J . Phys. Chem. 1989, 93, 2653-2658
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Study of Lipid Peroxyl Radicals In Urea Clathrate Crystals: Oxygen-17 Couplings and Rotational Averaging Michael D. Sevilla,* Mark Champagne, and David Becker Department of Chemistry, Oakland University, Rochester, Michigan 48309 (Received: August 30, 1988) This work reports an electron spin resonance investigation of radicals formed after irradiation of a variety of fatty acid esters trapped in urea clathrate structures. Irradiation of saturated fatty acid esters results in carbon-centered radicals at the a-carbon site, whereas for unsaturated species the expected allylic and bisallylic radicals are found. Exposure of the carbon-centered radicals to oxygen results in their conversion to the corresponding peroxyl radicals. Orientation- and temperature-dependence studies show that at room temperature both the lipid carbon centered and peroxyl radicals rotate or oscillate in urea channels about their long axis. The rotational jump mechanism found for peroxyl radicals in this work usually follows the trigonal symmetry of the low-temperature distorted urea host channels. At room temperature the entrapped molecules rotate freely on their long axis, and as a consequence orientation of samples at the magic angle (54.7') gives isotropic couplings. Oxygen-17 labeling of the peroxyl radicals results in the first reported anisotropic oxygen hyperfine couplings for lipid peroxyl radicals.
Introduction Cocrystallization of long-chain hydrocarbons with urea results in a crystalline structure with hexagonal urea channels occupied by the h y d r ~ c a r b o n . ' - ~At low temperature these channels are known to undergo a slight distortion to 120' ~ y m m e t r y . ~The irradiation of such crystalline structures results in radicals only on the trapped molecules. Thus, the trapping of long-chain structures in urea clathrate channels allows for the observation of radicals formed by radiation in a single-crystal-likeenvironment; further, the high melting point of the urea structure allows studies over a wide temperature range.5-9 In early work Griffith found that fatty acid esters could be trapped and studied in urea clathrate structure^.^ Faucitano et al. have investigated the free radical chemistry induced by irradiation of fatty acids in urea clathrates at low temperature^.^^^ Recently peroxyl radicals of hydrocarbon polymers have been investigated in these channel structure^.^^^*'^ Analysis of the electron spin resonance (ESR) spectra of the trapped peroxyl radicals with temperature gives details of the energetics and motion of the peroxyl probe. The effect of lipid peroxyl radical motion and mobility on reactivity and consequently on the autoxidation process has been of great interest to us.I3 In this work we investigated a number of saturated and unsaturated lipid structures trapped in urea clathrate single crystals and powders as a model system that constrains the lipid motion in a well-defined manner. Results show that the structure and motion of both the carbon radicals and their corresponding peroxyl analogues can be successfully studied.
Experimental Section The lipids employed in this study were obtained from Sigma and used without further purification. In a typical preparation of a clathrate 5 g of urea is dissolved in a warmed mixed solvent Zimmerschied, W. J.; Dinerstein, R. A.; Weitkamp, A. W.; Marschner, R. F. Ind. E m . Chem. 1950.42. 1300. (2) Redlich: 0.; Gable, C.'M.;'Dunlop, A. K.; Millar, R. W. J . Am. Chem. SOC.1950, 72, 4153. (3) Smith, A. E.Acta Crystallogr. 1952, 5 , 224. (4) Chatani, Y.;Taki, Y.; Tadokoro. H. Acta Crvsralloar. - 1977, B33.309. ( 5 ) Griffith, 0. H. J . Chem. Phys. 1964, 41, 1093. (6) Faucitano, A.; Perotti, A,; Allara, G.; Martinott, F. F. J . Phys. Chem. 1972, 76, 801. (7) Hori, Y.;Shimada, S.; Kashiwabara, H. Polymer 1977, 18, 1143. (8) Casal, H.L.;Giller, D.; Hartstock, F. W.; Kolt, R.; Northcott, D. J.; Park, J. M.; Wayner, D. D. M. J . Phys. Chem. 1987, 91, 2235. (9) Faucitano, A,; h t e l l i , P.; Perotti, A.; Martinotti, F. F. J . Chem. SOC., Perkin Trans. 2 1971, 1786. (10) Olivier, D.; Marachi, C.; Che, M. J . Chem. Phys. 1979, 71, 4688. (11) Hori, Y.; Aoyama, S.;Kashiwabara, H. J . Chem. Phys. 1981, 75, 1582. (12) (a) Kevan, L.;Schlick, S.J . Phys. Chem. 1986,90, 1998. (b) Schlick, S.;Kevan, L. J . Am. Chem. SOC.1980, 102, 1998. (13) (a) Becker, D.;Yanez, J.; Sevilla, M. D.; Alonso-Amigo, M. G.; Schlick, S.J . Phys. Chem. 1987, 91,492. (b) Yanez, J.; Sevilla, C. L.; Becker, D.; Sevilla, M. D. J . Phys. Chem. 1987, 91, 487. (c) Sevilla, C. L.; Becker, D.; Sevilla, M. D. J . Phys. Chem. 1986, 90, 2963. (d) Sevilla, C. L.; Swarts, S.; Sevilla, M. D. J . Am. Oil Chem. SOC.1983, 60, 950. (1)
0022-3654/89/2093-2653$01.50/0
of isopropyl alcohol (15 mL) and methanol (50 mL).' Approximately 0.5 g of the fatty acid or fatty acid ester is added to the urea solution, and the mixture set aside to cool. One or two crops of crystals are collected and air-dried, and the crystals physically separated on the basis of external appearance; in most cases, long needlelike crystals were obtained. Samples of each type of crystal are then y-irradiated a t room temperature for doses of ca. 0.5-1 Mrad and checked for an ESR signal. Peroxyl radicals are formed by exposing a small amount of sample with a good carbon-radical ESR signal to 50 atm of O2in a bomb calorimeter for a few hours. Powders gave a higher peroxyl radical concentration than larger crystals. (Large amounts of sample should not be exposed to high pressures of O2due to an explosion hazard.) For experiments with samples are evacuated to ca.0.015 Torr and then "0-enriched 02, sealed under approximately 0.6 atm of enriched O2 (50% I7O). A Varian Century ESR spectrometer with an E-4531 dual cavity was employed. Carbon-radical spectra were recorded at 2 m W of microwave power. Peroxyl spectra were recorded at powers up to 100 mW to saturate residual carbon-radical signals. Hyperfine values and g values are measured vs Fremy's salt with A(N) = 13.09G and g = 2.0056.
Results Carbon-Centered Radicals. Saturated Fatty Acids. Roomtemperature irradiation of saturated fatty acid esters in urea clathrates results in formation of the a-carbon radical, as shown in I for methyl stearate.
8 II
CH3-O-C--(CH,),,CH3
-He
I1
.
CH~-O-C-CH-[CHZ)~~CH~
I
ESR spectra obtained after irradiation at room temperature of a large crystal (7-mm length, 3-mm width, and 1-mm height) of the methyl stearate urea clathrate are shown in Figure 1. The urea channels that contain the fatty acid radicals are parallel to the long axis of the crystal, and the fatty acid chains align themselves in the hexagonal channels as shown in Figure ZL3 The spectra in Figure 1 were recorded with the long axis of the crystal at angles 0 of O o , 45', and 90' to the magnetic field (Figure 2). Figure 3 shows the hyperfine couplings of I at room temperature as 0 is changed in 15' increments. Two large couplings (32 and 26 G) do not vary significantly with orientation and are attributed to the two @-protonsin the radical. The smallest coupling due to three equivalent protons shows considerable anisotropy, never exceeds 3 G, and is assigned to the ester methyl group. The remaining a-proton coupling varies from 3 1 G a t 0' to 15 G a t 90°. Since the magnitude of the principal elements of the hyperfine tensor for the a-proton are approximately IO, 20, and 30 G (Figure 4A), it is clear that a t 0' (A,,= 31 G), the magnetic field is aligned parallel to the molecule chain axis, whereas at 90' ( A , = 15 G) the magnetic field is perpendicular to the chain axis.
0 1989 American Chemical Society
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Sevilla et al.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 A
A
chain axis I
i chain axis
0"
Figure 1. ESR spectra of a y-irradiated methyl stearate urea clathrate single crystal at room temperature. The crystals grow as long columns. The spectra are recorded with the magnetic field at angles of (A) Oo, (B) 4 5 O , and (C) 90° to the long axis of the crystal.
