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A series of spin labels has been empolyed to explore the environment of the sulfhydryl group in bovine plasma albumin. The spin labels consist of the nitroxide-free radical and a maleimide (or iodoacetamide)-attaching group separated by varying chain lenghths. Both sets of spin labels preferentially bind to the sulfhydryl group under appropriate conditions. From the change in the electron spin resonance spectra of these nitroxides as a function of chain length, we conclude that the sulfhydryl group is located in a crevice approx. 9.5 A in depth.
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