Dynamics of\nNO Release and Linkage Isomer Formation\nfrom S‑Nitroso-Mercaptoethanol in Aqueous Solutions: Insights\nfrom Femtosecond Infrared Spectroscopy

Understanding the photodynamics of\nS-nitroso-thiol (RSNO), an effective\nNO transporter in biological systems, is essential for its photochemical\napplications. S-nitroso-mercaptoethanol (MceSNO), a simple water-soluble\nRSNO, facilitates high-level quantum calculations. We investigated\nthe photoexcitation dynamics of MceSNO in an aqueous solution, focusing\non NO dissociation, recombination, and linkage isomerization using\nquantum calculations and femtosecond infrared spectroscopy. Upon excitation\nat 320 nm, MceSNO rapidly dissociates into NO and MceS radicals. Approximately\n31 ± 3% of MceS reacts with unexcited MceSNO molecules, forming\nMceSSMce and releasing additional NO. The remaining MceS undergoes\ngeminate recombination with NO, forming either MceSNO (41 ± 4%)\nor MceSON (28 ± 3%), the latter being a sulfur-ON linkage isomer\nobserved for the first time in a room-temperature solution. MceSON\nisomerizes back to MceSNO in 470 ± 30 ps. The formation mechanism\nof MceSON was verified through a potential energy surface constructed\nat the CASPT2D­(16,11)/cc-pVTZ level. The isomerization barrier was\ndetermined to be 3.3 ± 1.2 kcal/mol in water.