Photoamplification and Multiple Tag Release in a Linear Peptide‐Based Array of Dithiane Adducts

Abstract

Externally sensitized photoinduced fragmentation in dithiane adducts of carbonyl compounds [1] requires the presence of an electron-transfer sensitizer, and therefore can be made contingent on a molecular recognition event. This event "arms" the binary photoactive system by bringing the adduct and the sensitizer into the immediate proximity of each other.[2] Such conditional photorelease of dithianes, which are readily detectable at subpicomolar levels, presents new opportunities for useful bioanalytical applications.We have recently developed this concept into a fundamentally novel methodology for direct screening of solutionphase combinatorial libraries, in which various 2-alkylsubstituted dithianes are used as encoding digits.[3] By immobilizing dithiane-masked benzophenone units on polymeric beads or dendrimers, we have further shown that one molecular recognition event can trigger the release of multiple copies of the encoding dithiane tags, which amounts to amplification on the surface.[4] Such photoamplification is possible because each externally sensitized fragmentation of the benzophenone-dithiane adducts unmasks more sensitizer which, in turn, unmasks its neighbors carrying the amplification chain and therefore boosting the sensitivity.The next logical question is whether this photoamplification methodology can be implemented in a linear array of masked sensitizers, and-if such amplification is possiblewhether the propagation of the effect and the release of dithiane tags occurs sequentially (that is, not unlike the onedimensional propagation in the Bickford fuse), randomly, or in some other unusual order. Herein, we report on photoamplification in linear polypeptide scaffolds.The synthesis of masked benzophenone sensitizers tethered to Fmoc-protected lysine is outlined in Scheme 1. We chose four 2-substituted dithianes, ethyl (a), propyl (b), pentyl (c), and octyl (d), to positionally encode the lysine residues in the polypeptide chain. The lithiated dithianes were reacted with 3-benzoylbenzoic acid sodium salt (1), thus furnishing adducts 2 a-d, which were converted into N-hydroxysuccinimide (NHS) esters 3 a-d and tethered to Fmoc-protected lysine. The modified lysines 4 a-d, in the form of their NHS esters 5 a-d, were utilized in solid-state peptide synthesis on TentaGel beads (0.48 mmol g À1 ). The peptides were capped with lysine-tethered benzophenone as the photoinitiator. As linear amplification in a single strand was the focus of our study, we chose the low-loading beads as a scaffold mimicking infinite dilution in solvent (that is, preventing interstrand sensitization; see Supporting Information).The following tetra-, hepta-, and decapeptides, which contain photoactive lysine residues and spacers, were synthesized:Lys BP -Lys DT1 -Lys DT2 -Lys DT3 -BEAD (6 a) Lys BP -Gly-Lys DT1 -Gly-Lys DT2 -Gly-Lys DT3 -BEAD (6 b) Lys BP -Met-Lys DT1 -Met-Lys DT2 -Met-Lys DT3 -BEAD (6 c) Lys BP -Gly-Gly-Lys DT1 -Gly-Gly-Lys DT2 -Gly-Gly-Lys DT3 -BEAD (6 d) where Lys DTX represents a lysine residue outfitted ...

A diradical based on odd-electron σ-bonds

“…1), respectively. The concept of the odd-electron σ-bond was first proposed by Pauling 8 , and species with these intriguing bonds have been recognized as important intermediates in chemistry and biochemistry [9][10][11][12][13][14][15][16][17][18][19][20][21] . A number of radicals with a one-electron [22][23][24][25] or three-electron σ-bond 12,[26][27][28][29][30][31][32] have been isolated and structurally studied (Fig.…”