Alien pizza, anyone? Biochemistry may have taken a different turn on other worlds

Science News,  August 18, 2007  by Davide Castelvecchi

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Everything was ready for the celebratory feast. Weeks earlier, the alien fleet had entered Earth's orbit and made radio contact, and now the visitors would receive their official welcome. Dozens of heads of state would greet humanity's guests during an official dinner at the White House.

The aliens looked remarkably similar to us--apart from their green scales, that is. Moreover, the chemistry of their bodies and of ours was also very similar, scientists from the two worlds had concluded. Sure, the alien cells produced molecules not found in human biochemistry, but the building blocks of those molecules were essentially the same carbon-based amino acids and sugars as the ones in our bodies.

But on the eve of the great meeting, the science adviser to the President burst into the Oval Office. "The dinner must be called off, or the aliens might die!" the adviser told the startled President. "We forgot to check their chirality!"

That's a word that few politicians--indeed, few people outside science--will know. But chirality, or handedness, is an essential characteristic of the molecules of life. Most naturally occurring organic molecules are chiral, meaning that they are distinct from their mirror images in the same way that our right and left hands differ.

In the lab, chemical reactions that synthesize amino acids and sugars create the right- and left-handed versions of the molecules in equal amounts. Life on Earth, however, uses one version almost exclusively, preferring what are conventionally called right-handed sugars and left-handed amino acids. A molecular preference for only one handedness is what chemists call homochirality.

In principle, organisms could exist that use both kinds of molecules or that exclusively adopt the opposite forms from those used in life on Earth. This is what troubled the scientists in the alien state dinner scenario: Just as our bodies can't absorb organic matter of the wrong handedness, so too the aliens might find our food equally indigestible--and perhaps even toxic.

Most scientists believe that Earth life's choice of chemical handedness was purely random. "The most plausible idea is that it was an accident; says biochemist David Deamer of the University of California, Santa Cruz. It's possible, then, that the chemistry of some alien forms of carbon-based life--assuming such things exist--may well have the opposite chirality to ours. We might find alien pizza even harder to digest than deep-dish.

On the other hand, a few scientists say that something more fundamental might be going on. They argue that throughout the universe, nature might consistently choose one handedness over the other. That intrinsic preference, these scientists suggest, might originate from the influence of the weak nuclear force, the only fundamental force of nature that can tell left from right. In recent years, a number of experiments have provided tentative--if controversial--support for this proposal.

Whatever its origin, most scientists agree that homochirality must have begun with a tiny excess of one handedness in the chemistry of the primordial broth. Recently, chemists have discovered a mechanism that might explain how that slight imbalance could have developed into a complete predominance of one handedness, with the virtual absence of the other. Homochirality itself may not be so surprising after all--although knowing which of the two possibilities occurs on any given planet remains a toss-up.

CHANCE AND NECESSITY There is some reason to believe that nature favors homochirality: It takes extra metabolic energy to keep molecules of different handednesses separate, says Jack Szostak, a biochemist at Harvard University. "There is a selective pressure to have a system that is basically homochiral," he says. However, the laws of physics provide little reason to think that the original choice of biochemical chirality could have been anything other than random. Chemistry is ruled by electromagnetic forces--which determine how atoms, molecules, and electrons interact--and electromagnetism doesn't distinguish between left and right. For that reason, a reaction between, say, two left-handed molecules should proceed at exactly the same rate as the same reaction between the right-handed equivalents of those molecules.

Then how did our world end up with homochiral biochemistry?

Scientists have proposed various mechanisms that might have generated a small initial excess of one handedness over the other (SN5/5/01, p. 276; 9/6/03, p. 157). For example, right- and left-handed molecules respond differently to circularly polarized light, whose waves move in a corkscrew fashion that can have either a right- or a left-handed twist. If the organic matter that seeded life on Earth formed in space, as some scientists have suggested, astrophysical sources might have zapped it with circularly polarized light, perhaps selectively destroying molecules with a particular handedness.