Polarity contributions to hyperfine splittings of hydrogen-bonded nitroxides--the microenvironment of spin labels

A self-consistent treatment of reaction field effects on isotropic (14)N hyperfine coupling constants of nitroxide spin labels in mixtures of polar and apolar solvents is given based on the Onsager approach. It is shown that this works reasonably well for mixtures of water or methanol with dioxane, far better than do conventional approaches using the Clausius-Mossotti relation. Association constants, K(A,h), for hydrogen bonding of protic solvents to nitroxides are derived in this way from published EPR data. A value of K(A,h) approximately 1.0 M x (-1) is argued to be reasonable for water in a hydrophobic environment. Data from spin-labelled lipids can then be used to estimate effective water concentrations in biological membranes.