Vibrational signatures of curcumin’s chelation in copper(II) complexes: An appraisal by IRMPD spectroscopy

Abstract

Curcumin (Cur) is a natural polyphenol with a wide spectrum of biological activities and appealing therapeutic potential. Herein, it has been delivered by electrospray ionization as gaseous protonated species, [Cur + H] + , and as a Cu(II) complex, [Cu(Cur − H)] + , a promising antioxidant and radical scavenger. The gas phase structures were assayed by infrared multiple photon dissociation (IRMPD) spectroscopy in both the fingerprint (800-2000 cm −1) and hydrogen stretching (3100-3750 cm −1) ranges. Comparison between the experimental features and linear IR spectra of the lowest energy structures computed at the B3LYP/6-311+G(d,p) level reveals that bare [Cu(Cur − H)] + exists in a fully planar and symmetric arrangement, where the metal interacts with the two oxygens of the syn-enolate functionality of deprotonated Cur and both OCH 3 groups are engaged in H-bonding with the ortho OH. The effect of protonation on the energetic and geometric determinants of Cur has been explored as well, revealing that bare [Cur + H] + may exist as a mixture of two close-lying isomers associated with the most stable binding motifs. The additional proton is bound to either the diketo or the keto-enol configuration of Cur, in a bent or nearly planar arrangement, respectively.

IRMPD Spectroscopy of Bare Monodeprotonated Genistein, an Antioxidant Flavonoid

“…In addition, quantum chemical calculations have been performed to sample low-energy candidate isomers and to obtain optimized geometrical and electronic structures and linear IR absorption spectra. This inquiry originates from the current, increasing interest in the application of IR spectroscopy as a versatile analytical tool to clarify the structural features of small (bio)­molecules involved in relevant metabolic functions. ,− …”

Ligation Motifs in Zinc-Bound Sulfonamide Drugs Assayed by IR Ion Spectroscopy

“…IRMPD spectroscopy has been demonstrated to afford a valuable characterization of biologically active molecules and molecular complexes since its expansion as a structural diagnostic tool of mass-selected charged species [ 17 , 18 , 19 , 20 , 21 ]. In a considerable number of studies, including contributions from these authors, it has been applied to identify binding motifs and preferred geometries of transition metal complexes coordinating biomolecular ligands and targets [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Zinc complexes of biomolecules have also been assayed [ 33 , 34 , 35 , 36 , 37 , 38 ].…”

From Preassociation to Chelation: A Survey of Cisplatin Interaction with Methionine at Molecular Level by IR Ion Spectroscopy and Computations

“…It is worthy of note that the species is assayed in the gas phase, in the absence of solvent and associated band-broadening effects. IRMPD spectroscopy has been extensively used for the vibrational and structural characterization of transition metal complexes of both catalytic and biological relevance. − In particular, cisplatin related platinum­(II) complexes have been characterized. − Previous reports have highlighted the versatility of ESI-MS together with the specificity of IRMPD spectroscopy in delivering detailed structural information on the monofunctional complexes cis -[PtCl­(NH 3 ) 2 (AA)] + , where AA is either Met or His. − Furthermore, solutions of cisplatin and a simple ligand (L, chosen to model the nucleophilic functionalities of amino acids and nucleobases), assayed by ESI-MS and IRMPD spectroscopy have revealed the presence and role of the encounter complexes { cis -[PtCl­(NH 3 ) 2 (H 2 O)] + ·L}. , These species are formed by the diffusion controlled encounter of the aqua complex of cisplatin with the incoming ligand L and are the early actors in the substitution reaction of metal complexes in aqueous media, conforming to the Eigen–Wilkins reactant preassociation model. − In the reported cases, the role of the encounter complex along the substitution reaction path of the cisplatin aqua complex with model ligands was assessed by observing the reaction to occur in the isolated species after collisional- or photoactivation, while IRMPD spectroscopy has afforded its vibrational and structural features. Moving one step further in assaying the cisplatin reaction with nucleophiles approaching the biomolecular targets, herein is reported the isolation and characterization of the adduct formally obtained by the preassociation of cisplatin aqua complex with methionine, [PtCl­(NH 3 ) 2 (H 2 O)­(Met)] + .…”