Mechanistic analysis of the photolytic decomposition of solid-state S-nitroso-N-acetylpenicillamine

Abstract

S-Nitroso-N-acetylpenicillamine (SNAP) is among the most common nitric oxide (NO)-donor molecules and its solid-state photolytic decomposition has potential for inhaled nitric oxide (iNO) therapy. The photochemical NO release kinetics and mechanism were investigated by exposing solid-state SNAP to a narrow-band LED as a function of nominal wavelength and intensity of incident light. The photolytic efficiency, decomposition products, and the photolytic pathways of the SNAP were examined. The maximum light penetration depth through the solid layer of SNAP was determined by an optical microscope and found to be within 100-200 μm, depending on the wavelength of light. The photolysis of solid-state SNAP to generate NO along with the stable thiyl (RS·) radical was confirmed using Electron Spin Resonance (ESR) spectroscopy. The fate of the RS· radical in the solid phase was studied both in the presence and absence of O₂ using NMR, IR, ESR, and UPLC-MS. The changes in the morphology of SNAP due to its photolysis were examined using PXRD and SEM. The stable thiyl radical formed from the photolysis of solid SNAP was found to be reactive with another adjacent thiyl radical to form a disulfide (RSSR) or with oxygen to form various sulfonyl and sulfonyl peroxyl radicals {RS(O)_xO·, x = 0 to 7}. However, the thiyl radical did not recombine with NO to reform the SNAP. From the PXRD data, it was found that the SNAP loses its crystallinity by generating the NO after photolysis. The initial release of NO during photolysis was increased with increased intensity of light, whereas the maximum light penetration depth was unaffected by light intensity. The knowledge gained about the photochemical reactions of SNAP may provide important insight in designing portable photoinduced NO-releasing devices for iNO therapy.

Keywords: Kinetics; Nitric oxide; Nitrosothiols; Photolysis; S-Nitroso-N-acetylpenicillamine; Thiyl radical.

Figure 1:

Possible degradation and recombination pathways of solid state RSNO in absence and presence of O₂.

Figure 2.

Schematics of experimental set-up for the photolysis experiment with the RSNO tablet. (A) Photograph of SNAP tablet before photolysis (left) and after photolysis (right). (B) Cross-section of the photolysis cell. (C) Block diagram of the experimental setup.

Figure 3:

Photolytic degradation kinetics of SNAP in the solid state at various wavelengths and intensities (4mW/cm² and 40 mW/cm²)- (A) 340 nm (B) 385 nm, (C) 470 nm, (D) 565 nm, (E) 660 nm, (F) 940 nm.

Figure 4.

Light penetration depth in SNAP tablet in air- (A) 340 nm, 40 mW/cm²; (B) 340 nm, 80 mW/cm²; (C) 385 nm, 40 mW/cm²; (D) 385 nm, 80 mW/cm²; (E) 470 nm, 40 mW/cm²; and (F) 470 nm, 80 mW/cm².

Figure 5.

Characterization of the photolyzed products of SNAP- (A) IR, (B) UPLC-MS, (C) ESR, and (D) NMR.

Figure 6.

Morphological changes in SNAP tablet due to degradation–(A) PXRD data of NAP, SNAP, Photolyzed SNAP, and DNAP. (B) SEM image of the (I) SNAP tablet, (II) Photolyzed SNAP tablet, (III) SNAP powder, (IV) Photolyzed SNAP powder.

Figure 7.

Recombination mechanism of NO with the decomposed SNAP. (A) Schematics of SNAP-packed cartridge. (B) Bar diagram of remaining SNAP after NO recombines (number of experiments, n=3, p-value = 0.15).