Alared: environmentally sensitive fluorescent red amino acid for biomolecular interaction studies and live-cell imaging of bioactive peptides

20 December 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


To fill the need for environmentally sensitive fluorescent unnatural amino acids able to operate in the red region of the spectrum, we designed and synthesized Alared, a red solvatochromic and fluorogenic amino acid derived from the Nile Red chromophore. The new unnatural amino acid can be easily integrated into bioactive peptides by means of classical solid-phase peptide synthesis. The fluorescence quantum yield and the emission maximum of Alared-labeled peptides vary in a broad range depending on the peptide’s environment, making Alared a powerful reporter of biomolecular interactions. Due to its red-shifted absorption and emission spectra, Alared-labeled peptides could be monitored in living cells with minimal interference from cellular autofluorescence. Using ratiometric fluorescence microscopy, we were able to track the fate of the Alared-labeled peptide agonists of the apelin G protein-coupled receptor upon receptor activation and internalization. Due to its color-shifting environmentally sensitive emission, Alared allowed for distinguishing the fractions of peptides that are specifically bound to the receptor or unspecifically bound to different cellular membranes.


fluorescent amino acids
environment-sensitive fluorescence
cell imaging
G-protein coupled receptors
fluorescent probes

Supplementary materials

Supporting Information
Protocols and analytical data for the synthesis of Alared, bioavive peptides derived from apelin17, for the determination of critical micelle concentration of fluorescent peptides, NMR spectra and High Resolution Mass Spectrometry (HRMS) analyses


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