The mobility of 15N labeled proteins can be characterIzed by measuring the cross-correlation rates δN,NI that govern the conversion of Zeeman order Nz of an amide 15N nucleus into longitudinal two-spin order 2NzIz involving the amide 15N and 1H nuclei. This represents an alternative to the measurement of 15N self-relaxation rates 1/T1 and 1/T2 or 1/T1ρ. The rate of interconversion between Nz and 2NzIz 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/T1, 1/T2 and 1/T1ρ. 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(ω) 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.