents a step ahead in the investigation of the conformational space of peripheral residues that are not fixed by the protein folding.

Rotating-frame 15N relaxation rate (R1 rho) NMR experiments have been performed in order to study the dynamic behavior of the reduced recombinant high-potential iron-sulfur protein iso I (HiPIP I) from Ectothiorhodospira halophila, in the microsecond to ms time range. Measurements of R1 rho were performed as a function of the effective spinlock magnetic field amplitude by using both on and off-resonance radio frequency irradiation. The two data sets provided consistent results and were fit globally in order to identify possible exchange processes in an external loop of the reduced HiPIP I. The loop consists of residues 43-45 and the correlation time of the exchange process was determined to be 50 +/- 8 microseconds for the backbone nitrogen of Gln 44.

We have accounted for the effect of paramagnetism on the intensities of NOEs in a 73-residue paramagnetic metalloprotein, the reduced high-potential iron sulfur protein ISO I from Ectothiorhodospira halophila, whose solution structure had been recently solved by us. The paramagnetic effects were dealt with through a suitably modified complete relaxation matrix approach. We have then recalculated the structure through a distance geometry program by minimizing the difference between the sixth roots of the calculated and experimental NOEs. The average RMSD, calculated on residues 4-71, within the structures constituting the two families decreased from 0.67 to 0.46 angstrom for backbone atoms and from 1.23 to 1.06 angstroms for all heavy atoms. The structures in the new family are for the most part within the indetermination of the previous, less resolved, family. A few specific differences are detected and related to the presence of non-negligible paramagnetic effects, which are now properly evaluated.