Formation and Reaction Kinetics of Biradicals and Triplet States in a Series of Carboxylated 1,4,5,8-Naphthalene Diimides.

The singlet-state deactivation products in a series of alkyl carboxylate substituted 1,4,5,8-naphthalene diimide (NDI) compounds were characterized using fluorescence and phosphorescence spectroscopies, as well as nanosecond laser flash photolysis. The reactive intermediates were quantified as a function of the number of methylenes in the alkyl linker. Rate constants for photoinduced electron transfer (PET) to the singlet excited state of the NDIs varied from 1.2 × 1010 to 4.9 × 1010 s-1. The yield of the long-lived amino ketyl radical ranged from 15% to 60% in compounds having 4 to 1 intervening methylenes between the NDI chromophore and the covalently attached carboxylic acid. A predominantly triplet state was observed upon direct excitation of the compound having the slowest PET. This T1 state of the NDIs was reductively quenched by DABCO electron donor. The amino ketyl radical was unreactive toward electron donors but was found to react with the hydrogen atom donor ß-mercaptoethanol. The compounds comprise a novel class of long-wavelength and strongly absorbing UV-activated chromophores that generate carbon-centered biradicals via direct 355 nm excitation in the absence of a cosensitizer.

Enzyme Modification and Oxidative Cross-linking Using Carboxylate-, Phenol- and Catechol-Conjugated 1,8-Naphthalimides.

The ground- and excited-state interactions of ß-alanine, tyrosine and l-dopa substituted 1,8 naphthalimides (NI-Ala, NI-Tyr and NI-Dopa) with lysozyme and mushroom tyrosinase were evaluated to understand the mechanism of oxidative modification. Photooxidative cross-linking of lysozyme was observed for all three conjugates. The yield was significantly reduced for NI-Tyr and NI-Dopa due to intramolecular electron transfer to the excited singlet state of the 1,8-naphthalimide. Incubation of NI-Tyr and NI-Dopa with mushroom tyrosinase resulted in an increased fluorescence from the naphthalimide, suggesting that the phenol and catechol portion of the conjugates are oxidized by the enzyme. This result demonstrates that the compounds bind in the active site of mushroom tyrosinase. The catalytic activity of mushroom tyrosinase to oxidize both tyrosine (monophenolase) and l-dopa (diphenolase) was modified by NI-Tyr and NI-Dopa. Monophenolase activity was inhibited, and the diphenolase activity was enhanced in the presence of these conjugates. Detailed Michaelis-Menten studies show that both Vmax and Km are modified, consistent with a mixed inhibition mechanism. Collectively, the results show that the compounds interact in the enzyme's active site, but also modify the distribution of the enzyme's oxidation states that are responsible for catalysis.

Defining the conditional basis of silicon phthalocyanine near-IR ligand exchange.

Bond cleavage reactions initiated by long-wavelength light are needed to extend the scope of the caged-uncaged paradigm into complex physiological settings. Axially unsymmetrical silicon phthalocyanines (SiPcs) undergo efficient release of phenol ligands in a reaction contingent on three factors - near-IR light (690 nm), hypoxia, and a thiol reductant. These studies detail efforts to define the mechanistic basis for this unique conditionally-dependent bond cleavage reaction. Spectroscopic studies provide evidence for the formation of a key phthalocyanine radical anion intermediate formed from the triplet state in a reductant-dependent manner. Computational chemistry studies indicate that phenol ligand solvolysis proceeds through a heptacoordinate silicon transition state and that this solvolytic process is favored following SiPc radical anion formation. These results provide insight regarding the central role that radical anion intermediates formed through photoinduced electron transfer with biological reductants can play in long-wavelength uncaging reactions.