An ESR Study of Irradiated L-Tryptophan·HCl Single Crystals at 295 K

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Abstract
Single crystals of L-tryptophan .cntdot. HCl were irradiated with 4.0 MeV electrons and studied by ESR techniques at 295.degree. K. At least 2 different radical species are stabilized at this temperature. One is formed by a net H addition to the aromatic part of the molecule. In this radical the unpaired electron interacts mainly with 2 .beta.- and 2 .alpha.-protons. The .beta.-protons are almost equivalent to an isotropic splitting constant of 39.5 G. The principal values of the .alpha.-proton tensors are .alpha.1: -18.3, -10.2 and -4.2 G; .alpha.2: -18.6, -12.2 and -4.3 G. Additional hyperfine splitting was observed at selected orientations and attributed to 3 nuclei of the aromatic ring. The g-value was 2.0027. The H is added to the C7 position in the indole ring. The resonance of the other radical was not studied in HCl crystals due to severe overlap between the spectra from the 2 radicals. In crystals grown from DCl, however, the H-adduct (or the D-adduct) was not formed. Consequently, deuterated crystals were used to study the 2nd resonance. Interactions with 3 protons, 1 .alpha.- and 2 nonexchangeable .beta.-protons, were observed. The principal values for the .alpha.-proton tensor are -32.4, -20.3 and -11.9 G. The isotropic splitting constants for the .beta.-protons are 31.7 and 2.8 G. The g-value is almost isotropic around 2.0031. The responsible radical is formed by a deamination process whereby the unpaired electron becomes localized to the .alpha.-carbon atom of the amino acid.

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