Rotamer-Controlled Dual Emissive α-Amino Acids

The synthesis and photoluminescent properties of novel α-amino acids are described in which the biaryl benzotriazinone-containing chromophores were found to display dual emission fluorescence via locally excited (LE) and twisted intramolecular charge transfer (TICT) states. The intensity of each emission band could be controlled by the electronics and position of the substituents, and this led to the design of a 2-methoxyphenyl analogue that, due to twisting, displayed bright TICT fluorescence, solvatochromism, and pH sensitivity.


S3
Subscript ST signifies the quantities associated with the quantum yield standard. Subscript X signifies the quantities associated with the novel compound. Gradx is the determined gradient associated with the novel compound. GradST is the determined gradient associated with quantum yield standard. η is the refractive index of the solvent used in the fluorescence measurements. η = 1.333 for water, 1.361 for ethanol and 1.331 for methanol.
Purification by flash column chromatography, eluting with 30% ethyl acetate in hexane gave methyl

Synthesis of Peptide 12
Peptide 12 was synthesized on a Biotage Initiator+ Alstra peptide synthesizer using an Fmoc/ t Bu protecting group strategy (0.1 mmol synthetic scale) and Rink Amide ChemMatrix © resin (0.45 S15 mmol/g). Microwave-assisted SPPS was used for all coupling reactions. Following resin swelling in DMF at 70 °C for 20 min, the resin bound peptide was synthesized by first loading Fmoc-Arg(Pbf)-OH to the resin and by introduction of the Fmoc-protected amino acids (5 equiv., 0.2 M in DMF) successively by treatment with DIC (5 equiv., 0.5 M in DMF) and OxymaPure (5 equiv., 0.5 M in DMF) at 90 °C for 2 min. Arg residues were coupled at room temperature for 45 min, followed by 90 °C for 5 min, then double coupled at 90 °C for 10 min. Fmoc groups were removed using morpholine (20% in DMF with 5% formic acid, 4 mL) at 90 °C for 1 min. The resin was washed with DMF between deprotection and coupling (4 × 4 mL) and after coupling (
The excitation spectra were recorded on a Shimadzu RF-5301PC spectrofluorophotometer at each wavelength of the two emission maxima for amino acids 10a-10e and at a concentration of 15 M. All spectra were recorded using a concentration of 5 M.

Excitation at 267 nm:
Lippert-Magata Plot for 10e. To evaluate the effect of pH on the fluorescent properties of 10e, absorption and emission spectra were obtained in neutral methanol (pH 7). Quantities of concentrated hydrochloric acid (3 M in MeOH) were then added to the stock solution to obtain samples at pH 4 and pH 1. All spectra were recorded at room temperature and using a concentration of 5 μM.