ENDOR analysis of H-addition radicals and their conversion in irradiated crystals of tryptamine hydrochloride

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Abstract
In a previous study it was shown that the dominant radiation-induced room-temperature radical in single crystals of tryptamine⋅HC1 is formed by a net H addition to C7 of the indole ring. In the present work, it is demonstrated that this species is the precursor for two other indole H-addition radicals. By storage at room temperature, the C7 adduct slowly converts into the C4 adduct. The formation of the C4 adduct could easily be followed by ENDOR. The ENDOR spectra are characterized by hyperfine coupling to six protons bonded to the indole moiety. Upon annealing the crystal above 70 °C, the C7 adduct is more rapidly converted into the C4 adduct, but then the annealing also induces a simultaneous conversion into the C2 adduct. This species also is characterized by six proton hyperfine interactions in the strong-coupling region. The C7 add→C2 add process could be reversed by exposure to UV light at room temperature, whereas the C7 add→C4 add process is irreversible upon this procedure. Semiempirical molecular orbital calculations support the interpretations made. The mechanisms of the different processes is discussed in light of the crystal and molecular structure of the compound, and related to similar processes in other compounds, e.g., the purine and pyrimidine bases.