Electron Spin Relaxation of Photoexcited Porphyrin in Water—Glycerol Glass

Abstract

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/Tm) is noticeably affected by solvent nuclei and is substantially faster in protonated solvents than in deuterated solvents. The Tm is as large as 13–17 μs in deuterated solvent, potentially expanding the range of distances available for measurement by dipole spectroscopy with photoexcited porphyrin. The 1/Tm 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/Tm and electron spin echo envelope modulation (ESEEM) and found that porphyrin is quite exposed to solvent on the surface of HSA. The 1/Tm 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.

Structural properties of supercooled deep eutectic solvents: choline chloride–thiourea compared to reline

“…Then, spin relaxation of nearby nuclear and electron spins and spin–spin interaction between electron spins also induce ESE decay – see ref. 55–59 for more details. Fortunately, these additional processes mostly are field-independent and therefore they cannot manifest themselves in the experiment in which echo-detected EPR spectra are normalized like it is done in Fig.…”

Evidence for an Ordering Transition near 120 K in an Intrinsically Disordered Protein, Casein

“…Note that ESE signal decay, except of molecular motion of the spin-labeled molecules, is induced in solids also by other physical mechanisms—spectral diffusion due to spin relaxation of nearby nuclear and electron spins, spin-spin interaction between spins, spin-lattice relaxation of the spins under consideration—see [ 42 , 43 , 44 , 45 ] for more details. Fortunately, some of these additional mechanisms normally are too slow (spin-lattice relaxation) to influence the two-pulse echo decay; the others are mostly field-independent (spin relaxation of the nearby spins) and so may be excluded by comparison echo decays taken at two field possessing different spectral anisotropy.…”

Fullerene-based triplet spin labels: methodology aspects for pulsed dipolar EPR spectroscopy

“…16,17 The first work demonstrating the capabilities of dipolar EPR with photoexcited triplet labels was carried out using porphyrins, 16 and several other studies have followed in a few years. [18][19][20][21][22][23][24][25][26][27] The main advantage of porphyrins is their good water solubility, which allows spin labeling 16,20 or complexation with proteins. 23 The first application of triplet porphyrins was demonstrated for the 4-pulse DEER experiment, where porphyrins were photoexcited shortly before the microwave pulse sequence and used as an observer (laser-induced (Li) DEER).…”