Theory guides designed concerted electron-proton transfer for energy conversion


July 10, 2017

An important contributor to the efficiency in Photosystem II is mediation of charge separation by a tyrosine residue (Tyrz). Oxidation of Tyrz as part of the charge separation process occurs in concert with transfer of a proton from the oxidized tyrosine to a neighboring histidine (His) residue.

This concerted electron-proton transfer not only promotes charge separation, but also generates a proton with the thermodymanic potential to drive water oxidation. Proton-coupled electron transfer is thus critical to the overall light energy conversion process. In turn, understanding how to design and control concerted proton-electron transfer is also important in the development of artificial methods for solar fuel production based on Photosystem II, and other technologies that require managing proton activity. Download Full Image

Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It captures photons and via charge-separation uses the energy to oxidize water. The TyrZ-His redox-relay pair couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II.

In a recent collaborative study with researchers from the University of Illinois, Université Paris-Sud and the University of Virginia, Tom Moore, Ana Moore and Devens Gust of Arizona State University's School of Molecular Sciences recently showed how theory can be used to guide the design of a concerted electron–proton transfer process, inspired by the a TyrZ-His pair in Photosystem II.

The study used artificial redox relays based on phenol/benzimidazole pairs to test the theory that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol would take place when the benzimidazole substituents are strong proton acceptors, such as primary or tertiary amines. Theory also predicts a decrease in the redox potential of the phenol and a small kinetic isotope effect (KIE). Electrochemical, spectroelectrochemical and KIE experimental data provide strong evidence to support these predictions. The work was described in a recent paper in ACS Central Science.

As Tom Moore explains, “Inspired by nature, and working closely with Professor Hammes-Schiffer's group at the University of Illinois, we designed a proton coupled electron transfer construct and demonstrated that two proton transfer steps could be driven by one electron transfer step. … This work could be important in solar energy conversion to fuels, in the construction of artificial neural networks for artificial intelligence, and in general for the use of electricity to power proton currents and support lifelike processes in artificial cells.”

This study is significant because it shows that theory can be used to guide the design of artificial systems for energy conversion. Protons are important in myriad ways for solar fuel production, so the results of this study also have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.

This work was supported by a DOE grant to Gust, Moore and Moore and also involved SMS researchers S. Jimena Mora, Matias Villalba, Marley E. Tejeda-Ferrari, Paul A. Liddell and Brian R. Cherry. 

Ian Gould

President Professor, Associate Director of Communications, School of Molecular Sciences

480-965-7278

Propane pro: Ariaratnam named to national committee studying safety of pipeline infrastructure


July 10, 2017

Six million American households rely on propane gas to heat their homes and water, dry their clothes and barbeque their pork chops.

Farmers use propane to heat livestock housing and greenhouses, dry crops and power farm equipment and irrigation pumps. Samuel Ariaratnam, a construction engineering professor and program chair, was appointed for a 16-month term to serve on national committee studying the safety of the nation’s propane pipeline systems. Photo by Tim Trumble Download Full Image

Many businesses employ propane to power equipment ranging from forklifts to electric welders.

Transporting this fuel to homes and establishments across the United States requires an elaborate system of storage tanks and pipeline facilities. And since this volatile gas can displace oxygen to cause asphyxiation and is highly flammable and explosive, the safety of these systems is an imperative focus.

If the gas is allowed to pool in an area, a discharge of static electricity can be enough to cause an explosion.

Samuel Ariaratnam, construction engineering professor and program chair, was recently appointed for a 16-month term to serve on a United States’ National Academies of Science, Engineering and Medicine committee studying the safety of the nation’s propane pipeline systems.

The National Academies is composed of the National Academy of Sciences, the National Academy of Engineering, the National Research Council and the National Academy of Medicine, and serves as advisers to government and public leaders.

Commissioned by Congress, the committee’s “Study on Propane Gas Pipeline Facilities” is part of the Protecting our Infrastructure of Pipelines and Enhancing Safety Act of 2016, known as the PIPES Act, signed by former President Barack Obama in June 2016.

The PIPES Act renewed the Pipeline and Hazardous Materials Safety Administration, the federal government’s pipeline safety overseers within the U.S. Department of Transportation. Agencies like PHMSA and the National Fire Protection Association play an important role in monitoring propane storage and transportation methods.

Delivering propane to consumers involves transferring the fuel to the tank’s site, safely transferring the fuel from the delivery vehicle to the storage tank, and storing the fuel on the site where it can be channeled to homes via service lines.

However, these pipeline systems greatly differ in size — consider the propane storage needs for isolated rural properties compared with a large metropolitan city. A problem arises when federal regulators don’t adapt their compliance protocol for smaller systems.

To this end, Ariaratnam and the committee will be studying the current regulatory framework and making recommendations on possible modifications to reduce the compliance burdens of smaller systems — defined as 100 or fewer customers — while maintaining top safety.

“We have an opportunity to conduct a thorough study and to make recommendations concerning regulations, techniques and practices for propane pipeline systems,” Ariaratnam said.

Additional goals include reviewing techniques and best practices to ensure safe design, installation, operation and maintenance of these smaller systems; and examining the costs and benefits of the regulatory regime, as well as associated techniques and best practices.

The committee is composed of nine experts appointed by the National Academies. Members were drawn from academia, local and state government, and industry.

“It is important for academics to contribute to studies aimed at benefiting society. The opportunity to work with a diverse group of professionals is exciting,” said Ariaratnam.

From 2010-2011, Ariaratnam served on a National Academies’ committee and contributed to a book published by the National Academies in 2013 aimed at providing communities with guidance on underground engineering for sustainable development.

Propane-related fatalities, injuries and costs of property damage and loss have declined from 1987 to 2016. Ariaratnam is excited to play a role in continuing to decrease incidents and promote greater public safety across the country.

Rose Gochnour Serago

Communications Program Coordinator, Ira A. Fulton Schools of Engineering