Genesis: The Scientific Quest for Life's Origins by Robert Hazen

Author:Robert Hazen [Hazen, Robert]
Language: eng
Format: mobi, pdf
Tags: Explore Science
Publisher: National Academies Press
Published: 2005-09-08T16:00:00+00:00

The commonest crystal shape of the mineral calcite is the so-called “dogtooth.” All 12 faces of this form are chiral.

I wanted to find out whether chiral crystal surfaces would selectively attract chiral molecules. I had fist-sized calcite crystals with left- and right-handed faces, and I had D- and L-amino acids. Would D- or L-amino acids attach themselves preferentially to left- or right-handed calcite faces?

Seventy years of false claims and ambiguous results had set the bar high; a casual, sloppy experiment wouldn't do. I knew that many previous authors had started with concentrated D- or L-molecules, and then looked for a difference in behavior as those molecules interacted with a chiral crystal (usually quartz). That experimental path was fraught with difficulties, since every nook and cranny of the environment is already contaminated with life's excess L-amino acids and excess D-sugars. If you look for a chiral effect, you're probably going to find a chiral effect.

Instead, I prepared a 50:50 solution of D- and L-aspartic acid, an amino acid known to have special affinity for calcite in the proteins that add strength and resiliency to all sorts of shells. The idea was to expose both left- and right-handed calcite faces to this mixed solution, and then see which molecules stuck to which face. If crystal faces really do have the ability to select chiral molecules, then left- and right-handed calcite faces should attract equal and opposite excesses of L- and D-molecules.

The experiment sounded easy in principle, but it took months to get right. First we had to figure out a way to clean the crystals, which had been handled by dozens of people, from Tennessee miners to Geophysical Lab scientists. The slightest contamination—a fingerprint, a breath, even a speck of dust—could skew our results. We settled on a procedure of repeatedly washing each crystal in a sequence of strong solvents.

In the first failed round of experiments, postdoc Tim Filley (now a professor at Purdue) and I glued little plastic reservoirs directly onto the calcite crystal faces in hopes of minimizing contamination from dust, bacteria, and other chiral influences in the air. After some practice, we got the little plastic pieces to stick to the smooth calcite surfaces. What we hadn't realized is that our epoxy glue and our plastic containers were absolutely loaded with chiral molecules, which are part of the plastic-making process. Our careful procedure introduced far more chiral contamination than the atmosphere ever could.

Simple experiments are usually best, so I resorted to what seemed like an almost childish approach. I poured the 50:50 aspartic acid solution into glass beakers and just dunked the lustrous fist-sized calcite crystals into the liquid. Twenty-four hours seemed like a good amount of time to let the crystals soak. I came back the next day, removed the crystals, washed them off quickly in pure water, and then ever so carefully extracted the attached molecules from the crystal surfaces. It was an easy task, because calcite dissolves readily in dilute hydrochloric acid. I held a crystal in my gloved left hand and oriented one face parallel to the bench top.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.