Detection of Molecular Transitions with Nitrogen-Vacancy Centers and Electron-Spin Labels

We present a protocol that detects molecular conformational changes with two nitroxide electron-spin labels and a nitrogen-vacancy (NV) center in diamond. More specifically, we demonstrate that the NV can detect energy shifts induced by the coupling between electron-spin labels. The protocol relies on the judicious application of microwave and radiofrequency pulses in a range of parameters that ensures stable nitroxide resonances. Furthermore, we demonstrate that our scheme is optimized by using nitroxides with distinct nitrogen isotopes. We use detailed numerical simulations and Bayesian inference techniques to demonstrate that our method enables the detection of conformational changes under realistic conditions including strong NV dephasing rates as a consequence of the diamond surface proximity and nitroxide thermalization mechanisms. Finally, we show that random molecular tumbling can be exploited to extract the inter-label distance.