Photochemistry of Quinone-Bridged Amino Acids. Intramolecular Trapping of an Excited Charge-Transfer State

Abstract

Linkages to amino acids that feature a chlorinated p-benzoquinone as a bridging group have been prepared by reaction of 2,5-dichloro-p-benzoquinone or chloranil with the free amine groups of alanine and proline. Electron-transfer capabilities of these peptide conjugates have been assessed through cyclic voltammetry and laser flash photolysis measurements. Phototransients are observed (λ max ) 410 nm) that cannot be assigned to either triplet or radical anion intermediates normally associated with quinones. These intermediates are associated with novel biradical zwitterions that result from intramolecular trapping of an aminoquinone excited charge-transfer (CT) state.

Design, Synthesis and Structure of Novel Para‐Quinones and their Antibacterial Activity

“…Formation of the di‐esters 6 and 7 is confirmed by NMR, IR, elemental analysis and by a crystal structure of 6 . Formation of di‐esters rather than the free acid has been previously observed for several amino acids (30,31). Addition of the primary amine to the p ‐chloranil produced intensely coloured solutions that were filtered after 4 h. In the case of amines I, II, IV, V and IX, highly coloured powders of products 1 , 2 , 4 , 5 and 9 precipitated immediately and were collected by suction filtration and recrystallized from acetone or hot acetonitrile yielding X‐ray quality crystals of 1 , 2 , 6 and 9 (Table 1).…”

“…Electronic absorption spectra of 1-9, Table 3, typi-cally have k max around 300-390 nm, which is characteristic for p-benzoquinones (24). Compounds 6 and 7 also have a band at 354 nm characteristic of the ester group (30,31). In addition to these, data compounds 1, 2, 3, 6 and 9 were structurally characterized by single crystal diffraction.…”

Mechanistic Studies of the Photoinduced Quinone Trimethyl Lock Decaging Process

“…For the present purposes, we sought to employ an ET mechanism, as this seemed better suited for longer wavelengths. Although most quinone photoreductions involve amines, − we chose a sulfide as the potential electron donor in our initial design. We anticipated a more facile synthesis of the desired systems, more favorable redox properties, and perhaps greater stability in air and in a biological system.…”

Trimethyl Lock: A Multifunctional Molecular Tool for Drug Delivery, Cellular Imaging, and Stimuli‐Responsive Materials

“…[200] Compound 144 can be synthesized on as olid-phase support and consists of an octa-arginine cell-penetrating peptides equence that mediates cellular uptake, an uclear localization signal (NLS) that allows uptake into cell nuclei, and photosensitive amino acid 143 coupled to as erine bearingt wo peptide fragments of an uclear export signal (NES) as well as af luorescent moietyt hat allows localization of the compounds to be imaged. [202,203] The fact that quinone moieties can be photochemically reduced to their corresponding dihydroquinones is long known, [204][205][206][207] and severale xamples for which photoreduction is mediated by an adjacent sulfide [208,209] or amine [210][211][212][213][214][215][216][217] functionhave been reported. [200] Photochemicalc leavage of the compound by subsequent irradiation with UV light leads to the formation of a NES through an O-N-acyl shifti nr eleased N-terminal peptide 145,a si llustrated in Scheme 33.…”

“…[200] Recently,D oughertya nd co-workers have presented ag eneral strategy for the direct photoinduced decay of quinonebased TML systems. [202,203] The fact that quinone moieties can be photochemically reduced to their corresponding dihydroquinones is long known, [204][205][206][207] and severale xamples for which photoreduction is mediated by an adjacent sulfide [208,209] or amine [210][211][212][213][214][215][216][217] functionhave been reported.…”