<b>Direct observation and measurement of the average quantum methyl-tunneling frequency in a protein using </b><b><i>T</i></b>_{<strong>m</strong>}<b>-edited double electron-electron resonance EPR spectroscopy</b>

<b>Using a {U[</b>^{<strong>2</strong>}<b>H];[Leu-[CH</b>_{<strong>3</strong>}<b>]} isotopically labeled model protein, spin-labeled at two sites with the rigid R1p nitroxide sidechain, one of which is present in two distinct conformers, we show that oscillations in the ratio of integrated intensities of the two peaks in the DEER-derived probability distance distribution as a function of the second echo period time </b><b><i>T</i></b><b> arise from quantum methyl tunneling. Quantitative analysis yields an average methyl tunneling rate of 0.7 MHz.</b>