To anthropomorphize like crazy, let’s say the five chemical groups (5-4-3-2-1) are spectators at an outdoor concert. They are all wearing lap robes. A breeze lifts the lap robe off of 3. Number 2 is eager for another lap robe, so he steals it away, winding up with two lap robes. Number 1 is even more eager and steals the extra lap robe from 2. In the meantime, poor number 3 has lost his robe and is cold. A generous soul at his left, number 4, donates his robe. Number 5, being even more generous, gives 4 his robe. Now number 5 is robeless (positive charge) and number one has an extra robe (negative charge).
In the parlance of chemical graffiti, the lap-robe shuffle would look like this, with light energy moving left to right, and electrons moving left to right to neutralize holes left behind by donated electrons:
(*)= excitation of energy, (-)= extra electron, and (+)= hole left behind by a donated electron.
Step 1. Light excites Pzn
C Pzn* P Q Q
Step 2. Energy transfers from Pzn to P
C Pzn P* Q Q
Step 3. An electron transfers from P to Q
C Pzn P+ Q-Q
Step 4. An electron transfers from Pzn to P
C Pzn+ P Q-Q
Step 5. An electron transfers from C to P and Q to Q
C+ Pzn P Q Q-
Because the shape of a molecule and its interactions with neighbors determine just how likely it is to donate or accept an electron, pentad builders have a variety of “knobs” they can tweak to increase the rate of electron transfer. They can change the chemical structure of the molecules, their distances from one another, or even their interactions with the surrounding medium, which at this point is a liquid solution. Someday, Neal Woodbury speculates, they may even be able to embed the pentad in a membrane, surrounded by protein scaffolding that will further speed or slow down the transport of the electron.
The trick to pentad tweaking is to use a light touch, says Gust. “You don’t want to have the energy differences between steps too great, because with each step a little of the initial sun-energy that came into the system is lost. Too large a drop would mean the loss of too much energy. Instead, you want a shallow series of steps, each one dropping down only a little in the energy landscape. Say you begin with two volts from the sun. In our best efforts, we were able to keep a full fifty percent of the energy from each photon we put in. Two volts in, and at the end of the sequence, there is still one volt left to do work. That’s right up there with photosynthesis.”
By building the pentad, the ASU team proved an important principle. They showed that if you can get charges to travel far enough apart in space via steps that prove more seductive than the natural urge to recombine, those charges will stay separated for a long time. They further showed that if you make those steps shallow enough, you’ll buy yourself energy as well as time. The question is, energy and time to do what?
SPANNING A MEMBRANE: CATALYST WITH A POWER PACK
None of the scientists was too anxious to talk about applications. After their last piece of publicity (an article in Discover magazine), they began to get calls from people wondering when they would be able to buy molecular batteries at Wal-Mart. The team at ASU is quick to emphasize that their research is squarely in the basic realm, and they’re happy to leave the actual application work to engineers. “At the center, we are much more interested in perfecting our understanding of nature’s mechanisms than we are in building devices,” says Gust for the record.
Yes, I say, but if someone, someone else of course, were to build something, what might it be? Disclaimers out of the way, we begin to blue-sky.
None of them thinks that rooftop photovoltaics will be made out of pentads any day soon. In their current form, high temperatures may wilt them and cold temperatures may freeze them, so they’re unlikely to last twenty years on your roof. What about lichens, I ask, which contain algae able to photosynthesize at subzero temperatures, or desert plants able to survive quite nicely in the hellish temperatures of Death Valley? The difference, Gust reminds me, is that living plants can replace spare parts when they wear out. No matter how similar its function, a pentad can’t do that. Besides, if trends continue, he says, silicon photovoltaics are likely to keep coming down in price to the point where it might actually be economical to have them on your roof.
What sets the pentad apart from silicon cells, however, is its size—at eighty angstroms, a pentad is a very tiny double-A battery that is activated by light. In a world where machinery is fast approaching the molecular scale, there will be plenty of call for vanishingly small batteries. If you could find a way to hook them to a grid, I suggest, you could pour billions of pentads into a can of paint and layer your house with sun harvesters! Or paint the highway system with them! “Try doing that with a rooftop photovoltaic,” laughs Moore. Then he arches an eyebrow, looks both ways, and leans toward me. “Do you know what will be really amazing? When we find a way to get this thing to embed in an artificial membrane. Then we’ll be cooking.
“What we have now is essentially an electron transfer device,” he explains. “What we want to do next is what photosynthesis does next, which is to convert charge separation into membrane potential [he never misses a chance to bring this up]. To do this, we have to design an artificial cell, put the molecule in the membrane, and shine light on it. If we can do that, we will have converted light into a voltage across a membrane. Then we can make use of any of the biological paradigms for using potential. Pumping ions, making ATP (the gasoline of life), importing sugars—anything biochemistry does with potential, we can do once we learn to incorporate molecules into membranes.”
Scientists already know how to make an artificial cell—they put lipids (the molecules that make up cell membranes) in water and shake them up so that they self-assemble into watery spheres called liposomes. If Gust and the Moores could install their molecule in the skin of one of those bubbles, along with the toadstool-shaped coupling factor that makes ATP, they could shine light on it and make the fuel of life. “Just think,” says Tom Moore. “We would demonstrate the production of ATP in a light-driven system.”
What to do with it? Moore sighs. “Well, first, I’d stand back and admire it for a long time. Then I suppose we could mimic an uphill reaction that needs energy—like the assembly of a protein. Put in everything a cell needs to make proteins—a ribosome system, DNA, amino acids—and then shine a light on it and see if it will crank out a piece of protein, like insulin.” Right now insulin is made by genetically engineering E. coli bacteria. The day may come when we could dispense with the bacteria that have to be fed and kept at certain temperatures and instead have tiny nonliving factories—sacs with power packs in their skin. Being fearful of genetic tinkering, even when it’s done with E. coli, I like this alternative.
Ana Moore, the engineer, thinks of logistics. “Right now our pentad is too long to fit lengthwise across a membrane—the membranes have room for something thirty angstroms long and the pentad is eighty. For membranes, our best shot may be the shorter triads, but first we’ll have to improve yield and charge-separation times. Then we have to get the molecule to recognize the membrane, enter it, and line up in the right direction. Of course, we’ll have to deal with the interfacial relations between the triad and the proteins it will encounter in the membrane layer—right now it’s just floating in solution.” As she speaks, I see her mind whirring, plugging words into the grant application.
“Come back next year,” Tom Moore teases. “We’ll show you how it works.”
Neal Woodbury likes to imagine how chemistry might change even without a membrane, if we could find a way to hook pentads to catalysts, those workhorse proteins that float around inside cells, joining molecules together and splitting them apart. Like spot welders, catalysts work with amazing specificity, honed over eons of evolution.
Biochemists have a whole arsenal of nature’s catalysts that they can take off the shelf, compounds like DNA polymerase that zips along DNA, making thousands of copies. These
biochemical reactions are for the most part thermodynamically downhill. You just mix in the catalyst, and the reaction proceeds without enormous inputs of energy. Biochemistry is like that.
Unfortunately, a lot of the chemicals and pharmaceuticals we manufacture are uphill reactions, which we have to coerce with strong chemical baths, high heat, and extreme pressures. What if instead of bulk chemistry consisting of forty or fifty steps, you could go to the shelf and pick out a designer spot welder (a catalyst) with its own power pack (a pentad)? You could mix it with precursors A and B and hit it with light, and it would do uphill reactions, forming AB for you with the kind of specificity that nature achieves. In this way, we would be able to build chemicals efficiently and cleanly, in water, using sunlight as the energy source and producing no noxious byproducts. Now there’s something we could stand back and admire for a while.
HYDROGEN DREAMS
Finally, if we are to mimic a green plant’s real planetary coup, we must find a way to use the light of the sun to run a chemical reaction that would net us a storable, high-energy fuel. With all due respect to plants, sugar and starch are not what we humans had in mind (plants already do a fine job of making those for us). What does interest us is the possibility of producing hydrogen gas from sunlight and water.
Hydrogen is the world’s cleanest storable fuel—it can be derived from water, and when you burn it, you release pure water again. Hydrogen is also the fuel of choice in fuel-cell technology. Fuel cells are portable devices that take hydrogen gas and use it to generate electricity, right in your car, for instance. At this point, fuel-cell technology is still an elusive goal—no one can get the chemical reaction to work for more than a few hours. If and when the barriers are overcome, the demand for hydrogen gas will be immense.
The alchemy needed to “crack” water and extract hydrogen gas does not look difficult on paper. Nature does it all the time with the help of an enzyme called hydrogenase. Hydrogenase takes hydrogen ions (H+) and, with the addition of electrons, makes H2 gas, which can be bubbled out of the solution. Photosynthesis produces all the needed ingredients. It releases hydrogen ions from water and shuttles electrons into the hands of NADP+, which becomes the electron carrier NADPH. As long as we have hydrogen ions and this constant source of electrons, we should be able to add hydrogenase and collect our H2 gas for free, right? Unfortunately, it’s not that simple. Hydrogenase is not comfortable in the presence of oxygen, and after a few hours of pumping out a product, it is overcome by oxygen, and the reaction grinds to a halt. Technology watchers predict that it’s just a matter of time, however, before someone perfects the side reactions. When they do, the world will come looking for a sun-harvesting power pack to provide the charge separation. Chances are the pentad, or an even newer and improved model based on the reaction center, will be on the short list of candidates.
COMPUTING AT THE SPEED OF LIGHT
In the meantime, the most likely application on the horizon is a marriage that’s hard to picture: technology mimicked from the world’s most ancient organisms breathing life into a brand-new generation of computers. These organic-silicon hybrids, sporting switches the size of a molecule, will make Pentium PCs seem as plodding as the vacuum-tubed ENIAC from the fifties.
Today’s computers use a series of switches to store and transmit electronic bits—the zeros and ones of digital code. The switches act like those in a railroad yard. They open to let trains of electrons pass through whenever they receive the right signals. Conversely, some switches can be shut down to block the flow of electrons. What most of us don’t realize is how slow and labored this process really is—with a linear series of switches, the computer can do only one calculation at a time, in sequence. Computers of the future will be more like brains—they will have three-dimensional webs of switches. The signals, instead of traveling via electron flow, will be encoded on light waves traveling at, well, the speed of light. Say you want to send the Encyclopaedia Britannica—all thirty or so volumes—from Boston to Baltimore. If you send it on today’s copper wires and squeeze it into your computer’s 28.8-baud modern, it would tie up your phone lines for half the day. That same transmission sent via light waves in a hair-thin optical fiber would show up in less than one second.
To equip these optical wunderkinds, technologists will need light-sensitive switches, the smaller the better. A device like the pentad, which changes its charge distribution (how its electrons and holes are positioned) in response to a certain frequency of light, makes an ideal switch. Hit it with light, and the negative and positive charges will zip to opposite ends of the pentad. When the switch is in this charge-separated state, it actually changes shape and therefore absorbs light from a different part of the light spectrum (the moodring phenomenon). This means that the pentad can be controlled—it can be flipped back and forth from a state in which it absorbs only red light, for instance, to a state in which it absorbs only green light. In computer lingo, those states are called off and on, zero and one.
Gust and Tom Moore have been daydreaming publicly about the possibility of installing pentad switches by the millions in a durable material. Their articles in computing journals outline the specifics of molecular “OR” gates and “NAND” gates. Here’s one scenario: In its charge-separated state (C+ Pzn P Q Q-), the pentad will absorb light at wavelengths measuring 960 nanometers (nm). In a switch where light is hurtling through at 960 nm, a charge-separated pentad would block that light by absorbing it and stopping its transmission. Essentially, it would switch off the flow. Conversely, a pentad in its relaxed state wouldn’t absorb the 960 nm light and would therefore let it pass. We could toggle these molecular switches from a relaxed state to a charge-separated state by hitting them with a preparatory pulse of light, essentially opening or closing the gates to the transmission of bits and bytes.
This last application may seem far from the inspiration of photosynthesis, until you remember that by finding a new application for the machinery of photosynthesis, we are being the ultimate biomimics. “Nature is famous for retrofitting an existing technology to accomplish many different things,” Tom Moore reminds me. With a few modifications, he says, the same mechanism that turns carbon dioxide plus water plus energy into sugar and oxygen is simply run in reverse whenever we eat a salad or a stroganoff. We take sugar and oxygen and break them down to energy, carbon dioxide, and water. What these mirror reactions have in common is what cuts across the plant and animal kingdoms: the miracle of membrane polarization. In fact (I’m sounding more like Tom Moore every minute), it’s a common theme in all biological functions, including thinking. As you read this sentence, the membrane potential in your nerve cells is helping you send signals, process information, and, in short, compute. Suddenly, playing a game of Tetris on an artificial photosynthesis computer doesn’t seem so strange—it’s just another acorn that has fallen, and not all that far from the tree.
Later that week, thinking about all this as I make my way among the Anasazi ruins that Devens Gust had turned me on to, I begin to smile. After all these years, we are only now looking to leaves as a source of inspiration. Unlike the Anasazi, we have built too many of our labs facing the wrong direction, away from the sun. “I hope you get your grant,” I say out loud, and then I lean back against the circular wall of a ceremonial kiva, and fall asleep in the soft rays.
PHOTOZYMES
Months later, when I mention ASU’s pentad light-harvesting efforts to James Guillet of the University of Toronto, he nods his head. “They’re impressive, and they work well.” A polite silence. “As long as you have something to plug your laser into. But what happens when you walk outside and hold them up to ordinary, northern Canadian sunlight? Can you get an electrical current? Even better, can you get fuel out of them? That’s what I want to do.”
And to do that, Guillet is tackling a different part of the photosynthetic machinery. While Gust and Moore are modeling the reaction center, Guillet attempts to build what he thinks every reaction center will
ultimately need: a way to get the diffuse drizzle of sunlight to hit home. In plants it’s done with pigment antennas, and if Guillet succeeds, the ASU team may be able to marry a full-fledged antenna to its pentad and do chemistry in water. But that’s getting ahead of the story.
For now, Guillet has good reason to be seeking his own part of the artificial photosynthesis Grail. He lives in a cold, dark country that uses more energy per capita than any other in the world. Because sunny days are not terribly plentiful, Guillet is interested in finding a way to wring a storable fuel out of the sun, something to burn during those winter months—something like hydrogen. Though he can be suspenseful about his plans, I think he may be onto something. His track record—papers, awards, patents, businesses—speaks of a man who lets no moss grow on a good idea.
Though he retired from teaching several years ago, Guillet maintains an office at the University of Toronto and still comes in regularly. Here he keeps one foot in academia and another in private industry, where his career began. “I was trained in the private sector, where practical applications were king,” he tells me. “But when I transferred to the university, they wanted me to give all that up.” It was in an era when the real prestige in science lay in the field of physics, where elegant theories and unifying concepts were badges you could shine. Deep in the throes of “physics envy,” as it’s called, the head of his department actually told him he shouldn’t be producing anything patentable. To Guillet’s credit, he flatly ignored the advice and has been patenting inventions and fledging companies ever since.
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