Protein thiyl radicals in disordered systems: A comparative EPR study at low temperature

A systematic study concerning the EPR spectroscopic properties of alkyl thiyl radicals in disordered systems under various conditions in proteins and in low molecular weight thiols is reported. Thiyl radicals were generated by UV photolysis at 77 K and studied at this temperature in frozen aqueous solution of the R1 protein of Escherichia coli ribonucleotide reductase (RNR), and of bovine serum albumin (BSA), in lyophilized BSA, and in a BSA film, and for comparison, in frozen aqueous solution of cysteine and in polycrystalline cysteine. A correlation of the g-tensor with the polarity of the environment (hydrogen bonding to the sulfur) of the thiyl radicals, as well as unusual anisotropic relaxation of the radicals have been observed. The stability of the thiyl radicals in various matrices was studied, including their reactions with oxygen and thiols in the matrix, which give rise to secondary radicals. The EPR line shape of alkyl thiyl radicals in disordered systems is characterized by an axial symmetric large g-anisotropy with an intense g⊥ component and a broad and weak g∥ component. A considerable g-strain, and in proteins, a number of different thiyl radical sites strongly broaden the g∥ component in spectra of frozen glasses. Nevertheless, g∥ is completely resolved in X-band spectra also in disordered systems. Protein thiyl radicals in BSA and R1 protein exhibit similar EPR spectroscopic properties in frozen glassy solution as thiyl radicals derived from low molecular weight thiols. The g∥ component of about 2.10–2.30 is a unique feature and thus characteristic for thiyl radicals in disordered systems. However, this fingerprint is particularly broad and weak in protein thiyl radicals and can most clearly be detected in absorption display. Protein-based thiyl radicals, as well as thiyl radicals in low molecular weight systems, exhibit a very anisotropic relaxation behavior. The microwave power for half-saturation of the g∥ component at 80 K is about 100 times larger than that of the g⊥ component indicating much faster relaxation for the g∥ component. This relaxation behavior was observed for the first time in disordered systems and represents a general property of thiyl radicals. Upon UV irradiation of proteins and cysteine in frozen solution at 77 K, thiyl radicals (major species) as well as perthiyl and C-centered radicals (accompanying minor species) have been identified as primary photolysis products. Thiyl radicals in the different systems are generally stable up to 120 K; at temperatures above 270 K, however, they are unstable. The stability of thiyl radicals is larger in crystalline samples than in frozen aqueous solution. Protein-associated thiyl radicals of R1 protein and BSA in frozen aqueous glasses are considerably more stable than thiyl radicals from cysteine in the same frozen solution, which is traced back to the effect of the protein matrix. Thiyl radicals from proteins and from cysteine in a frozen solid glass exhibit a temperature-dependent irreversible reaction with oxygen and with sulfhydryl groups of cysteine. Thereby, sulfinyl, sulfonyl, thiyl peroxyl, and perthiyl radicals have been observed as secondary reaction products.