Electron Spin Relaxation of Photoexcited Porphyrin in Water-Glycerol Glass


Recently, the photoexcited triplet state of porphyrin was proposed as a promising spin-label for pulsed dipolar electron paramagnetic resonance (EPR). Herein, we report the factors that determine the electron spin echo dephasing of the photoexcited porphyrin in a water-glycerol matrix. The electron spin relaxation of a water-soluble porphyrin was measured by Q-band EPR, and the temperature dependence and the effect of solvent deuteration on the relaxation times were studied. The phase memory relaxation rate (1/T-m) is noticeably affected by solvent nuclei and is substantially faster in protonated solvents than in deuterated solvents. The T-m is as large as 13-17 mu s in deuterated solvent, potentially expanding the range of distances available for measurement by dipole spectroscopy with photoexcited porphyrin. The 1/T-m depends linearly on the degree of solvent deuteration and can be used to probe the environment of a porphyrin in or near a biopolymer, including the solvent accessibility of porphyrins used in photodynamic therapy. We characterized the noncovalent binding of porphyrin to human serum albumin (HSA) from 1/T-m and electron spin echo envelope modulation (ESEEM) and found that porphyrin is quite exposed to solvent on the surface of HSA. The 1/T-m and ESEEM are equally effective and provide complementary methods to determine the solvent accessibility of a porphyrin bound to protein or to determine the location of the porphyrin.

spin labels, EPR spectroscopy, porphyrin, electron spin relaxation, distance measurements, DEER, PELDOR, DISTANCE MEASUREMENTS, TRIPLET-STATE, DIPOLAR SPECTROSCOPY, ESEEM SPECTROSCOPY, EPR, ECHO, MOLECULES, RESONANCE, PEPTIDES, RADICALS, Biochemistry & Molecular Biology, Chemistry, Multidisciplinary
Sannikova, N., Timofeev, I., Bagryanskaya, E., Bowman, M., Fedin, M., & Krumkacheva, O. (2020). Electron Spin Relaxation of Photoexcited Porphyrin in Water—Glycerol Glass. In Molecules (Vol. 25, Issue 11, p. 2677). MDPI AG. https://doi.org/10.3390/molecules25112677