The aim of this thesis was to use state-of-the-art quantum chemical calculations of electron paramagnetic resonance (EPR) with applications to radicals in biomolecular systems. Special emphasis has been devoted to investigations of structure-to-property relations of amino acid radicals and spin labels in proteins. The g-tensor calculations constitute the principal point employing novel response theory and density functional theory (DFT) algorithms. The A-tensor calculations supply additional means for a comprehensive comparison with experimental results. Calculations on substituted benzene radicals as a test set showed that restricted open-shell Hartree-Fock (ROHF) wave functions provide accurate results for the g-tensor. Some radicals, however, like the phenoxyl radical, call for a more sophisticated approach such as the multi-configuration self-consistent field (MCSCF) method. This method provide accurate results for the g-tensor in every case. The main contributions to the g-tensor could be traced qualitatively to the organization of the few lowest excited states of the radical. However, to obtain quantitative accurate results the contributions from all excited states must be accounted for, as attained by the response theory method.

The EPR parameters are sensitive probes of molecular interactions in the close vicinity of the radical. In a combined theoretical and experimental study it was possible to specify the causes of the parameter shifts in detail. The calculations confirmed that 9xx decreases and Azz increases at hydrogen bonding of the label. MCSCF calculations on tyrosyl radicals resulted in a lower 9xx at hydrogen bonding as observed experimentally in ribonucleotide reductase (RNR). A sulfur substitution, modeling the tyrosyl radical in galactose oxidase (GO), did unexpectedly not significantly influence the g-tensor or the spin density distribution. Calculations on sulfur centered radicals with different structures and charges showed that the g-tensor pattern is significantly different among these radicals. Thus the calculations provided means to distinguish different radical structures from each other. By DFT it is possible to make EPR calculations on large molecular systems. This was shown by calculations on a nitroxide spin label and nearby amino acids in a protein complex. The combination of EPR spectroscopy and DFT calculations provide a prospect of obtaining detailed knowledge about protein radicals.