Comparative Studies of Different Quinolinone Derivatives as Donors in Fluorescence‐Resonance‐Energy Transfer (FRET) – Systems in Combination with a (Bathophenanthroline)ruthenium(II) Complex as Acceptor

Abstract

We describe the preparation as well as a detailed photophysical study of Fmoc-amino acid building blocks carrying different carbostyril (¼ quinolin-2(1H)-one) heterocycles as donors in a FRET (fluorescence-resonance-energy transfer) system in combination with a [Ru II (bathophenanthroline)] complex (bathophenanthroline ¼ 4,7-diphenyl-1,10-phenanthroline). The efforts resulted in a clear preference for building block 16 due to its ease of synthesis (Scheme 2), its chemical robustness, and the FRET efficiency when incorporated into peptides.Introduction. -Luminescent chromophores as entities of fluorescence-resonanceenergy transfer (FRET) systems are important tools to study supramolecular interactions with a special emphasis in the realm of biomolecules like DNA, RNA, and proteins [1]. FRET Systems allow to monitor distance-dependent interactions on the molecular level, and in a real-time mode. Therefore, they are especially suited for the characterization of biochemical events both in vitro and in vivo. Meanwhile, a myriad of applications has been reported. It involves binding of ligands to their pertinent protein receptors [2], DNA -protein complexation [3], and RNA-folding and catalysis [4]. Other applications are enzyme assays based, e.g., on the Fçrster resonance-energy-transfer principle [5] and monitoring of polymerase chain reactions (PCR) [6]. The FRET technology is based on the nonemissive transfer of energy between a donor (D) and an acceptor (A) fluorophore. It decreases with r À6 , r being the distance between the donor and the acceptor [7]. A prerequisite for an efficient transfer is an intensive overlap between the emission of the donor with the absorption of the acceptor as well as the correct orientation of their dipole transition moments relative to each other. A plethora of different donor -acceptor pairs have been reported up to date, but despite the multitude of available systems, sensitivityespecially in the presence of background luminescence from matrix constituents -still remains an issue. A further concern is robustness of the applied dyes as well as the possibility to employ them in a modular way as broadly as possible via stable covalent bonds and without interference of the spectral properties of the labelled molecules.

6‐Methoxy‐2‐oxo‐1,2‐dihydroquinoline‐3,4‐dicarbonitriles, A Red Compound Class with Solvent and pH Independent Green Fluorescence Maxima

“…shown in Ref. ) suitable to outclass similar and wide‐used coumarin derivatives . In a recent project, we reported about the interesting fluorescence properties of 3‐aryl‐4‐cyano‐6‐methoxycarbostyrils with excitation wavelengths above 400 nm and emission wavelengths above 500 nm, with quantum yields of about Φ = 0.23.…”

3‐aryl‐6‐methoxy‐2‐oxo‐1,2‐dihydroquinoline‐4‐carbonitriles as Solvent and pH Independent Green Fluorescent Dyes

“…Furthermore, these 6,7‐4 push–pull substituted quinolone dyes show high thermal and photochemical stability and no oxygen quenching different from many other fluorophores. Recently, another work group incorporated our 4‐trifluoromethyl and 4‐cyanocarbostyrils into peptides and got an efficient fluorescence‐resonance‐energy transfer (FRET) system .…”

Novel Three-Color FRET Tool Box for Advanced Protein and DNA Analysis

“…In a comparative study of different carbostyril derivatives, the chromophore in 1 showed the highest quantum efficiency Φ F . Therefore, 1 yielded an improved FRET performance in combination with the Ru (II) bathophenanthroline complex as acceptor due to a larger R 0 . In addition to that, 1 was accessible in just five synthetic steps in a straightforward manner starting from 3,4-dimethoxy aniline .…”

“…Therefore, 1 yielded an improved FRET performance in combination with the Ru (II) bathophenanthroline complex as acceptor due to a larger R 0 . 32 In addition to that, 1 was accessible in just five synthetic steps in a straightforward manner starting from 3,4-dimethoxy aniline. 32 In order to demonstrate the efficient incorporation of the three chromophore-modified building blocks 1À3, we have synthesized peptides 4À8 (Figure 4) using standard Fmoc-building blocks for the natural amino acids.…”