Photoinduced Electron Transfer Involving Eosin-Tryptophan Conjugates. Long-Lived Radical Pair States for Systems Incorporating Aromatic Amino Acid Side Chains

Abstract

The electron-transfer photochemistry of the covalent derivatives of the dye eosin, in which the xanthene dye is covalently attached to the amino acid L-tryptophan via the thiohydantoin derivative, the tryptophan dipeptide, and an ethyl ester derivative, has been investigated. The singlet excited state of the dye is significantly quenched on attachment of the aromatic amino acid residue. Dye triplet states are also intercepted through intramolecular interaction of excited dye and amino acid pendants. Flash photolysis experiments verify that this interaction involves electron transfer from the indole side chains of tryptophan. Rate constants for electron transfer are discussed in terms of the distance relationships for the eosin chromophore and aromatic redox sites on peptide derivatives, the pathway for u--'IT through-bond interaction between redox sites, and the multiplicity and state of protonation for electron-transfer intermediates. Selected electron-transfer photoreactions were studied under conditions of binding of the peptide derivatives in a high molecular weight, water-soluble, globular polymer, poly(viny1-2-pyrrolidinone).

Reversible Photobleaching of Fluorescein Conjugates in Air‐Saturated Viscous Solutions: Singlet and Triplet State Quenching by Tryptophan

“…tested to confirm involvement of the triplet state and to attempt to modify the kinetics and/or efficiency of reversible fluorescein photobleaching. Laser flash photolysis studies have shown that the triplet state of eosin (a fluorescein-like chromophore with high quantum yield for triplet formation) undergoes efficient electron transfer redox reaction with free tryptophan (8) and in eosin-tryptophan conjugates (9). The possibility of a similar interaction of triplet fluorescein with tryptophan was tested.…”

“…Such mechanisms yielding recovery times of microseconds and greater are of interest for studies on biological systems. Photochemical studies of organic dye molecules (14), including fluorescein and fluorescein-like molecules (8,9,15), suggest several general mechanisms of ground-state regeneration, the most important of which is triplet decay (3,16,17).…”

“…High quenching rates were also observed using lysozyme, which contains six tryptophans per protein (8,23). Laser flash photolysis experiments of several tryptophan-eosin conjugates have also revealed an electron transfer mechanism for efficient fluorescence quenching by tryptophan when chromophore-tryptophan distance is less than [10][11][12][13][14][15] A (9). The majority of nanosecond laser flash photolysis experiments have been done using eosin because of its high triplet quantum yield (0.33 (21)).…”

Improving gelation efficiency and cytocompatibility of visible light polymerized thiol-norbornene hydrogels via addition of soluble tyrosine

“…could increase the half-life of eosin-Y radicals [49,50]. Therefore, we hypothesized that the addition of soluble tyrosine might enhance the crosslinking efficiency of visible light-based thiolnorbornene hydrogels through improving eosin-Y mediated cysteine deprotonation.…”

“…However, gelation efficiency was influenced significantly by the sequence of the biscysteine peptides. To improve the crosslinking efficiency of visible light initiated thiol-norbornene hydrogels using bis-cysteine peptide linkers, we explored the addition of soluble amino acids capable of altering the photo-excitability of eosin-Y [49,50]. It was discovered that the addition of soluble tyrosine could increase the degree of cysteine deprotonation, which led to an improvement in thiol-norbornene gelation efficiency.…”