Discovery of ravenelin B from Exserohilum rostratum: structural elucidation of a scarce xanthone via integrated NMR/DFT-GIAO approach and comprehensive excited-state characterization.

CONTEXT: This study reports the isolation and structural characterization of ravenelin (RVL) and ravenelin B (RVL B), a novel xanthone derivative produced by the endophytic fungus Exserohilum rostratum from the Brazilian Amazon. The compound was obtained in minute quantities with partial purity, presenting significant challenges for traditional structural characterization. Through the combined application of NMR spectroscopy (1D/2D) and DFT/GIAO calculations, we achieved unequivocal structural determination, demonstrating an effective strategy for investigating scarce natural products. Extensive computational analysis revealed key insights into the molecular properties: RVL B exhibits a predominantly locally excited character (61%) compared to RVL's mixed charge-transfer/locally excited nature and shows promising two-photon absorption (TPA) properties with a high cross-section (351 GM in water) for higher excited states. Further TD-DFT calculations elucidated the solvent-dependent photophysical behavior, including absorption/emission spectra and Stokes shifts. These findings not only expand the chemical diversity of Amazonian endophytes but also establish a comprehensive framework for characterizing low-abundance metabolites using integrated experimental and theoretical approaches, while revealing potential applications as substituent design in nonlinear optics and photodynamic therapy.

METHODOLOGY: This work discusses the procedure for the isolation, characterization, and theoretical insights into the electronic properties of a novel xanthone isolated from an Amazonian fungus. High-resolution mass spectra were obtained in positive ion mode using a MicroTOF-Q mass spectrometer (Bruker Daltonics, USA) equipped with an electrospray ionization (ESI) source. NMR spectra were recorded on a Bruker Ascend 400 using the solvent signal (d-chloroform) as a reference. Chemical shifts are given in delta ( δ ) values, and coupling constants (J) are given in Hertz (Hz). Concerning theoretical methods, all molecular optimizations and NMR calculations were performed within the DFT/GIAO framework using ω B97X-D/6-311++G(d, p). When necessary, the solvent was accounted for by the integral equation formalism of the polarizable continuum model. The electronic excitations of the ground and first excited state, as well as two-photon absorption cross-sections, were calculated within the TD-DFT framework by using different degrees of Hartree-Fock exchange (CAM-B3LYP, ω B97X-D, and M06-2H), also associated with the Pople 6-311++G(d, p) basis set and the continuum model of solvation.