  In-cell distance determination by EPR reveals essential structural
information about biomacromolecules under native conditions. We demonstrate
that the pulsed EPR technique RIDME (relaxation induced dipolar modulation
enhancement) can be utilized for such distance determination. The performance
of in-cell RIDME has been assessed at Q band using stiff molecular rulers
labelled with Gd(III)-PyMTA tags and microinjected into X. laevis oocytes. The
overtone coefficients are determined to be the same for protonated aqueous
solutions and inside cells. As compared to in-cell DEER (double
electron-electron resonance, also abbreviated as PELDOR), in-cell RIDME
features approximately 5 times larger modulation depth and does not show
artificial broadening in the distance distributions due to the effect of
pseudo-secular terms.
