Electron Spin Resonance of an Irradiated Single Crystal of L-Tyrosine-HCL

Abstract

and Summary.-Analysis of the electron spin resonance (ESR) of a 'y-irradiated single crystal of L-tyrosine-HCl reveals a free radical formed by loss of an H from the hydroxyl group attached to the ring. Principal g values of the radical are g. = 2.0023 A 0.0005, g, = 2.0067 + 0.0005, and g, = 2.0045 ± 0.0005, with gx perpendicular to the plane of the ring and g, along the CO bond to the ring. Analysis of the hyperfine structure of the spectra shows electron spin density on C(2) and C(6) to be 0.24 and on C(4) to be 0.32. Corresponding spin densities calculated from molecular orbital theory are 0.26 and 0.31, respectively.Radiation effects on various derivatives of tyrosine in the powdered or polycrystalline form have been investigated before with electron spin resonance.1-' By bombardment of the sample with H atoms, Liming and Gordy4 recently showed that one type of radical produced results from H addition on the tyrosine ring. However, there are other ESR signals produced by ionizing irradiation of tyrosine and its derivatives which cannot be attributed to H-addition radicals. Also, when H-addition radicals are formed by ionizing irradiation of pure tyrosine, it is evident that the added H atoms must be released from some other part of the molecule. As the possible explanation of the ESR patterns observed in 7y-irradiated samples of powdered tyrosine and polytyrosine, radicals formed by loss of the hydrogen from the OH group on the ring have been postulated,1' 2 but this postulate could not be proved from the unresolved structure of the resonance of the powdered samples. We have undertaken this study on single crystals in the hope of correctly identifying the radicals formed and obtaining quantitative information about them.The Tyrosine-HCl Crystal.-Single crystals of L-tyrosine-HCl were grown at room temperature by slow evaporation of L-tyrosine dissolved in 30 per cent hydrochloric acid.The dimensions of the crystal used in this study were 1 X 1 X 8 mm. When the crystal was rotated about the c axis (see Fig. 1), six faces were observed. The approximate angles between these faces were measured by an optical goniometer, and the values were compared with the ones calculated from the unit cell dimensions.

Angular orientation of the stable tyrosyl radical within photosystem II by high-field 245-GHz electron paramagnetic resonance.

“…4, was estimated independently from a study of the power saturation of the tyrosyl radical. The orientation of the principal axis system of the g anisotropy was taken from the literature (21,31) and the g values were adjusted to give the best fit. Using Eq.…”

Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: The tyrosine radical Y\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{_{D}^{{\bullet}}}}\end{equation*}\end{document}

“…Because the g tensor is closely related (50) to the geometrical structure of phenoxyl type radicals (Fig. 1), the orientation of the phenoxyl ring itself can be derived.…”

High‐Field2H‐Mims‐ENDOR Spectroscopy on PSII Single Crystals: Hydrogen Bonding of YD.

“…The principal axes of the g-matrix for a tyrosine radical are virtually collinear with the molecular coordinate system defined by the CÀO bond direction and the ring plane [34] (depicted in Figure 1) and, as we reported earlier, [25] with these molecular axes in the most recent structural model of PSII. [26] Representing the direction of the principal axis for the largest deuterium hfc component for the H-bonded proton in the molecular coordinate system of the PSIIcc with the Y D C oxygen as the origin results in a vector pointing towards D2-His189 as visualised in Figure 5.…”