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Hyperfine Structures of Doxyl-Labeled n-Alkyl Chains by NMR and EPR
The proton hyperfine coupling constants in a series of n-alkyl chains with a doxyl group attached at various points from the end of the chain have been measured by NMR and EPR spectroscopies. The hyperfine structure shows no further change when the attachment point is four or more carbon?carbon bonds from the end of the chain. The high resolution afforded by 500 MHz NMR reveals small magnetic inequivalencies in the chain methylene hyperfine coupling constants located at the same distance from the attachment point. Protons in the same chain methylene group are shown to have different hyperfine coupling constants while protons on different chain methylene groups, symmetrically placed with respect to attachment point, are the same, in every case except one. EPR spectra simulated from hyperfine coupling constants derived from NMR are in excellent agreement with experiment. Inhomogeneous EPR line broadening is found to be in excellent agreement with a previously derived universal hyperfine pattern (B. L. Bales, in "Biological Magnetic Resonance" (L. J. Berliner and J. Reuben, Eds.), Vol. 8, p. 77, Plenum, New York, 1989), so no new correction procedures are necessary. Strategies for selectively deuterating n-alkyl spin probes are developed and compared with some results taken from the literature. Deuterating the chain methylene groups two to three carbon?carbon bonds from the attachment point is necessary for the maximum gain in resolution and sensitivity.