Density functional theory calculations of electron paramagnetic resonance parameters of a nitroxide spin label in tissue factor and factor VIIa protein complex

The electron paramagnetic resonance (EPR) g and N-14 hyperfine coupling (A) tensors of a nitroxide spin label are calculated with density-functional theory (DFT). The influence on the spin label from nearby amino acids in the extracellular part of tissue factor (sTF) and activated factor VII (FVIIa) protein complex is investigated. For that purpose, the nitroxide unit and six surrounding amino acids within 5 Angstrom are selected on the basis of a molecular mechanics structure of the protein complex. The effects of the surroundings on the EPR parameters of the spin label can be divided into indirect effects caused by the induced structure changes of the spin label and direct effects. The structural changes are larger in the present case. The experimentally measurable hyperfine tensor component perpendicular to the molecular plane of the spin label, A(zz), as well as the g tensor component along the NO direction, g,,, are significant probes of the intramolecular structure of the spin label. This indicates the possibility of relating EPR properties to the geometric structure of radical sites. The direct environmental effects on the g tensor from the surrounding amino acids mainly affect the second-order spin-orbit/orbital Zeeman cross-term contributions from the spin label itself. The direct effects originating elsewhere in the model are small. Neither the g nor A tensors display additivity of the effects of individual amino acids on the final observable. The results underline the feasibility of DFT calculations of the EPR parameters in large molecular systems, such as spin labels and other radicals in proteins.