ESR and ENDOR studies of radicals produced in hydroxyproline single crystals by x irradiation at low temperature

X irradiation of hydroxyproline CH2CHOHCH2CHNH+2COO− at ∼15 K produces a complex ESR dominated by a two-line absorption resulting from the carboxyl anion. ENDOR shows the interaction to be from a nonexchangeable proton similar to those observed in other amino acids, with coupling constants A1=67.9±0.3 MHz, A2=58.9±0.2 MHz, and A3=56.3±0.3 MHz. The results indicate also that the anion is unprotonated at the carboxyl group. When the crystal is warmed to 77 K, the ESR rapidly transforms to one dominated by a three-line absorption, and further warming of the crystal to ∼125 K leads to rapid transformation of the ESR to one dominated by an eight-line pattern. Both absorptions are found to be the result of a radical formed by rupture of the Cα–N bond, a reaction similar to deamination in other amino acids. The transformation at 125 K is evidently the result of an irreversible conformation change. All interactions are found to result from nonexchangeable protons. At 77 K, Aα1=−89.6±0.2 MHz; Aα2=−55.3±0.5 MHz; Aα3=−26.6±0.4 MHz; Aβ1=86.3±0.3 MHz; Aβ2=74.6±0.3 MHz; Aβ3=73.2±0.4 MHz. At 125 K the couplings are Aα1=−88.9±0.7 MHz; Aα2=−55.2±1.3 MHz; Aα3=−25.4±1.3 MHz; Aβ11=130.5±0.4 MHz; Aβ21=120.9±0.4 MHz; Aβ31=118.0±0.3 MHz; Aβ12=29.0±0.3 MHz; Aβ22=18.2±0.4 MHz; Aβ32=17.1±0.4 MHz.