Date of Degree

2-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Gustavo Lopez

Committee Members

Donna McGregor

Nicolas Giovambattista

Moira Sauane

Subject Categories

Chemistry

Keywords

Nanotechnology, Materials Science, Histidine, Nanostructures, Computational Chemistry

Abstract

Over the past two decades, proton transport has been extensively studied because of the many applications to the development of new technological devices. In particular, understanding proton transport permits the design of new materials with desired properties for constructing effective proton exchange membranes (PEM) in fuel cells. Current PEM technology involves biocummulative perfluorinated membranes that operate under hydrated conditions, for which the proton conductivity is limited by the usually high temperature that fuel cells operate. Therefore, it is of great interest to study the alternative systems that have the potential to conduct protons, in particular, those that are able to operate through a non-solvent mediated mechanism in a wider temperature range. This doctoral project studies the fundamental process of proton translocation in various histidine-based systems.

In the design of potential proton-conducting systems, amphiprotic species are to be incorporated within the membrane. These molecules act as both proton donor/acceptor moieties that permit efficient proton transport through the Grotthuss shuttling mechanism (GSM). Here, protons typically translocate along a chain of H-bonds formed by the amphiprotic molecules, with the formation and cleavage of H-bonds occurring between neighboring molecules separated by distances of approximately 5-6 Å. These structures can then effectively act as non-aqueous proton wires. The rate of proton migration depends on the details of the energetics defined by a translocation coordinate between the molecules donating and accepting the protons.

The amino acid histidine is amphoteric and a promising building block for short aromatic peptides containing a proton donor/acceptor moiety. Experimental studies showed His-containing materials can be synthesized in helical secondary structures and form tripeptides via self-assembly, both of which could favor the formation of proton wires. In addition, the macroscopic dipole moment that systems possess could facilitate the unidirectional proton translocation through the amphiprotic side chains of the peptides.

This dissertation presents first-principle calculations of both non-aqueous His-containing helical peptides and His-based self-assembled nanostructures. The results demonstrate that efficient deprotonation-controlled proton wires can be formed within all systems. Overall, this doctoral work establishes the general parameters for consideration in various His-based systems that have the potential to conduct protons by better understanding the proton adsorption, diffusion, and deprotonation.

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