A New Colorimetric Test for Solid-Phase Amines and Thiols

Colorimetric and Fluorescent Dual-Signal Chemosensor for Lysine and Arginine and Its Application to Detect Amines in Solid-Phase Peptide Synthesis

Lysine (Lys) and arginine (Arg), as two of the most alkaline amino acids among 20 common amino acids, are closely involved in many vital biological processes and biomaterial synthesis. Abnormal levels of Lys and Arg can lead to various diseases. Although a limited number of fluorescent probes for Lys and Arg have been reported, many of them are not sensitive enough due to the moderate fluorescence signal and on-off mode. In addition, none of them were applied for detecting amine groups in solid-phase peptide synthesis. In this study, we designed and synthesized optical fluorescent probe 1 based on the benzoxadiazole fluorophore, which could undergo an accelerated hydrolysis reaction under basic conditions. Probe 1 revealed excellent selectivity toward alkaline Lys and Arg over other common amino acids with both fluorometric and colorimetric readouts. After treatment with Lys and Arg, probe 1 could emit a turn-on fluorescent response at 580 nm with a distinct color change from pink to yellow. The limit of detection for Lys and Arg was calculated to be 1.1 and 1.39 μM, respectively. We also successfully applied probe 1 for the visualization of Arg in living cells. Moreover, to the best of our knowledge, probe 1 provided the first fluorescent platform to detect -NH₂ groups in solid-phase synthesis of peptides with distinct fluorescent and colorimetric changes. We envision that the probe can provide an alternative method for the traditional Kaiser test.

Solid Phase Synthesis of Helically Folded Aromatic Oligoamides

Aromatic amide foldamers constitute a growing class of oligomers that adopt remarkably stable folded conformations. The folded structures possess largely predictable shapes and open the way toward the design of synthetic mimics of proteins. Important examples of aromatic amide foldamers include oligomers of 7- or 8-amino-2-quinoline carboxylic acid that have been shown to exist predominantly as well-defined helices, including when they are combined with α-amino acids to which they may impose their folding behavior. To rapidly iterate their synthesis, solid phase synthesis (SPS) protocols have been developed and optimized for overcoming synthetic difficulties inherent to these backbones such as low nucleophilicity of amine groups on electron poor aromatic rings and a strong propensity of even short sequences to fold on the solid phase during synthesis. For example, acid chloride activation and the use of microwaves are required to bring coupling at aromatic amines to completion. Here, we report detailed SPS protocols for the rapid production of: (1) oligomers of 8-amino-2-quinolinecarboxylic acid; (2) oligomers containing 7-amino-8-fluoro-2-quinolinecarboxylic acid; and (3) heteromeric oligomers of 8-amino-2-quinolinecarboxylic acid and α-amino acids. SPS brings the advantage to quickly produce sequences having varied main chain or side chain components without having to purify multiple intermediates as in solution phase synthesis. With these protocols, an octamer could easily be synthesized and purified within one to two weeks from Fmoc protected amino acid monomer precursors.