The intramolecular sulfur-nitrogen bond in aqueous 3-(methylthio)propylamine radical cation

Kinetic and structural properties of the intramolecular sulfur-nitrogen bon d (>S therefore NH2+.-) in aqueous 3-(methylthio)propylamine (3-MTPA) radic al cation have been examined as a model for the cyclized radical state of L -methionine, an essential aminoacid in protein biosynthesis. The (OH)-O-. r adical 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 hydrog en abstraction and water elimination from the (OH)-O-. adduct (3-MTPAH(+)-O H. --> 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 w ith its broad 390-nm absorption on the microsecond times, exhibits a surpri singly simple spectrum dominated by a strongly enhanced S thereforeN stretc hing 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 com pletely confined to the S thereforeN bond. The equilibrium S-N separation i n the radical is estimated as similar to2.5 Angstrom. The hypothetical inte rmediate, SNOH., presumed to be formed on oxidation of methionine and deriv atives in basic solutions, has been identified with the > S therefore NH.- species.