Antioxidant, Protein-Binding, and Copper(II) Ion Sensing Activities of Rhus alata Phytochemicals: An Integrated...

The genus Rhus, recognized for its wide range of biological activities, is a rich reservoir of bioactive phytochemicals. Chemical investigation of Rhus alata (R. alata) leaves led to the isolation and characterization of two new compounds, 4-octadecylphenol (Ra-01) and 2-(7-hydroxy-2-oxo-2H-chromen-4-yl)-2-methylpropanoic acid (Ra-02), alongside five known compounds: lupeol, dimethyl ester of terephthalic acid, ethyl gallate, friedelin, and oleanolic acid. The structures of all compounds were elucidated by using spectroscopic techniques (IR, UV, NMR, and mass spectrometry). Notably, 4-octadecylphenol (Ra-01) is reported here for the first time as a natural product; it was previously only known as a synthetic compound. Compound Ra-02 represents a new compound, with no prior reports of natural isolation or synthesis. Circular dichroism (CD) spectroscopy revealed nonspecific binding of compound Ra-01 to both bovine and human serum albumin (BSA and HSA), suggesting a general interaction governed by weak intermolecular forces. Fluorescence studies corroborated these binding interactions between Ra-01 and serum albumins, and molecular docking analysis indicated comparable binding affinities of Ra-01 for HSA (domain IIIB, -8.4 kcal/mol) and BSA (central domain, -8.0 kcal/mol), accompanied by conformational changes in the proteins. Furthermore, isolated compounds exhibited significant antioxidant activity. The photophysical properties of compound Ra-02 were investigated using absorption and fluorescence spectroscopy, revealing its potential as a selective "turn-on" fluorescent sensor for Cu²⁺ ions over other metal ions. Photoluminescence (PL) spectroscopy further characterized the optical properties and polarity sensitivity of Ra-02. In addition, a computational study using DFT (B3LYP/LANL2DZ/6-31G-(d,p)) with PCM solvation identified Site 1 as the most energetically favorable binding site for Cu²⁺ in Ra-02, displaying stronger and more stable interactions. In silico ADMET and toxicity analysis supported the drug-likeness and biosafety of Ra-02 based on key pharmacokinetic parameters. These findings underscore the diverse bioactivities of R. alata constituents, including antioxidant effects, protein interactions, and the potential for the development of novel fluorescent sensors.