Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Asparagine Complexes with Zn2+ and Cd2+ and Their Deamidation Processes

Spectroscopic Investigation of the Metal Coordination of the Aromatic Amino Acids with Zinc and Cadmium

The aromatic amino acids (AAA), phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp), were cationized with ZnCl⁺ and CdCl⁺, and the complexes were evaluated using infrared multiple photon dissociation (IRMPD) action spectroscopy. Specifically, the ZnCl⁺(Phe), CdCl⁺(Phe), ZnCl⁺(Tyr), CdCl⁺(Tyr), and ZnCl⁺(Trp) species were examined because the CdCl⁺(Trp) IRMPD spectrum is available in the literature. Several low-energy conformers for all complexes were found using quantum chemical calculations, and their simulated vibrational spectra were compared to the experimental IRMPD spectra to identify dominant isomers formed. In the case of MCl⁺(Phe) and MCl⁺(Tyr), these comparisons indicated the dominant binding motif is a tridentate structure, where the metal atom coordinates with the backbone amino nitrogen and carbonyl oxygen, as well as the aryl ring. These observations are consistent with the predicted ground states at the B3LYP, B3P86, B3LYP-GD3BJ, and MP2 levels of theory. For the ZnCl⁺(Trp) system, the experimental spectrum indicates a similar binding motif, with the zinc atom coordinating with the backbone nitrogen and carbonyl oxygen and either the pyrrole ring or the benzene ring of the indole side chain. These observations are consistent with the predicted low-lying conformers identified by the aforementioned levels of theory, with the B3LYP and B3P86 levels predicting the metal-pyrrole ring interaction is more favorable than the metal-benzene ring interactions and the opposite at the B3LYP-GD3BJ and MP2 levels.

Complexation of Amino Acids with Cadmium and Their Application for Cadmium-Contaminated Soil Remediation

The interaction of amino acids with toxic heavy metals influences their immobilization and bioavailability in soils. However, the complexation ability of amino acids with Cd has not been well studied. The complexes of amino acids and cadmium were investigated by density functional theory (DFT) calculations and Fourier transform infrared spectrometry (FTIR) analyses. The complex structures were found to be [COc, COc] for fatty amino-cadmium and PheCd2+, [COc, COc, COs] for GluCd2+ and ThrCd2+, respectively. The complex energy of these conformers followed the order PheCd2+> AlaCd2+ > LeuCd2+ > GluCd2+ > GlyCd2+ > ThrCd2+. Importantly, all of the complex energy values were less than zero, indicating that these complexes could be easily dissolved in water. The Cd2+ concentration decreased with increasing amino acid concentration in aqueous solution. The complex stability constants (logβ) followed the order PheCd2+> AlaCd2+ > LeuCd2+ > GluCd2+ > GlyCd2+ > ThrCd2+, consistent with the order of the calculated complex energy values. The Cd removal efficiencies by Thr, Glu, Gly, Ala, Leu, and Phe were 38.88%, 37.47%, 35.5%, 34.72%, 34.04%, and 31.99%, respectively. In soil batch tests, the total Cd concentration in soil decreased in the presence of amino acids, while the Cd concentration in water increased from 231.97 μg/L to 652.94~793.51 μg/L. The results of sequential extraction showed that the acid-extractable fraction and the reducible fraction of Cd sharply decreased. Consequently, the significant features of amino acids along with their biocompatibility make them potentially applicable chelators in Cd-contaminated soil remediation processes.

IRMPD Spectroscopic and Theoretical Structural Investigations of Zinc and Cadmium Dications Bound to Histidine Dimers

Metallated gas-phase structures consisting of a deprotonated and an intact histidine (His) ligand, yielding M(His-H)(His)⁺, where M = Zn and Cd, were examined with infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). In parallel, quantum chemical calculations identified several low-energy isomers for each complex. Experimental action spectra were compared to linear spectra calculated at the B3LYP level of theory using the 6-311+G(d,p) and def2-TZVP basis sets for the zinc and cadmium complexes, respectively. For both Zn and Cd species, the definitive assignment is complicated by conflicting relative energetics, which were calculated at B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels. Spectral comparison for both species indicates that the dominant conformation, [N_α,N_π,CO^-][CO₂^-](N_πH⁺), has the deprotonated His chelating the metal at the amine nitrogen, π nitrogen of the imidazole ring, and the deprotonated carbonyl oxygen and that the intact His ligand adopts a salt-bridge bidentate binding motif, coordinating the metal with both carboxylate oxygens. There is also evidence for a conformation where the deprotonated His coordination is maintained, but the intact His ligand adopts a more canonical structure, coordinating with the metal atom at the amine nitrogen and π nitrogen, [N_α,N_π,CO^-][N_α,N_π]gtgg. For both metallated species, B3LYP, B3P86, and B3LYP-GD3BJ levels of theory appear to describe the relative stability of the dominant zwitterionic species more accurately than the MP2(full) level.

Zinc and cadmium complexation of L-methionine: An infrared multiple photon dissociation spectroscopy and theoretical study

Methionine (Met) cationized with Zn2+ , forming Zn (Met-H)+ (ACN) where ACN = acetonitrile, Zn (Met-H)+ , and ZnCl+ (Met), as well as Cd2+ , forming CdCl+ (Met), were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from the FELIX free electron laser. A series of low-energy conformers for each complex was found using quantum-chemical calculations in order to identify the structures formed experimentally. For all four complexes, spectral comparison indicated that the main binding motif observed is a charge solvated, tridentate structure where the metal center binds to the backbone amino group nitrogen, backbone carbonyl oxygen (where the carboxylic acid is deprotonated in two of the Zn2+ complexes), and side-chain sulfur. For all species, the predicted ground structures reproduce the experimental spectra well, although low-lying conformers characterized by similar binding motifs may also contribute in each system. The current work provides valuable information regarding the binding interaction between Met and biologically relevant metals. Further, the comparison between the current work and previous analyses involving alkali metal cationized Met as well as cysteine (the other sulfur containing amino acid) cationized with Zn2+ and Cd2+ allows for the elucidation of important metal dependent trends associated with physiologically important metal-sulfur binding.

Infrared multiple photon dissociation action spectroscopy of protonated glycine, histidine, lysine, and arginine complexed with 18-crown-6 ether

Complexes of 18-crown-6 ether (18C6) with four protonated amino acids (AAs) are examined using infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by the infrared free electron laser at the Centre Laser Infrarouge d'Orsay (CLIO). The AAs examined in this work include glycine (Gly) and the three basic AAs: histidine (His), lysine (Lys), and arginine (Arg). To identify the (AA)H⁺(18C6) conformations present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311+G(d,p) level of theory. Relative energies of various conformers and isomers are provided by single point energy calculations carried out at the B3LYP, B3P86, M06, and MP2(full) levels using the 6-311+G(2p,2d) basis set. The comparisons between the IRMPD and theoretical IR spectra indicate that 18C6 binds to Gly and His via the protonated backbone amino group, whereas protonated Lys prefers binding via the protonated side-chain amino group. Results for Arg are less definitive with strong evidence for binding to the protonated guanidino side chain (the calculated ground conformer at most levels of theory), but contributions from backbone binding to a zwitterionic structure are likely.

Experimental and theoretical investigations of infrared multiple photon dissociation spectra of lysine complexes with Zn2+ and Cd2

The gas-phase structures of zinc and cadmium complexes of lysine (Lys) are investigated via a combination of infrared multiple photon dissociation action spectroscopy and ab initio quantum chemical calculations. In order to unambiguously identify the experimentally observed species, [Zn(Lys-H)]⁺ and CdCl⁺(Lys), the action spectra were compared to linear absorption spectra calculated at the B3LYP level of theory, using 6-311+G(d,p) and def2-TVZP basis sets for the zinc and cadmium systems, respectively. Single point energies were also calculated at the B3LYP, B3P86, MP2, and B3LYP-GD3BJ (accounting for empirical dispersion) levels of theory using larger basis sets. Identification of the experimentally formed isomers is possible through good agreement between infrared multiple photon dissociation action spectra and the theoretically predicted spectra. The [Zn(Lys-H)]⁺ complex adopts a tridentate orientation involving the amino acid backbone amine and deprotonated carboxylic acid groups as well as the side-chain amine group, [N_α,CO^-,N_ɛ]. The CdCl⁺(Lys) complex similarly adopts a tridentate chelation involving the amino acid backbone amine and carbonyl groups, as well as the side-chain amine group, [N_α,CO,N_ɛ]. In both cases, the identified complexes are the lowest energy gas-phase structures at all levels of theory.

Spectroscopy and formation of lanthanum-hydrocarbon radicals formed by association and carbon-carbon bond cleavage of isoprene

La atom reaction with isoprene is carried out in a laser-vaporization molecular beam source. The reaction yields an adduct as the major product and C-C cleaved and dehydrogenated species as the minor ones. La(C₅H₈), La(C₂H₂), and La(C₃H₄) are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of all three species exhibit a strong origin band and several weak vibronic bands corresponding to La-ligand stretch and ligand-based bend excitations. La(C₅H₈) is a five-membered metallacycle, whereas La(C₂H₂) and La(C₃H₄) are three-membered rings. All three metallacycles prefer a doublet ground state with a La 6s¹-based valence electron configuration and a singlet ion. The five-membered metallacycle is formed through La addition and isoprene isomerization, whereas the two three-membered rings are produced by La addition and insertion, hydrogen migration, and carbon-carbon bond cleavage.

Infrared Multiphoton Dissociation Spectroscopy with Free-Electron Lasers: On the Road from Small Molecules to Biomolecules

Infrared multiphoton dissociation (IRMPD) spectroscopy is commonly used to determine the structure of isolated, mass-selected ions in the gas phase. This method has been widely used since it became available at free-electron laser (FEL) user facilities. Thus, in this Minireview, we examine the use of IRMPD/FEL spectroscopy for investigating ions derived from small molecules, metal complexes, organometallic compounds and biorelevant ions. Furthermore, we outline new applications of IRMPD spectroscopy to study biomolecules.

Experimental and theoretical investigations of infrared multiple photon dissociation spectra of arginine complexes with Zn2+ and Cd2+

Arginine (Arg) complexes with Zn²⁺ and Cd²⁺ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light from a free electron laser.

Experimental and theoretical investigations of infrared multiple photon dissociation spectra of glutamic acid complexes with Zn2+and Cd2+

IRMPD of [Zn(Glu-H)ACN]⁺was particularly interesting because fragmentation of the amino acid was favored, rather than dissociation of the ACN ligand.