Experimental and Computational Investigations of Selective Fluorogenic Recognition of Tryptophan Using a New Mixed Macrocyclic Ligand Complex

Abstract

A fluorescence (FL) molecular probe based on a naphthalene macrocycle, anchored to two porphyrin subunits (Zn2p), selective for L-tryptophan (L-Trp) has been synthesized. Single-crystal X-ray diffraction, FT-IR, ¹H, ¹³C NMR, and microelemental analyses were applied to characterize Zn2p. The FL spectroscopy represented Zn2p as a sensitive (LOD = 5 × 10⁻⁷ M) and highly selective sensor toward L-Trp in DMF among other selected amino acids. The L-Trp recognition mechanism was studied by ¹H NMR, representing a downfield shift for the proton signals of the OCH₂CH₂O moiety on Zn2p in the presence of L-Trp in CDCl₃ solution. Applying the supramolecular chemical analysis method, the K₁₁ value was calculated as 2.7496 (± 2.1963) × 10⁴ M⁻¹, covering the least computed error. Computational studies were carried out by employing density functional theory (DFT calculations, which were performed in the gas phase using the B3LYP-D3/def2-TZVP basis set in the ORCA 4.1.0 quantum chemistry program, supporting the ¹H NMR result on the involvement of the ethereal oxygen in the supramolecular interaction with L-Trp.