Understanding peptide structure and dynamics is of great interest in chemistry and biology. EPR methods have been used to rank distances between side chains in doubly labeled peptides, ?thereby revealing local folding geometry. Recently, EPR of site-specific spin labeled peptides has been shown to be an excellent probe of-position-dependent dynamics. Although these EPR experiments have added considerably to our understanding of peptide motions, many important details remain unresolved. In particular, conventional 9 GHz EPR spectra are not particularly sensitive to the anisotropy of the spin label motion. Thorough characterization of the local label motion is necessary in order to clarify the interpretation of the position-dependent dynamics and to aid in the determination of distances in double label experiments. High frequency EPR has been demonstrated to be more sensitive than 9 GHz spectra to motional anisotropy at short correlation times. We have obtained high frequency EPR spectra of the alanine based 3K- 11 helical peptide at temperatures from 275-305 K. The 3K- I I helical peptide, shown below, is labeled at the cysteine with a methanethiosulfonate (MTSSL) spin label. Ac-AAAAKAAAAKCAAAKA-NH2 The high frequency EPR spectra exhibit greater sensitivity to motional anisotropy than low frequency spectra but remain amenable to simple line shape analysis according to motional line narrowing theory. Simulations of the experimental spectra using different models for the anisotropic motion of the spin label chain attached to the peptide backbone are in progress.
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