Electron spin .resonance spectroscopy has been used to identify
free radicals formed in powdered amino acids by the exposure to
thermal hydrogen atoms at 77°K. The low temperature ESR spectra
of the radicals produced at 77°K are different from the spectra at
room temperature. The primary radicals are those produced by the
abstraction of H atoms from the common part of amino acids (in
the zwitter-ion compounds) or from some position of the amino
acid residue (in neutral compounds). In most amino acids a reversible
change of the radical conformation was observed. The irreversible
change of the isoleucine resonance pattern is attributed to
the change of the unpaired electron from the sp3 to the p orbital.
Different conformations of radicals at room temperature when they
are produced at 77° and at 300°K are interpreted by the difference
in the crystal lattice arrangement in the vicinity of the radical