EPR measurements of weak exchange interactions coupling unpaired spins in model compounds

Abstract

I review electron paramagnetic resonance (EPR) measurements performed to evaluate very weak exchange interactions (defined as rYr -J,jS,$j, with 10 -3 cm-t< IJ,~l < I cm-') between unpaired spins, transmitted through long and weak chemical pathways typical of protein structures. They are performed in appropriate model compounds, mainly copper derivatives of amino acids and peptides, making use of the phenomenon of exchange narrowing and collapse of the resonances. I describe the theoretical basis and the implementations of the method to different situations, including selected experimental values of the exchange couplings J between metal centers, and briefly discuss correlations between J and the structure of the paths. Results obtained in relatively simple EPR experiments performed at room temperature in single-crystal samples are compared with those obtained from thermodynamic magnetic measurements having higher experimental difficulties. The experimental information allows describing the role of molecular segments typieal of biomolecules (H bonds, aromatic ring stacking, cation-Ÿ contacts, etc.) in the transmission of the exchange interaction. The values of J obtained in some model compounds are compared with those obtained in proteins to conclude that the magnitudes of the exchange interactions are useful to characterize long and weak biologically relevant chemical pathways. One observes that these exchange couplings are weakly dependent on the nature of the unpaired spins and strongly dependent on the chemical pathway. Thus, measurements of exchange couplings in model compounds may provide usefui information about biological function, particularly about electron transfer in proteins.

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