Spin-labeled transfer RNA.

The alpha-amino group of valyl-tRNA has been combined chemically with a nitroxide spin label through an amide linkage. This spin-labeled aminoacyl-tRNA has an electron paramagnetic resonance spectrum which reflects the mobility of the aminoacyl end of the tRNA molecule. The rate of tumbling of the spin label has been measured as a function of temperature and ionic strength. An abrupt transition in the nature of the motion occurs at a temperature which is sensitive to the ionic strength of the medium. This change in behavior of the aminoacylated spin label occurs at the same temperature as the midpoint of the optical density melting curves in dilute salt solutions. However, when denaturing agents such as dimethylsulfoxide or urea are added to the solution, the spin melting temperature is lower than the melting temperature as measured by the change in optical density. This suggests that under these conditions the local region of the aminoacyl end of the molecule denatures before the rest of the molecule is disrupted.