 | Mark Johnson (foreground), holding a model of a water molecule, is pictured with graduate students (from left) Eric Diken, Joseph Roscioli and Joseph Bopp. |
The prestigious journal Science recently cited research on the structure and chemical behavior of water conducted by Mark A. Johnson, professor of chemistry, and his laboratory as one of the 10 most important discoveries in 2004.
While water is known as the fluid of life, it has not always been clear why that is the case. Studies published by Johnson and his colleagues this year focused on how protons and electrons dissolve in or are held by water molecules. It was one of several studies on the liquid highlighted by Science.
The editors of Science said the Yale researchers' 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," notes 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."
Johnson spoke with the Yale Bulletin & Calendar recently to provide some background about the chemistry and structure of water that explains the particular interest in the research published this year.
The mechanism for proton transport in water has been around for almost 200 years, and we are only now getting a molecular-level look at how it works. This charge propagation lies at the heart of many biological processes including proton conduction in vision and photosynthesis.
There is a lot of water in biology that is not very wet -- in biological membranes, for example. In this regime, each water molecule counts and 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.
Ever since the simple chemical formula for water became clear almost 200 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 that are denser than their liquids 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 100 years ago, a major cause for water's rigidity and high melting point was identified as the "hydrogen bond." This is an unusually strong interaction that occurs when a hydrogen atom is shared between two atoms that hang on especially strongly to their surrounding electrons. It's what holds DNA together in a double helix. It's very important because a lot of H-bonds can hold the DNA strands together very tightly until the genetic information is needed. But, they are also individually weak enough that you can "unzip" the H-bonds to get the two strands apart by snapping them one-by-one from the end. It's like molecular Velcro.
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.
When you put a lot of water molecules together, this cooperation phenomenon acts to enhance the stickiness of the H-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.
A group of us around the world are finally establishing the dynamics of water on the molecular level, and how the underlying mechanics of H-bonding control important processes like charge conduction and accommodation.
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 H-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 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.
My students and I are euphoric about the selection of our work among the top research results in the world. 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.
-- By Janet Emanuel
T H I S
Yale scientists hailed for research on H20
For those of us who are not chemists, how can we understand your research?
Why study water? What is so special about water?
But what is this "molecular Velcro?" How does it work?
There were a number of important studies of water this year, how do they fit together?
How do you go about studying these bonds?
What have you discovered?
How does it feel to have your work cited as one of the year's top discoveries?
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