Local mobility of 15N labeled biomolecules characterized through cross-correlation rates: Applications to paramagnetic proteins.

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.