The binding of calmodulin (CaM) to neuronal nitric oxide synthase (nNOS) enables formation of the output state of nNOS for nitric oxide production. Essential to NOS function is the geometry and dynamics of CaM docking to the NOS oxygenase domain, but little is known about these details. In the present work, the domain docking in a CaM-bound oxygenase/FMN (oxyFMN) construct of nNOS was investigated using the relaxation-induced dipolar modulation enhancement (RIDME) technique, which is a pulsed electron paramagnetic resonance technique sensitive to the magnetic dipole interaction between the electron spins. A cysteine was introduced at position 110 of CaM, after which a nitroxide spin label was attached at the position. The RIDME study of the magnetic dipole interaction between the spin label and the ferric heme centers in the oxygenase domain of nNOS revealed that, with increasing [Ca2+], the concentration of nNOS center dot CaM complexes increases and reaches a maximum at [Ca2+]/[CaM] >= 4. The RIDME kinetics of CaM-bound nNOS represented monotonous decays without well-defined oscillations. The analysis of these kinetics based on the structural models for the open and docked states has shown that only about 15 +/- 3% of the CaM-bound nNOS is in the docked state at any given time, while the remaining 85 +/- 3% of the protein is in the open conformations characterized by a wide distribution of distances between the bound CaM and the oxygenase domain. The results of this investigation are consistent with a model that the Ca2+-CaM interaction causes CaM docking with the oxygenase domain. The low population of the docked state indicates that the CaM-controlled docking between the FMN and heme domains is highly dynamic.