Electron-paramagnetic-resonance spectroscopy of spin-labelled cowpea-chlorotic-mottle virus.

Cowpea chlorotic mottle virus has been labelled with a maleimide spin label when the nucleoprotein particles were in the swollen state. After the spin-labelling reaction, the virus was dialyzed to bring the nucleoprotein particles into different conformational states. The mobility of the labelled part of the protein was monitored by electron paramagnetic resonance (EPR) spectroscopy and by the saturation-transfer EPR technique. The latter technique especially showed sensitivity to conditions that influence the RNA conformation and the RNA-protein interaction. The shape of the EPR and saturation-transfer EPR spectra can only be explained by an anisotropic motion of the whole spin-labelled viral protein or the spin-labelled part of the protein subunit in the core of the virus.