Side-Chain Protonation States of a Fluorescent Arginine.

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 p of the side-chain guanidinium group by several orders of magnitude, to 9.0 (±0.