Characterization of V-IV biomolecules relies strongly on electron paramagnetic resonance (EPR) spectroscopy, particularly the application of the additivity relationship of A(z) values. It has been shown experimentally that the A(z) values of (VO2+)-O-IV imidazole species have a critical angular dependence. Density-functional theory (DFT) calculations elucidate the dependence of V-51 A(z) value on the orientation of the aromatic ring in (VO2+)-O-IV pyridine complexes, following closely the functional dependence observed for (VO2+)-O-IV imidazole species, [A(z)(pyr) = 42.23 + 1.80 x sin(2 theta - 90)], with A(z) measured in 10(4) cm(-1). A DFT re-examination of (VO2+)-O-IV imidazole complexes gives an equation very similar [A(z)(imid) = 42.35 + 2.34 x sin(2 theta - 90)] to that experimentally found. These results generalize the application of the additivity relationship for (VO2+)-O-IV complexes containing aromatic nitrogen ligands such as pyridine or imidazole. The increase of the absolute value of A(z), vertical bar A(z)vertical bar, when the dihedral angle theta between the V=O and N-pyr-C or N-imid-C bonds varies from a parallel to a perpendicular orientation is due to an increase of the d vanadium orbital contribution and to a decrease of the pi aromatic system participation in the singly occupied molecular orbital.