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. 2024 Mar 21;15(16):5944-5949.
doi: 10.1039/d4sc01173b. eCollection 2024 Apr 24.

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

Rebecca Clarke  1 , Liyao Zeng  1 , Bethany C Atkinson  1 , Malcolm Kadodwala  1 , Andrew R Thomson  1 , Andrew Sutherland  1
Affiliations

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

Rebecca Clarke et al. Chem Sci. .

Abstract

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.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. (a) l-Tryptophan (1) and selected fluorescent unnatural analogues. (b) Design of carbazole-derived α-amino acids for incorporation into peptides.
Scheme 1
Scheme 1. Chemical synthesis of carbazole-derived α-amino acids.
Fig. 2
Fig. 2. CD spectra for TrpZip1 and modified peptide 14.
Fig. 3
Fig. 3. (a) Absorption spectra of amino acids 13a–13e (5 μM) in methanol. (b) Normalised emission spectra of amino acids 13a–13e (5 μM) in methanol. (c) Normalised emission spectra of 13d (5 μM) in various solvents. (d) Emission spectra of 13d (5 μM) at various pH.
Fig. 4
Fig. 4. Proposed strategy for the use of amino acid 13d as a fluorescent reporter group for the binding of a WW domain with its peptide ligand.
Fig. 5
Fig. 5. Fluorescence titration data of binding of modified peptide ligand 16 with WW domain 15.
See this image and copyright information in PMC

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