Infrared Spectroscopy of Phenylalanine Ag(I) and Zn(II) Complexes in the Gas Phase

Abstract

Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag + and Zn 2+ with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag] + the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn 2+ forms a deprotonated dimer complex with Phe, [Zn+Phe2-H] + , in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly supporting this as the predominant binding geometry. This most stable calculated complex is characterized by N/ O/ring metal chelation with a tetrahedral-type coordination core of Zn 2+ to N and O of both ligands. Another similar tightly chelated structure shows a square-planar-type coordination core, but this structure is computed to be less stable and gives a less satisfactory match to the experimental spectrum. This preference for the tetrahedral geometry of the Lewis-basic atomic ligands parallels the common Zn(II) coordination geometry in proteins. The number of clearly identifiable peaks resolved in the IR-MPD spectra as well as the much-improved matches between the observed spectra and the DFT-calculated spectra of the most stable geometries compared to previous studies are noteworthy for systems of this size and complexity. These results demonstrate that IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment. In addition, we present a novel ion-capturing method for Fourier transform ion cyclotron resonance mass spectrometry which removes the necessity of a buffer gas pulse, while allowing ion trapping at moderate voltages with apparently reduced collisional excitation of the ions.

2,4-Dihydroxy and O2 Protonated Tautomers of dThd and Thd Coexist in the Gas Phase: Methylation Alters Protonation Preferences versus dUrd and Urd

“…+ and its modified form [Thd + H] + were measured using a 4.7 T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) [24][25][26]. Photodissociation was induced by the widely-tunable free electron laser for infrared experiments (FELIX) [27] and an OPO/ OPA laser system to examine the IR fingerprint and hydrogenstretching regions, respectively.…”

“…The solutions were delivered to a Micromass "Z-spray" electrospray ionization (ESI) source at a flow rate in the range of 3. [25]. Although significant heating of the ions during transmission from the hexapole to the ICR cell was not expected, the ions were stored in the ICR cell at 10 -8 Torr for at least 300 ms to enable any excess internal energy to be dissipated by radiative emission.…”

Observation of zwitterion formation in the gas-phase H/D-exchange with CH3OD: Solution-phase structures in the gas phase

“…Upon entering the octopole, the kinetic energy of the ions is typically about ϳ35 eV. By pulsing the octopole DC bias from ground to about Ϫ35 V while the ions are traveling through, the potential energy of the ions can be rebiased without affecting their kinetic energy [26]. As the ions enter the ICR cell, they have to overcome the potential energy difference from Ϫ35 V to ground, thereby reducing their kinetic energy.…”

IRMPD spectroscopy of metalated flavins: structure and bonding of Mq+–lumichrome complexes (Mq+ = Li+–Cs+, Ag+, Mg2+)

“…This combined strategy has proven to be a powerful tool to characterize metal ion complexes of a large variety of (bio-)molecules. [21][22][23][24][25][26][27][28][29][30][31][32][33] Similar to protonated flavins, 18 the CO stretch modes serve as the main indicator of the metal binding sites. The comparison of the measured IRMPD spectra with the calculated linear absorption spectra provides insight into their respective structures and bonding mechanisms.…”