Exploring Redox Properties of Aromatic Amino Acids in Water: Contrasting Single Photon vs Resonant Multiphoton Ionization in Aqueous Solutions

Abstract

Direct measurements of the valence ionization energies and the reorganization energies of the three aromatic amino acids, l-tyrosine, l-tryptophan, and l-phenylalanine, in aqueous solution using the liquid microjet technique and two different photoemission methods-X-ray photoelectron spectroscopy (XPS) at 175 eV photon energy and resonant two-photon ionization (R2PI) using 2 × 267 nm (2 × 4.64 eV) UV laser light-are reported. l-Tryptophan has the lowest vertical ionization energy, 7.3 eV, followed by tyrosine (7.8 eV) and phenylalanine (∼8.7 eV). Essentially, no variation in recovered orbital energies is observed comparing near threshold ionization to X-ray ionization. Superior sensitivity of the (background-free) R2PI scheme for solutions with very low solute concentration (<2 mM) is demonstrated in contrast to the single-photon XPS measurements, which often requires solute concentrations of 0.1-1 molar. This higher sensitivity along with chemical selectivity of the R2PI technique can be exploited for both spectroscopic assignment and as an analytical tool. The nature of the adiabatic ionization energy for the three aromatic amino acids has been explored by the R2PI approach and by empirically formulating the correlation between the estimated ionization onset with electronic and nuclear relaxation on the excited state surface. Our results have implications for understanding one-electron transfer within enzymes and in redox situations where (ir)reversible deprotonation occurs such as those manifest in the biochemistry of oxidation damage.

Redox Activity of Ce(IV)-Substituted Polyoxometalates toward Amino Acids and Peptides

“…In fact, reactivity in the presence of His (10%) and Phe (5%) was only observed after 7 days of incubation at 60 °C. Although the redox potential of amino acids depends on the medium, these results follow a general trend that can be extracted from the literature, with the redox potential increasing in the following order: Cys , < Trp ≈ Tyr − < His ,, < Phe . Given the striking difference in the Ce IV K reduction in the presence of different amino acids, a detailed kinetic study was performed with each amino acid that exhibited a redox behavior.…”

Oxalate decarboxylase uses electron hole hopping for catalysis

“…To test the hypothesis of W-mediated hopping transport in OxDC, site-directed mutants were prepared for W96 and W274. In order to protect the quaternary structure, we used the aromatic amino acid phenylalanine, which we hypothesized would maintain a π-stacking interaction with the neighboring indole, while disrupting the hole-hopping chain due to its higher reduction potential ( 54 , 55 ). We find, indeed, that the W→F mutations significantly depress catalytic activity while the corresponding W→Y mutations partially rescue catalysis.…”

Design and characterization of a recirculating liquid-microjet photoelectron spectrometer for multiphoton ultraviolet photoelectron spectroscopy

“…8.0 ± 0.1 eV and 8.5 ± 0.1 eV, which also agree well with the XUV measurements. 28 Their analysis was based on two assumptions: (1) that the Franck-Condon overlap between the first 1 ππ * state and D 0 is the same as that between S 0 and D 0 and (2) that inelastic scattering shifts the spectrum in a similar way to photoelectron spectra of solvated electrons in aqueous solution. 53 In Fig.…”

“…35 This contrasts with the 267 nm MPI photoelectron spectrum reported by Roy et al in which the area of the peak corresponding to 1 1 ππ * − D 0 ionization was less than the area of the peak corresponding to 1 1 ππ * − D 1 ionization. 28 If we were to assume that the VIEs from S 0 could be determined by adding the S 0 − 1 1 ππ * adiabatic excitation energy, estimated as 4.46 eV from the uv-vis absorption spectrum, 35 to the VIEs from the 1 1 ππ * state and that inelastic scattering shifts the spectrum in a similar way to photoelectron spectra of solvated electrons in aqueous solution, 53 we would obtain S 0 − D 0 VIEs of 7.6 ± 0.1 eV and 8.5 ± 0.1 eV. These values agree with the BESSY II data, within the experimental errors of both measurements; however, our values are slightly lower, which could be the result of solvent relaxation during the REMPI process.…”

“…10,51,53 In order to be able to unravel the contribution of electron scattering, it is essential to have high quality photoelectron spectra and to account for, or eliminate, uncertainties arising from streaming potentials. We suspect that comparing high quality multiphoton ultraviolet liquidmicrojet photoelectron spectra and time-resolved photoelectron spectra [28][29][30][31][32] with analogous measurements in the gas-phase 35 will provide a new gold standard against which calculations can be benchmarked and can improve our understanding of the role a solvent or protein environment plays in determining electron binding energies and the timescales of relaxation processes of photoexcited molecules.…”