NITROXIDE SIDE CHAIN DYNAMICS IN SPIN LABELED HELIX FORMING PEPTIDE

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High frequency electron paramagnetic resonance has been performed on a nitroxide spin labeled peptide in fluid aqueous solution. The peptide, which follows the single letter sequence Ac-(AAAAK)2CAAAKA-NH2 was reacted with the methanethiosulfonate spin label at the cystein sulfur. The spin sensitivity of high frequency EPR is excellent with less than fifty nanograms of sample required to obtain spectra with good signal-to-noise ratios. Simulation of the temperature-dependent spectral lineshapes reveals the existence of local anisotropic motion about the nitroxide N-0 bond with a motional anisotropy ,rl/,rll (=- N) approaching 2.6 at 306 K. Comparison with previous work on rigidly labeled peptides suggests that the spin label is reorienting about its side chain tether. The study demonstrated the feasibility of performing 140 GHz EPR on biological samples in fluid solution.