Spin-labeled hemoglobin derivatives in solution and in single crystals.

The paramagnetic resonance of a spin-label attached to a protein in solution, or in a single crystal, depends on static as well as on the dynamic features of its molecular environment.' This conformation-dependent paramagnetic resonance offers the possibility of relating structural properties of proteins in single crystals to those in solutions. The present paper is a brief report of a study of this type. Here we compare the paramagnetic resonance spectra of spin-labeled horse hemoglobin in solution, and in single crystals, for the carbonmonoxyand acid met (ferric) derivatives. The present work was stimulated by an earlier, puzzling observation that the paramagnetic resonance spectra of spin-labeled horse oxyand acid methemoglobin in solution are significantly different,2 whereas Perutz and co-workers have shown that these two molecules must have very similar secondary and tertiary structures, and identical quaternary structures in the crystalline state.3 4 Materials and Methods.-The spin-label used in the present work is N-(1-oxyl-2,2,6,6tetramethyl-4-piperidinyl) iodoacetamide (V),