[5] Spin labeling of proteins

Spin labels usually refer to compounds that contain the paramagnetic nitroxide free radical (stabilized by full alkyl substitution at the a-carbons) and a second group that can covalently bind another molecule. Some spin labels modify the side chains of proteins. These reagents can be subclassified either as affinity labels, with structural complementarity to binding pockets on proteins, or as reagents that covalently modify specific types of side chains, but do not depend on a protein specific pocket as a prerequisite for labeling. This chapter discusses: (1) to outline experimental protocols that have been used in one- and two-dimensional IH NMR studies of spin-labeled proteins; (2) to discuss spin labels, which react with the side chains of lysines (and a-amino groups), tyrosines, cysteines, histidines, and methionines––the utility, limitations, and reaction conditions of these reagents will be evaluated; and (3) to examine purification procedures that have been used to prepare mono-labeled proteins.