Geology Reference
In-Depth Information
Perhaps the most daunting challenge in molecular selection is chirality, the pervasive
“handedness”oflife.Manyoflife'smoleculescomeinmirror-imagepairs—left-andright-
handed variants like your two hands. Chiral pairs of molecules are identical in many re-
spects:theyhavethesamechemicalcompositions,thesamemeltingandboilingpoints,the
samecoloranddensity,andthesameelectricalconductivity.Butleft-andright-handedmo-
lecules have different, incompatible shapes—a familiar characteristic if you've ever tried
to put a left-handed glove on your right hand. It turns out that life is incredibly picky: cells
almost exclusively employ left-handed amino acids and right-handed sugars.
Chirality matters. In the curious case of the artificial fragrance limonene, the right-
handed form smells like an orange, whereas the left-handed version of this simple ring-
shapedmoleculesmellslikealemon.Thesmellreceptorsinyournosearesensitivetochir-
ality,soright-andleft-limonenetransmitslightlydifferentsignalstoyourbrain.Tastebuds
are less sensitive to the differences between right- and left-handed sugars. They both taste
sweet, butourbody'sfine-tuned digestive system can process onlythe right-handed forms.
The artificial sweetener tagatose, a zero-calorie left-handed sugar substitute, exploits these
properties. The tragic story of thalidomide also rests on handedness. The right-handed ver-
sion of this drug alleviated morning sickness in pregnant women, but the left-handed vari-
antthatinevitablytaggedalongcausedbirthdefects.TodaytheFDAimposesstrictrequire-
ments for chirally pure drugs—regulations that save lives but cost consumers an estimated
$200 billion per year in added manufacturing costs.
Most experiments that synthesize biomolecules (including Miller-Urey and hydrotherm-
al experiments) produce equal amounts of left- and right-handed molecules, and most nat-
ural processes treat left- and right-handed molecules exactly the same. Indeed, the nonliv-
ing natural world is for the most part indifferent to the distinction between left and right.
But life absolutely requires the correct shape: left-handed amino acids and right-handed
sugars are essential. The opposite-handed molecules simply will not do. So our research
team tackled the question of how life selected left-handed amino acids almost exclusively
over right, and right-handed sugars over left.
Our recent experiments have explored the possibility that chiral mineral surfaces played
the starring role in selecting handed molecules, and perhaps in the origins of life as well.
In 2000 my colleagues and I realized what was then surprising but is now obvious: chir-
al mineral surfaces are everywhere in nature. The commonest minerals in every rock and
every soil abound with surfaces where atoms form molecular-scale “handholds,” some left
and some right. In the natural world, these left- and right-handed mineral surfaces occur
in statistically equal proportions, so Earth on a global scale doesn't appear to be biased
for either left or right. But each individual molecule does care where it winds up. Our ex-
periments showed that certain left-handed molecules can aggregate on one set of crystal
surfaces,whilethemirror-image,right-handedcounterpartmoleculesaggregatejustaseas-
