Photoinduced Signal Amplification through Controlled Externally Sensitized Fragmentation in Masked Sensitizers

Abstract

Detection of molecular recognition events has always been an area of primary focus in bioanalytical sciences, with methods based on fluorescence being of preference due to their sensitivity. However, high throughput fluorescence binding assays require the tested ligands to be either spatially addressable or segregated on support beads for mechanical sorting. Solution phase libraries, either unsupported or immobilized on sub-micron carriers, present a challenge, as there were no direct methods to assay them. Our recent methodology for direct screening of solution phase libraries, encoded with photolabile tags, resolves this limitation. 1 It uses dithiane-based molecular systems, capable of photoinduced fragmentation, but only in the presence of external sensitizers, i.e. the fragmentation in such systems is made contingent on a molecular recognition event, which brings the sensitizer in the proximity of the photolabile unit (Scheme 1).The next logical question is whether a molecular recognition event can trigger not just one fragmentation, but rather set off a progression of photochemical events, releasing multiple copies of the encoding dithiane tags, and thus enhancing the sensitivity of detection. In this Communication we prove that such photoamplification can be achieved.Non-PCR amplification of molecular recognition has been implemented via enzymatic catalysis, 2a polymerization, 2b or massive liquid crystal reorientation 2c -all in a spatially addressable fashion. Amplification of electrophoretic tags via sensitized generation of singlet oxygen, with the effect localized due to limited diffusion of 1 O 2 , has also been proposed. 3 Our amplification strategy is fundamentally different. Addition of lithiated dithianes to diaryl ketones -potential sensitizers -disrupts conjugation between the two aromatic moieties, effectively masking the sensitizer. The photosensitized fragmentation in such adducts releases more diaryl ketone, capable of sensitization. In the experiment shown in Figure 1, 3 mol % of 4,4′-dimethoxybenzophenone was added to its dithiane adduct in acetonitrile and irradiated, while UV absorption of the solution was monitored at 360 nm. A typical autocatalytic curve was obtained, indicating that photorelease of the masked ketone accelerated this bimolecular fragmentation. While this process resembles a chain reaction, with the released sensitizer carrying the chain, it is controlled better than radical polymerizations, as the photoamplification chain can be stopped and/or re-initiated at any time by removing or applying the UV source. The amplification uses a source of UV photons to amplify the amount of sensitizer, which often needs to be replenished in photoinduced ET reactions to compensate for irreversible reduction. Another outcome of the amplification is the mass release of dithianes, triggered with a very small amount of the initiator.akutatel@du.edu. Supporting Information AvailableExperimental procedures and spectra. This material is available free of charge via the Internet at http://p...

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