Kinetic and structural properties of the intramolecular sulfur-nitrogen bond (>S therefore NH2+.-) in aqueous 3-(methylthio)propylamine (3-MTPA) radical cation have been examined as a model for the cyclized radical state of L-methionine, an essential aminoacid in protein biosynthesis. The (OH)-O-. radical reacts with amine-protonated 3-MTPA at a rate constant of 8.3 x 10(9) M-1 s(1), primarily by addition at the sulfur site. Intramolecular hydrogen abstraction and water elimination from the (OH)-O-. adduct (3-MTPAH(+)-OH. --> 3-MTPA(+.) + H2O) occurs at a rate of 9.4 x 10(7) s(-1), forming the radical cation. The Raman scattering of 3-MTPA(+.), excited in resonance with its broad 390-nm absorption on the microsecond times, exhibits a surprisingly simple spectrum dominated by a strongly enhanced S thereforeN stretching vibration at 288 cm(-1), observed for the first time. This observation provides vibrational spectroscopic evidence of intramolecular bonding in 3-MTPA(+.) and shows that the 390-nm absorption of the radical is almost completely confined to the S thereforeN bond. The equilibrium S-N separation in the radical is estimated as similar to2.5 Angstrom. The hypothetical intermediate, SNOH., presumed to be formed on oxidation of methionine and derivatives in basic solutions, has been identified with the > S therefore NH.- species.