Synthesis and characterization of a spin‐labeled aminoacyl transfer ribonucleic acid

Unfractionated yeast transfer ribonucleic acid (tRNA) was reacted in aqueous acetone solution with the sulfhydryl spin‐labeling reagnent, N‐(l‐oxyl‐2,2,5,5‐tetramethyl‐3‐Pyrrolidinyl)iodoacetamide. Whereas tRNA stripped of amino acids reacted only slowly, there were sites on tRNA charged with cysteine which combined rapidly with the reagent. The latter class of spin‐label was quickly cleaved from the tRNA upon incubation in mildly alkaline solution (pH 8.0), suggesting that it was attached to the cysteinyl side chains. The paramagnetic resonance spectrum of column‐purified spin‐labeled cysteinyl tRNA showed that the spin‐label was partially immobilized as a result of its interaction with the tRNA. When the tRNA was slowly heated, an abrupt increase occurred in the rotational mobility of the paramagnetic amino‐acid side chain.