Spin-label studies of glycogen phosphorylase hosted in microemulsion droplets

ESR spectra of Phosphorylase b labelled with two different paramagnetic probes were recorded in microemulsions of various compositions, formulated with AOT and isooctane. Measurements of the hyperfine coupling constant aN and of the degree of anisotropy H1/H0, showed that: i) the polarity of the microenvironment of both spin-labels bound to the enzyme, expressed in terms of aN, is increased with increasing the hydration ratio of the reversed micelles, R; ii) a transition of the polarity between bound and free water is observed at R = 10; iii) the mobilities of the spin-labelled moieties of the enzyme are different, depending on the hydrophobicity of the site where the probe is bound and on the size of the microdroplets.