Thursday, December 02, 2010

Arsenic, Phosphorus, Not Such a Big Difference

I'm not as impressed as I guess I was supposed to be with this news, and certainly not as impressed as the New York Times headline-writer.
Subsisting on Arsenic, Microbe May Redefine Life
Yes, we don't like arsenic because, as Dennis Overbye nicely explains in that article, it's a lot like phosphorus chemically, phosphorus is essential to life as we know it, and arsenic is enough not like phosphorus to get into its place and bollix things up.

However, poultry farms feed arsenic to chickens, which seems to improve their growth. Lots of things, like selenium, which had a run in health-food stores a while back, may be essential to life in small amounts and damaging in larger amounts. That's even true of iron. So the "poison" label on arsenic isn't that important chemically, although it has a lot of resonance in literature.

But the reason I'm not impressed is that I've seen something very much like this before, up close and personal, my very first introduction to chemical research. Or official chemical research, anyway. I did a bunch of things earlier on my own.

I worked in Harold Strain's laboratory at Argonne National Laboratory. Harold was one of the most skillful separator of plant pigments in the world. He used big (a couple of feet high by a few inches across) columns of packed confectioner's sugar. The plants were mashed up, extracted with light petroleum ether (boils in the palm of your hand), and adsorbed on the column, "developed" with additions of alcohols to the solvent, and then sculpturally scraped, sugar and all, out of the column by color and re-extracted from the sugar. The process typically gave a few milligrams of each pigment. Spinach was a good starter material for the standard pigments.

Those were the days when differences among isotopes were a big deal, nuclear magnetic resonance was new, and it looked like it might give more information about the processes by which chlorophyll converts light into chemical energy. If you had normal chlorophyll and chlorophyll with deuterium everywhere that normal chlorophyll had a hydrogen. Then you could use NMR to watch particular deuterium atoms exchanging with hydrogen atoms and get some of the kinetics.

Deuterium is an isotope of hydrogen. Whereas arsenic is below phosphorus on the periodic chart, deuterium shares hydrogen's box. But, because hydrogen is the lightest element, deuterium is approximately twice as heavy as hydrogen, a much bigger mass difference than between arsenic and phosphorus. This mass difference makes some difference in the chemical reactions of hydrogen and deuterium, although phosphorus and arsenic have greater differences. Further, deuterium is very uncommon, about 150 atoms in a million hydrogen atoms. So how does one go about obtaining chlorophyll with 72 deuterium atoms in place of its hydrogen?

The way it was done by Joseph Katz at Argonne was to grow algae in deuterated water, also known as heavy water, commonly used in certain types of nuclear reactors, and certainly, at that time, used in nuclear research. As Joe told it, the algae didn't like it at first, and got sort of fat and bulgy, a few died, but most eventually settled down and looked pretty much normal. But their chlorophyll was deuterated.

So Joe's lab grew the algae, and then dried and processed them to get a separable extract, which Harold then put on his sugar columns. The deuterated pigments were sealed into glass tubes under vacuum, to be opened for the appropriate experiments. We speculated on how much per pound those pigments were worth. A lot, but not many potential buyers.

Algae are bigger, with more enzyme systems and biochemical conversions than the arsenic-based bacteria. It's probably even up as to difficulty for the little guys to survive in such alien media, though. Which aren't nearly as alien as what other planets have to offer. They're still carbon based. And some of us have seen something very much like that before, although it was a long time ago.

If you google "algae deuterium oxide", you'll find a fair number of publications from Katz's group. Apparently Argonne is still growing stuff in deuterated water. And I think there's a black and white video of me talking about what I was doing in Harold's lab somewhere out there on the intertubes, although I couldn't find it with a quick google.

Update: And we have to add in XKCD's contribution. (h/t to @jfleck)

Further update: More here.


Ed Gaffney said...

Neat stuff, Cheryl. But I think that As vs. P has to be considered a larger change than D vs. H. The atomic weight ratio is about 2 for each pair (slightly larger for As/P) but the AS/P pair has different electron structure, as well.

Cheryl Rofer said...

Thanks, Ed. I should have checked the atomic weights.

Totally agree that the addition of electron orbitals makes a bigger difference than the difference between isotopes.

Joel said...

It is true that ingestion of trace minerals can be essential to certain enzymatic processes (selenium) or toxic to rapidly growing, potentially cancerous cells (arsenic). However, in no previously demonstrated case has anyone been able to replace one of the six fundamental elements for life with another, non-essential element and still sustain life.

Members of the same periodic group share a lot of similarities, but in no way are they the same. This is especially true in biological processes. Sodium and potassium are both critical regulators of membrane potential and fall within the same periodic group, but are in no way biologically interchangable. They are transported through specialized ion channels, have different (often opposing) physiological effects, and have dramatically different pharmacology (injecting sodium chloride, otherwise known as saline, is completely harmless, whereas injected potassium chloride, a component of the lethal injection mixture, will totally kill you).

You really have to get into the nitty-gritty of the discovery to see what's so fascinating about it. Phosphorous is an essential component of signal transduction, transcription and translation. Think about it this way; without phosphorous, in every case but this one, there's no DNA. That's a huge advancement in biochemistry, however you slice it.

MT said...

Huffington ran it front page on top with a photo from space and crediting NASA for discovering a new form of life and I ran to the radio before reading more. Of course NPR had more important things to talk about like FIFA's assignment of the next two World Cups and when I read their gloss on the press release from NASA I wanted to scream. I bookmarked the Daily Beast and hope never to read Huffington again. I'm sure I would have to read 18 column inches of Overbye before I appreciated what's so cool about this, if anything. Does it suggest that no important enzymes act directly on phosphorous atoms, or else their kinetics would be so different targeting arsenic that the bugs would be like mutants or even nonviable? Wait, did the researchers even look for such effects? How do they even know to what extent they've replaced all the P's with As's, and are they really saying it's 100%? I don't care enough to look.

MT said...

I suppose from the astrobiology angle, subbing As for P could look like another kettle of fish from subbing D for H, because H is everywhere and D is supposed to predominate nowhere, while arsenic could be abundant where phosphorous isn't, suggesting life could be places we haven't bothered looking...until now. I don't know how much that would matter to the methods with which we've looked so far, or could have looked, if we'd had more respect for arsenic.

MT said...

i.e. not much, I'm guessing.

Cheryl Rofer said...

Joel, I agree that it's intriguing. But I am wondering about some of the questions mt raised, particularly about the DNA. Have all the P's been replaced by As's? That's going to be interesting DNA. Is it reproducing itself normally?

And I just like to see credit given for earlier work. Some of the articles sounded like the approach was to substitute As nutrients for all P nutrients. That's entirely analogous to substituting deuterated water for normal water.

Anonymous said...

They found their bacteria in Mono Lake, which, like all the (remaining) lakes in the great Basin desert is highly saline. However "merely" ~10,000 years ago, these lakes were not saline. Are the biologists suggesting that the bacteria evolved an arsenic-accepting metabolism during the Holocene? Or are they saying something else???

Cheryl Rofer said...

More here.

Joel said...

They were able to subsist on an arsenic-free diet and show a lack of radiolabeled (radioactive) phosphorus incorporation, so the demonstration of arsenic substituion is by inference. I don't have the access to AAAS at home, but it's in the current issue of Science if you want to read the primary literature. Of all the academic journals, Science is probably the most approachable for the layman.

Joel said...

Excuse me, phosphorus-free diet.