Yale Chemist's Research in Science Top 10 for 2004

Research by Yale professor of Chemistry, Mark A. Johnson on the structure and chemical behavior of water was highlighted as one of the ten most important of discoveries in 2004 by the preeminent journal Science.

While water is considered the fluid of life, scientists are only now unraveling how intricately its unusual properties are integrated into essential biological processes. Studies published by Johnson and his colleagues this year focused on the mechanism of charge transport in water. They specifically determined how protons and electrons are held when only a few water molecules are available, a situation common when charges are transported through biological membranes.

“A group of us around the world are finally establishing the dynamics of water on the molecular level, and how the underlying mechanics of hydrogen-bonding control important processes like charge conduction and accommodation,” said Johnson. “My students and I are euphoric about the selection of our work among the top research results in the world. We started and nurtured a way of studying water on the small scale, with only 10 or so molecules frozen in a tiny crystal. These are just big enough to see how water works from the bottom up. To see this approach yield significant results is very satisfying - and we are not done! We are feverishly working at the moment on how acids dissociate upon contact with water. There will be lots of surprises to keep us on our toes.”

According to the editors of Science, their results on the structure and chemical behavior of water “could reshape fields from chemistry to atmospheric science.”

“Over the past few years the Johnson group has been doing outstanding work to define the structure of water when an extra electron or proton is added. This is of fundamental importance for processes in aqueous solution that occur in biological systems,” noted Gary W. Brudvig, professor and chair of the Department of Chemistry at Yale. “I’m very pleased that this work is gaining the recognition it deserves. It is a reflection of the strong program in physical chemistry that we have at Yale.”

Citations: Science 306: 2010-2017 (December 17, 2004)
Science 304: 1137-1140 (May 21, 2004); published online 04/29/2004
Science 306: 675-679 (October 22, 2004); published online 09/16/2004
Yale Press Release: http://www.yale.edu/opa/newsr/04-04-30-03.all.html

An interview with Mark A. Johnson follows:

Mark A. Johnson, Yale professor of Chemistry, provided some background about the chemistry and structure of water that explains the particular interest in the research published this year.

While most of us take water for granted and think little about how unusual it is as a chemical substance, our world would be vastly different if its rules were the norm rather than a singular exception. Professor Johnson gives us a new perspective on this very familiar substance:

For those of us who are not chemists, how can we think about this?
Many biological processes, including proton conduction in photosynthesis and vision, rely on charge propagation through membranes. This is an unusual environment because the water there is not very wet. That is, there is not much of it and each water molecule counts - each plays a unique role to execute a function. It is not just a supporting medium; it is an intrinsic part of the process. We are sorting out the rules that govern how a particular environment enables a water molecule to play one of these roles.

Why water - what is so special about water?
“Ever since the simple chemical formula for water became clear almost two hundred years ago, its peculiar behavior has confounded chemists. The properties that make it unusual are pervasive! For example, ice floats on water, while most solids are denser than their liquids and fall to the bottom. And it takes a lot of energy to melt ice, but most molecules as small as H2O (like methane, CH4) remain weakly interacting gases until they reach very low temperature. You may have noticed that it takes a long time for ice to melt, even when the temperature is clearly above freezing.”

“About one hundred years ago, a major cause for its rigidity and high melting point was identified as the ‘hydrogen bond.’ This is an unusually strong interaction that occurs when a partially positively charged hydrogen atom becomes strongly attracted to an oxygen atom on another water molecule. It’s what helps DNA molecules come together in just the right way to make the double helix, like a lock and key. The DNA strands are held together very tightly until the genetic information is needed. But, the hydrogen bonds are also individually weak enough that they can “unzip” to allow the two strands to separate by snapping them one-by-one from the end. It’s like molecular Velcro.”

But what is this “Velcro”? How does it work?
“We now know that this property is intimately connected with how water molecules break apart and reform, exchanging their hydrogen atoms. One of the key modern questions is whether the electrons that belong to two nearby water molecules are shared between the two of them or whether they stay put on one. In the past 20 years, physical chemists learned to freeze just two molecules together and study the hydrogen-bonding interaction in exquisite detail. So you might think that solved the question. But, the plot thickens when you start putting many water molecules together. They have a way of cooperating with each other so that one feels the effect of another that may be separated by several intervening water molecules.”

There were a number of important studies of water this year, how do they fit together?
“When you put a lot of water molecules together, this cooperation phenomenon acts to enhance the stickiness of the hydrogen bond, and this enhancement is extremely sensitive to the local orientation of the interlocking water molecules. When the arrangement is just right, the electrons can intermingle but when the molecule jostles just a bit, its electrons settle back down. Two of the papers cited along with us worked on just how the electrons in the water molecules begin to be shared depending on orientation. One of these papers even argues that everybody has been wrong about the hydrogen bonding character of water, and that there are only half as many of them at any given time as people thought. That conclusion is still controversial.”

What does your research show about how these bonds work?
“What we have done is to bring water molecules together in small ‘clusters’ or tiny ice crystals. The shapes of these nanocrystals naturally explore most of the arrangements at play in water, and we can look at how distorted each water molecule becomes when placed in a particular location within an extended network. When you work with water, everyone is at first amazed at how ‘stretchy’ it is. Water molecules do not act like little tinker toys hard-linked together. The hydrogen atoms are very elastic so each water molecule actually looks quite different depending on how it is placed in the crystal.”

“This also makes it extremely difficult to predict the properties of real water with simple theoretical models. Problems show up immediately in any situation when there is only a little bit of water, like the environment around proteins or at the interface of sea salt in aerosol particles near the coastlines. At the molecular level, the water molecules just at the surface are not like the ones deeper in the droplet. And these surface water molecules are the ones that are important in ozone-related chemistry.”

“The work cited in Science, involved putting fundamental charges-protons and electrons-onto these small networks and monitoring how the fabric of the water molecules becomes disrupted to accommodate it. It’s as if the molecules decide among themselves how to make an interconnected structure in which one molecule is sacrificed to absorb the perturbation presented by an unwanted guest. This disruption is central to how charge moves through water.”

“Charges don’t just plow through a bunch of ‘spectator’ water molecules that give way to let them through. Protons, for example, simply shuttle between two water molecules, each time completely chemically changing the water molecule to which it is attached. The extra proton does not really move very far per se; but the excess charge can be displaced great distances when another proton is released at the end of a water ‘wire’ or chain, passing through a succession of transiently compromised water molecules. At a crude level, it’s like a five-ball executive desk toy – when you swing and drop the ball on one end, the ball on the far end pops out.”

“Right now we can only see the overall effect. What we are doing in our field is to identify the conditions that initiate such a hopping process, and to understand how long it takes to make each individual jump. To understand this, we need to push both our experiments and theoretical approaches to the next level. Happily, there is abundant enthusiasm in our young research team, which is comprised mostly of graduate students, to tackle these challenges.