Two-Pulse EPR COSY (Correlation Spectroscopy) Sequence: Feasibility for Distance Measurements in Biological Systems

Abstract

The feasibility of one-dimensional two-pulse correlation spectroscopy (COSY) Electron Paramagnetic Resonance (EPR) sequence for distance measurements in biological systems using nitroxide biradicals is investigated numerically at Ku-band. It is found that the COSY sequences can be exploited to measure distances between the two nitroxides in the range $17.3 \AA \lesssim r \lesssim 47.0 \AA (0.5 \mathrm{MHz}\le d\le 10 \mathrm{MHz})$, where $d=\frac{2}{3}D$, with $D$ being the dipolar-coupling constant. Taking into account the dead time after the second pulse, it is found that the modulation depth can only be measured for $0.5 \mathrm{MHz}\le d \le 7.0$ MHz. However, for $d>7.0 \mathrm{MHz}$, for which a significant part of the initial signal is lost in the dead time, the Fourier transform of the observable part of the signal after the dead time as a function of ${t}_{1}-{t}_{d}$, where ${t}_{1}$ is the time of the echo after the second pulse and ${t}_{d}$ is the dead time, still provides undistorted Pake doublets centered at $\pm d$. It is shown here numerically that the amplitudes of the Pake doublets of the COSY signal are the most intense, one-to-two orders of magnitude larger, as compared to those of the four-, five-, six- pulse double quantum coherence (DQC), two-pulse double quantum (DQ), and five-pulse DQM (double quantum modulation) sequences. Another advantage of the COSY technique is that it provides a measurement of ${T}_{2}^{S}$, the spin–spin relaxation time over the $p=\pm 1$ coherence pathways.