Increasing the Modulation Depth of Gd(III)-Based Pulsed Dipolar EPR Spectroscopy (PDS) with Porphyrin–Gd(III) Laser-Induced Magnetic Dipole Spectroscopy

[Image: see text] Distance determination with pulsed EPR has become an important technique for the structural investigation of biomacromolecules, with double electron–electron resonance spectroscopy (DEER) as the most important method. Gd(III)-based spin labels are one of the most frequently used spin labels for DEER owing to their stability against reduction, high magnetic moment, and absence of orientation selection. A disadvantage of Gd(III)–Gd(III) DEER is the low modulation depth due to the broad EPR spectrum of Gd(III). Here, we introduce laser-induced magnetic dipole spectroscopy (LaserIMD) with a spin pair consisting of Gd(III)(PymiMTA) and a photoexcited porphyrin as an alternative technique. We show that the excited state of the porphyrin is not disturbed by the presence of the Gd(III) complex and that herewith modulation depths of almost 40% are possible. This is significantly higher than the value of 7.2% that was achieved with Gd(III)–Gd(III) DEER.