Exploring the influence of non-covalent interactions on electron transfer properties of tyrosine and tryptophan

Biological electron transfer (ET) is an intricate process, and in many cases, these ET events are coupled with proton transfer reactions, which can add complexity to reaction mechanisms. Importantly, there is a distance dependence at which these reactions can occur. Proton transfer must occur over short reaction distances. In contrast, the rate of ET decreases exponentially as the distance between the electron donor and acceptor increases. It is these long-range ET (or proton-coupled ET) reactions that require the use of reactive intermediates in functional biological systems, and in the context of this thesis, these reactive intermediates can be either tryptophan (Trp) or tyrosine (Tyr). Proton-coupled ET via Trp and Tyr residues in proteins has been deeply explored within the literature, however, much less is known about how nearby non-covalent interactions impact the properties of Trp and Tyr. Herein, an artificial protein model system was constructed using blue-copper azurin from Pseudomonas aeruginosa, such that we can study how substitution of the amino acids alanine, leucine, lysine, arginine, glutamine, and methionine can introduce or remove non-covalent interactions within the microenvironment surrounding a Trp or Tyr residue. Their effect on tuning the electronic properties of either of these aromatic residues is explored also using protein absorption and fluorescence, electron paramagnetic resonance, electrochemistry, and transient absorption spectroscopy.