The cation radicals formed when methionine and N-acetylmethionine quench the photo-excited triplet state of 4-carboxybenzophenone in aqueous solution are characterized by their g-factors and hyperfine coupling constants. It is shown that the radicals exist as either cyclic or linear structures depending on the amino acid and the pH of the medium. The study is based on the analysis of the dynamic nuclear polarization generated in the spin-selective decay of the transient radicals and its dependence on the external magnetic field. The requirements and scope of this method are discussed.