Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4'-bipyridyl-3,3'-disulfide (1) and the di-N-methylated derivative (2(2+)) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S-S bond to form the dithiolate. In contrast, compound 2(2+) is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S-S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S-S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

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A new efficient photocaging system with a fluorescence reporting function has been developed. The photolabile latch is based on adducts of C-nucleophiles with aromatic ketones, such as thioxanthones and xanthones. The system is designed to quantify the release of biological effectors and to monitor their spatial distribution and localization by single- and two-photon fluorescence microscopy. In the armed state the ketone's conjugation is disrupted by nucleophilic addition, resulting in a blue shift of the absorption maxima and a dramatic decrease in fluorescence intensity. The mechanism of the photoinduced uncaging involves homolytic C-C bond fragmentation followed by radical disproportionation, regenerating the carbonyl moiety and restoring fluorescence. The uncaging can be initiated via either a one- or two-photon process, offering a new powerful tool for molecular life sciences. The synthesis and uncaging of dendrimer- and polymeric bead-based model systems are described.

dithiane tag ͉ photoinduced externally sensitized fragmentation ͉ solution library encoding and screening H igh-throughput combinatorial chemistry is critical for modern drug discovery, with the most prolific adaptation being the split-and-mix synthesis of libraries immobilized on polymeric beads (1, 2). These one-bead-one-compound libraries are assayed for binding of biological targets through fluorescenceguided mechanical segregation of the winning beads. The required mechanical manipulations impose a lower limit on the size of the particle used as a solid support. It is further recognized that solution phase combinatorial chemistry holds even greater promise, as it is compatible with both divergent and convergent multistep synthetic schemes, and not constrained to linear synthesis (3). However, its immense potential has not yet been fully realized, partly due to the complexity of assaying solution mixtures for binding. Although various iterative deconvolution methods (4) have been developed to synthesize and screen soluble sublibraries, all of them require redundant synthetic steps and are time consuming and expensive.We have developed a methodology for direct screening of solution phase libraries encoded with photolabile tags that does not depend on mechanical manipulation or iterative deconvolution. Results and DiscussionOur tagging methodology is based on dithiane adducts of carbonyl compounds, which are capable of efficient photoinduced fragmentation, but only in the presence of an external electron transfer (ET) sensitizer. At the core of this approach is the concept we term photolabile scaffolds for molecular recognition: binary molecular systems in which the sensitizer and the dithiane-based photocleavable fragments are each tethered to the respective components of a host-guest molecular recognition pair (5-7). In these pairs, photoinduced fragmentation is contingent on a molecular recognition event, which is necessary to arm the system, making it photolabile. This general concept has now been developed into a methodology for screening of encoded libraries, both unsupported or immobilized on nanosized carriers too small for mechanical handling. Scheme 1 gives a general outline for screening the micelle-solubilized library of ligands L, encoded with the tethered tags T. The receptor R is outfitted with an ET-sensitizer S, and incubated with the library (A). The host-guest binding brings the sensitizer into the vicinity of the adduct (B), which, upon irradiation (C), triggers the release of dithiane tags (D).Our strategy is to encode individual molecules one tag at a time. The kth library member L k can be encoded with a set of tags {T} k, for example, T k1 , T k2 , and T k3 , in the following fashion: one fraction of L k molecules is encoded with T k1 , another fraction is encoded with T k2 , and yet another is encoded with T k3 , such that L k is present in the solution as three subpopulations: L k -tether-T k1 , L k -tether-T k2, and L k -tether-T k3 . In this case, irradiation yields the desired resu...

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...