The Role of Tryptophan and Its Derivatives in Elucidating A PCET Mechanism in the Model Protein Azurin

Abstract

The role and properties of electron-rich tyrosine in the proton-coupled electron transport (PCET) pathway has been extensively studied, specifically relating to ribonucleotide reductase. Tryptophan is another electron-rich amino acid that can be derivatized into unnatural amino acids (UAAs) to further explore the mechanism of PCET. The purpose of this study is to computationally study the properties of tryptophan and its derivatives, modeled as simplified indole rings, to rationalize key experimental findings. Calculations were performed using density functional theory (DFT) to provide information on structure, energetics, and properties in a reliable, efficient manner. Ionization potentials of the halogenated and hydroxylated indole rings were studied with and without a solvent environment to understand certain features of the experimentally derived Pourbaix (potential vs. pH) diagram. Spin density distributions of the radical cations were examined regarding their relationship to the signature fine structure in the electron paramagnetic resonance spectra of the fluorinated derivatives of tryptophan. Electrostatic potential maps were calculated to provide a simple yet reliable means of evaluating charge distribution in the neutral and radical species. Results indicate that DFT mimicked the trends found in the Pourbaix diagram and gave insight on the properties of UAAs such as charge stabilization. Ultimately, a computationally-guided synthesis of an UAA, with its properties explicitly known, into the bacterial protein azurin could provide further insight into the PCET pathway.