Switching the Redox Mechanism:  Models for Proton-Coupled Electron Transfer from Tyrosine and Tryptophan

Abstract

The coupling of electron and proton transfer is an important controlling factor in radical proteins, such as photosystem II, ribinucleotide reductase, cytochrome oxidases, and DNA photolyase. This was investigated in model complexes in which a tyrosine or tryptophan residue was oxidized by a laser-flash generated trisbipyridine-Ru(III) moiety in an intramolecular, proton-coupled electron transfer (PCET) reaction. The PCET was found to proceed in a competition between a stepwise reaction, in which electron transfer is followed by deprotonation of the amino acid radical (ETPT), and a concerted reaction, in which both the electron and proton are transferred in a single reaction step (CEP). Moreover, we found that we could analyze the kinetic data for PCET by Marcus' theory for electron transfer. By altering the solution pH, the strength of the Ru(III) oxidant, or the identity of the amino acid, we could induce a switch between the two mechanisms and obtain quantitative data for the parameters that control which one will dominate. The characteristic pH-dependence of the CEP rate (M. Sjodin et al. J. Am. Chem. Soc. 2000, 122, 3932) reflects the pH-dependence of the driving force caused by proton release to the bulk. For the pH-independent ETPT on the other hand, the driving force of the rate-determining ET step is pH-independent and smaller. On the other hand, temperature-dependent data showed that the reorganization energy was higher for CEP, while the pre-exponential factors showed no significant difference between the mechanisms. Thus, the opposing effect of the differences in driving force and reorganization energy determines which of the mechanisms will dominate. Our results show that a concerted mechanism is in general quite likely and provides a low-barrier reaction pathway for weakly exoergonic reactions. In addition, the kinetic isotope effect was much higher for CEP (kH/kD > 10) than for ETPT (kH/kD = 2), consistent with significant changes along the proton reaction coordinate in the rate-determining step of CEP.

Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics

“…Sjödin et al explained the pH dependence of the experimentally measured rate constant in terms of a PCET mechanism in which the proton transfers from tyrosine to bulk water. [15][16][17][18] Our theoretical calculations on this model system were consistent with this interpretation. [32] Alternative interpretations of this type of pH-dependence have also been proposed.…”

“…[11][12][13][14][15][16][17][18] For example, Sjodin et al studied tyrosine-bound ruthenium-tris-bipyridine model systems with the flash-quench method. [15][16][17][18] In these experiments, the excited state of Ru is quenched by an external quencher, followed by electron transfer from the tyrosine to the photo-oxidized Ru. Sjödin et al explained the pH dependence of the experimentally measured rate constant in terms of a PCET mechanism in which the proton transfers from tyrosine to bulk water.…”

“…[11][12][13][14][15][16][17][18] In some cases, photoexcitation of the metal in tyrosine-bound metal polypyridyl systems induces electron transfer from the tyrosine to the metal with concurrent proton transfer from the tyrosine to the solvent or the buffer. [14][15][16][17][18] Other experiments involve the chemical oxidation of phenols in solution. [11,12] Often the data are interpreted in the context of Marcus theory for electron transfer.…”

Photo-induced cytotoxicity and anti-metastatic activity of ruthenium(ii)–polypyridyl complexes functionalized with tyrosine or tryptophan

“…18,19 Both complexes were characterized using a range of spectroscopic and analytical techniques. All these spectroscopic data, as well as the results of elemental analysis, confirm the desired purity for both complexes utilized in our studies ( fig.…”

A Selective Chemosensor for Mercuric Ions Based on 4-Aminothiophenol-Ruthenium(II) Bis(bipyridine) Complex

“…Polypyridine-Ru(II) complexes are interesting chromophores. The assembly of oligopyridyl groups around Ru(II) center yields mono-or polymetallic complex ions that have been studied extensively for their potential ability as fluorescent chemodetectors [41][42][43] and energy or electron transformers [44][45][46][47]. Furthermore, these Ru(II)-metallo complexes, in some cases, have demonstrated good photophysical and colorimetric sensing properties.…”