Single crystals of glycylglycine grown from aqueous and heavy-water solutions have been irradiated at 77°K with <tex-math>${}^{60}{\rm Co}\ \gamma \text{-rays}$</tex-math> in the dark, and the electron paramagnetic resonance of free radicals produced have been studied under various conditions. The radical initially produced shows complicated hyperfine patterns depending on the direction of the applied magnetic field. Analysis of angular dependence of major hyperfine splittings gives the principal values, <tex-math>$A_{x}=17$</tex-math>, <tex-math>$A_{y}=27$</tex-math>, and <tex-math>$A_{z}=9$</tex-math> gauss, for the coupling tensors of two α-protons. Comparison of the observed hyperfine coupling constant with those of similar radicals and those from the spin-density calculations suggests <tex-math>${\rm NH}_{3}{}^{+}-{\rm CH}_{2}-{\rm CO}-{\rm NH}-\dot{{\rm C}}{\rm H}_{2}$</tex-math> as the most probable structure. Upon warming, this is converted to a stable radical, <tex-math>${\rm NH}_{3}{}^{+}-{\rm CH}_{2}-{\rm CO}-{\rm NH}-\dot{{\rm C}}{\rm H}-{\rm COO}^{-}$</tex-math>, which is also obtained on visible light illumination at 77°K through the first order reaction with respect to the initial radical.

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