The Rate Ladder of Proton-Coupled Tyrosine Oxidation in Water: A Systematic Dependence on Hydrogen Bonds and Protonation State

Abstract

Proton coupled electron transfer (PCET) from tyrosine covalently linked to Ru(bpy)32+ has been studied with laser flash-quench techniques. Two new complexes with internal hydrogen bonding bases to the phenolic proton have been synthesized. Depending on the hydrogen bonding and protonation situation the rate constant of PCET spanned over 5 orders of magnitude and revealed a systematic dependence on pH. This resulted in a previously predicted "rate ladder" scheme: (i) pH dependent concerted electron-proton transfer (CEP) with deprotonation to bulk water, giving low PCET rates, (ii) pH independent CEP with deprotonation to the internal base, giving intermediate PCET rates, and (iii) pure electron transfer from tyrosinate, giving high rates. This behavior is reminiscent of Yz oxidation in Mn-depleted and native photosystem II. The study also revealed important differences in rates between phenols with strong and weak hydrogen bonds, and for the latter a hydrogen bond-gated PCET was observed.

Site-Specific Incorporation of 3-Nitrotyrosine as a Probe of pKa Perturbation of Redox-Active Tyrosines in Ribonucleotide Reductase

“…The importance of this residue in nucleotide reduction was demonstrated by previous studies of Climent et al16 with a E 350 A mutant and our recent studies with E 350 Q mutant (unublished results). As noted above, the model studies of Hammarström may thus provide a model of PCET at this center 56,66…”

“…H bonded phenols, for examle, have been designed to understand the oxidation of the Y Z in photosystem II in which a histidine functions as the proton acceptor from Y Z during its oxidation 68,69. phenols with amine and imidazole bases62,63,70 and carboxylates56,65,66 appended ortho to the phenol have been synthesized and structurally and sectroscoically characterized. The rate constants for oxidation and the deuterium kinetic isotope effects on oxidation have been measured as a function of temperature 56,62,65,66,70.…”

“…phenols with amine and imidazole bases62,63,70 and carboxylates56,65,66 appended ortho to the phenol have been synthesized and structurally and sectroscoically characterized. The rate constants for oxidation and the deuterium kinetic isotope effects on oxidation have been measured as a function of temperature 56,62,65,66,70. The Hammarström model seems particularly appropriate for Y 356 oxidation in RNR where E 350 is proposed to be the initial proton acceptor (Fig.…”

Proton Coupled Electron Transfer and Redox-Active Tyrosine Z in the Photosynthetic Oxygen-Evolving Complex

“…37–40,48,49 These reactions avoid the formation of high energy intermediates, but occur at the expense of a high reorganization energy, which is defined as the energy to reorganize from the initial to final coordinates, without charge transfer. 37–40 We propose that the extensive hydrogen bonding network around YZ • and the calcium ion facilitates CPET at low pH.…”

Hangman Salen Platforms Containing Dibenzofuran Scaffolds

“…[6][7][8][9] PCET is crucial to radical generation and the transport of the tyrosine radical [10][11][12][13][14] and substrate oxidation at a variety of enzyme active sites. [10,15,16] This PCET-induced oxidation catalysis may be captured at synthetic active sites that "hang" an acid-base group over a redox platform.…”