Photodissociation Dynamics of Nitric Oxide from N-Acetylcysteine- or N-Acetylpenicillamine-bound Roussin's Red Ester.

Abstract

The photochemical release of nitric oxide (NO) from a NO precursor is advantageous in terms of spatial, temporal, and dosage control of NO delivery to target sites. To realize full control of the quantitative NO administration from photoactivated NO precursors, it is necessary to have detailed dynamical information on the photodissociation of NO from NO precursors. We synthesized two new water-soluble Roussin’s red esters (RREs), [Fe2(μ-N-acetylcysteine)2(NO)4] and [Fe2(μ-N-acetylpenicillamine)2(NO)4], which have five times longer lifetime than the well-known [Fe2(μ-cysteine)2(NO)4]. The photodissociation dynamics of NO from these RREs in water were investigated over a broad time range from 0.3 ps to 10 μs after excitation at 310 and 400 nm using femtosecond time-resolved infrared (IR) spectroscopy. When these RREs are excited, they either release one NO, producing a radical species deficient in one NO (R), [Fe2(μ-RS)2(NO)3], or relax into the ground state without photodeligation via an electronically excited intermediate state (M). R appears immediately after photoexcitation, suggesting that one NO is photodissociated faster than 0.3 ps. A certain fraction of R undergoes geminate recombination (GR) with NO with a time constant of 7–9 ps, while the remaining R competitively binds to the solvent. Solvent-bound R eventually bimolecularly recombines with NO with a rate constant of (1.3–1.6) × 108 M–1 s–1. For a given RRE molecule, the fractional yield of M (0.62–0.76) depends on the excitation wavelength (λex); however, the relaxation time of M (6 ± 1 ns) is independent of λex. Although the primary quantum yield of NO photodissociation (Φ1) was found to be 0.24–0.38, the final yield of NO suitable for other reactions (Φ2) was reduced to 0.14–0.29 due to the picosecond GR of the dissociated NO with R. Detailed photoexcitation dynamics of RRE can be utilized in the quantitative control of NO administration at a specific site and time, promoting pin-point usage of NO in chemistry and biology. We demonstrate that femtosecond IR spectroscopy combined with quantum chemical calculations is a powerful method for obtaining detailed dynamic information on photoactivated NO precursors such as Φ1 and Φ2, the GR yield, and secondary reactions of the nascent photoproducts, which are essential information for the design of efficient photoactivated NO precursors and their quantitative utilization.