An arginine derivative with a fluorescent side-chain, Boc-Arg(Nap)-OH, was prepared by palladium(0)-catalyzed coupling of Boc-Arg-OH with a 4-bromonaphthalimide. The presence of the fluorophore lowers the pKa of the side-chain guanidinium group by several orders of magnitude, to 9.0 (± 0.1), allowing the derivative to access an electrically neutral protonation state that is not generally available to arginine itself. Computational modelling (DFT) predicts that protonation takes place at the side-chain C=N atom that bears the fluorophore. Calculated electronic absorptions for the protonated (356 nm) and neutral species (440 nm) are in good agreement with experiment. When irradiated with light, excited-state proton transfer (ESPT) occurs from cationic side-chains to suitably basic solvents, resulting in fluorescence emission from the neutral species. Arg(Nap) can be incorporated into peptides with sterically accessible N-termini using specially-adapted conditions of solid-phase peptide synthesis.

Side-chain protonation states of a fluorescent arginine

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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