Synthesis and Fluorescent Properties of β-Pyridyl α-Amino Acids

Abstract

RESULTS AND DISCUSSIONThe first stage of the synthesis of the -pyridyl -amino acids involved the preparation of suitably protected enone-derived -amino acids (Scheme 1). Initially, N-trityl protected phosphonate ester 3 was prepared in three-steps (92% overall yield) from L-aspartic acid 1. 13 Following protection of the amino and carboxylic acid groups, the key step involved the highly regioselective reaction of the anion of dimethyl methylphosphonate with the sterically accessible -methyl ester of 2 to give phosphonate ester 3. Horner-Wadsworth-Emmons reaction of 3 with a wide range of aldehydes under mild conditions, gave enones 4a-j, exclusively as the E-isomers. 14,15 The focus of this project was to prepare aryl substituted pyridyl side-chains and therefore, benzaldehyde derivatives were mainly used to explore the scope of this reaction. As expected, the majority of these gave high yields of the E-enones (66-95%), although highly electron-rich aldehydes required longer reaction times (120 h) and gave more modest yields (e.g. 4e). Scheme 1. Synthesis of Enone-Derived -Amino Acids 4a-j a a Isolated yields are shown. peptide synthesizer using Rink Amide ChemMatrix® resin as the polymer support 22 and routine SPPS methodology (Scheme 3). On coupling Fmoc-L-Lys(Boc)-OH onto the polymer support using HCTU activation, successive rounds of piperidine-mediated N-deprotection and coupling using the next amino acid gave the pentapeptide. Following a subsequent N-terminal Fmoc-deprotection, the Fmoc-protected version of -pyridyl -amino acid 8d, compound 9 was coupled onto the polymer-supported pentapeptide. 23 After a final Fmoc-deprotection step, the N-terminus was acetyl capped and a TFA cleavage cocktail that included triisopropylsilane (TIPS) as a cation scavenger was used to remove the side-chain protecting groups and release the peptide from the polymer support. The resulting hexapeptide, 10 was purified by reverse phase-HPLC (>95% purity) and characterized by high-resolution electrospray ionization mass spectrometry (see Supporting Information for data). Scheme 3. SPPS of a -Pyridyl -Amino Acid Containing HexapeptideAs the -amino acids contained -deficient pyridine moieties, it was proposed that electron-rich conjugating groups may confer fluorescent properties through a charge-transfer (push-pull) mechanism. 24 Accordingly, the optical properties of -pyridyl -amino acids 8a-l were measured. As expected, pyridines with non-conjugating or electron-deficient substituents showed weak fluorescence, while compounds with electron-rich substituents showed strong fluorescence (Figure 2b and Table 1). In

Miniaturized Chemical Tags for Optical Imaging

“…[51] Alternative scaffolds have been successfully employed to prepare fluorescent amino acids with suitable properties for in vitro biomolecular and bioimaging studies (Figure 4). These include xanthone-based amino acids for studying peptide folding by triplet-triplet energy transfer, [52] acridon-2-ylalanines for lifetime and Förster resonance energy transfer studies, [53] conformationally rigid pyrazoloquinazoline α-amino acids for twophoton NIR excitation, [54] phospholyl and benzotriazoles amino acids with large Stokes shifts (>160 nm), [55] and solvatochromic amino acids containing β-pyridyl [56] or phthalimide groups for the preparation of environmentally sensitive peptides retaining bioactivity (e.g., transmembrane fragments of the epidermal growth factor receptor). [57] The miniaturized size of fluorescent amino acids has also been exploited to utilise the machinery of living cells for site-specific labeling of proteins under physiological conditions.…”

Fluorescent amino acids as versatile building blocks for chemical biology

“…Another FlAA with a built-in fluorophore, benzoacridone-modified alanine, was synthesized by an Ullmann-type coupling between 3-chloro-2-naphthoic acid and aminophenylalanine, followed by cyclization of 2-aminonapthoic acid to yield green-fluorescent benzoacridone amino acid (λem.= 500-550 nm, quantum yield ~50%) 74 . Furthermore, a non-natural FlAAs have been synthesised by building aryl-substituted pyridyl, pyrazole, benzotriazole and pyrazoloquinazoline (24) heterocyclic chromophores on aspartic acid and asparagine structures (λem.= 348-460 nm) [75][76][77][78] (Figure 3b).…”

Triphenylamine/4,4′-Dimethoxytriphenylamine-Functionalized Thieno[3,2-b]thiophene Fluorophores with a High Quantum Efficiency: Synthesis and Photophysical Properties

“…It is well recognized that fluorescent materials have been immensely popular in display technologies due to their unique advantages in energy economy with low driving voltage, high brightness, and high contrast display for full-color fluorescence. − Conjugated organic molecules are one of the most popular materials used in display technologies along with other applications such as light-emitting diodes, lighting lasers, photovoltaics, nonlinear optical (NLO) materials, security marking, and biomedical imaging. − Organic fluorophores with high quantum efficiency, strong brightness, , long-lasting fluorescence lifetime, chemical and thermal robustness, and color tunability − are potential compounds for fluorescence applications. An important advantage is that fluorescence properties can be finely tuned through chemical modification of their structures. − In particular, the geometrical design and conformation of the fluorophores have a crucial influence on their fluorescence and electroluminescence properties. − Thiophene-derived conjugated compounds, such as thienothiophenes (TTs), have particular importance as they are easy to modify in addition to their electron-rich, flat, and good electron delocalized structures. Thus, they are widely used in optoelectronic applications owing to their notable optic, electronic, and redox properties. − …”