Synthesis and Fluorescent Properties of Alkynyl- and Alkenyl-Fused Benzotriazole-Derived α-Amino Acids

Fluorescent carbazole-derived α-amino acids: structural mimics of tryptophan

Fluorescent tags are commonly used for imaging of proteins and peptides during biological events; however, the large size of dyes can disrupt protein structure and function, and typically require the use of a chemical spacer. Herein, we report the synthesis of a new class of fluorescent unnatural α-amino acid, containing carbazole side-chains designed to mimic l-tryptophan and thus, readily incorporated into peptides. The amino acids were constructed using a Negishi cross-coupling reaction as the key step and exhibited strong fluorescent emission, with high quantum yields in both organic solvents and water. Compatible with solid phase peptide synthesis, the carbazole amino acids were used to replace tryptophan in a β-hairpin model peptide and shown to be a close structural mimic with retention of conformation. They were also found to be effective fluorescent molecular reporters for biological events. Incorporation into a proline-rich ligand of the WW domain protein demonstrated that the fluorescent properties of a carbazole amino acid could be used to measure the protein-protein binding interaction of this important biological signalling process.

Synthesis of Thiazoloindole α-Amino Acids: Chromophores Amenable to One- and Two-Photon Induced Fluorescence

Thiazoloindole α-amino acids have been synthesized in four steps from tryptophan using a dual-catalytic thiolation reaction and a copper-mediated intramolecular N-arylation process. Late-stage diversification of the thiazoloindole core with electron-deficient aryl substituents produced chromophores that on one-photon excitation displayed blue-green emission, mega-Stokes shifts, and high quantum yields. The thiazoloindole amino acids could also be excited via two-photon absorption in the near-infrared, demonstrating their potential for biomedical imaging applications.

Synthesis and Photophysical Properties of Charge-Transfer-Based Pyrimidine-Derived α-Amino Acids

The four-step synthesis of fluorescent pyrimidine-derived α-amino acids from an l-aspartic acid derivative is described. The key synthetic steps involved preparation of ynone intermediates via the reaction of alkynyl lithium salts with a Weinreb amide, followed by an ytterbium-catalyzed heterocyclization reaction with amidines. Variation of substituents at the C2- and C4-position of the pyrimidine ring allowed tuning of the photoluminescent properties of the α-amino acids. This revealed that a combination of highly conjugated or electron-rich aryl substituents with the π-deficient pyrimidine motif resulted in fluorophores with the highest quantum yields and overall brightness. Further analysis of the most fluorogenic α-amino acid demonstrated solvatochromism and sensitivity to pH.

Fluorescent α-amino acids via Heck-Matsuda reactions of phenylalanine-derived arenediazonium salts

The Heck-Matsuda coupling reaction of arenediazonium salts derived from L-phenylalanine with various alkenes has been developed. A two-step process involving the preparation of a tetrafluoroborate diazonium salt from a 4-aminophenylalanine derivative, followed by a palladium(0)-catalysed Heck-Matsuda coupling reaction allowed access to a range of unnatural α-amino acids with cinnamate, vinylsulfone and stilbene side-chains. Analysis of the photophysical properties of these unnatural α-amino acids demonstrated that the (E)-stilbene analogues exhibited fluorescent properties with red-shifted absorption and emission spectra and larger quantum yields than L-phenylalanine.

Rotamer-Controlled Dual Emissive α-Amino Acids

The synthesis and photoluminescent properties of novel α-amino acids are described in which the biaryl benzotriazinone-containing chromophores were found to display dual emission fluorescence via locally excited (LE) and twisted intramolecular charge transfer (TICT) states. The intensity of each emission band could be controlled by the electronics and position of the substituents, and this led to the design of a 2-methoxyphenyl analogue that, due to twisting, displayed bright TICT fluorescence, solvatochromism, and pH sensitivity.