H00102--00A, Front mat Genesis
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ment revealed a small preference for L or D molecules, there would be
no hope at all of determining which crystal surface was doing the
selecting.
I decided to try a different approach.
Big crystals are the key—fist-sized crystals with fine flat faces. That’s
the only way to understand the atomic-scale interaction between mol-
ecule and crystal. But what mineral fits the bill?
My crystals had to be big because a layer of molecules adsorbed
onto a surface an inch square weighs at most a few billionths of a
gram—a daunting analytical challenge. To measure that effect, I had to
find crystals with faces at least a few square inches in area. Big crystals
of most minerals tend to be astronomically expensive, thanks to the
voracious appetite of mineral collectors; so I had to find a common
rock-forming mineral that collectors don’t covet. As an added benefit,
the commonest minerals are also likely to be the most relevant to ori-
gin scenarios. Most important, the crystals had to possess faces with
surface structures that lack mirror symmetry. Only a chiral face could
accomplish the chiral selection task.
I thumbed through my favorite mineralogy book, a frayed, dog-
eared copy of Edward Dana’s A Textbook of Mineralogy, purchased at the American Museum of Natural History when I was 14. Classic line
drawings of crystal forms decorate almost every page. The vast major-
ity of crystal faces, I realized, aren’t chiral. Many of the commonest
minerals—garnet, olivine, mica, pyrite—won’t do.
Then I turned to page 512—calcite, an abundant mineral and the
one most closely associated with life. Calcite is the mineral of clamshells
and snail shells, of pearls and coral. Lo and behold, its commonest
crystal faces are chiral. Most calcite crystals feature a graceful, six-sided,
pointed form with the fancy scientific name of scalenohedron. Every-
body in the business calls it a dogtooth.
A quick search of eBay confirmed that dogtooth calcite crystals are
both common and cheap. I bid on three pretty specimens from the
then thriving (but now defunct) Elmwood lead-zinc mine outside
Carthage, Tennessee. A week and $40 later I had the beginnings of what
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The commonest crystal shape of the mineral cal-
cite is the so-called “dogtooth.” All 12 faces of this
form are chiral.
has become a sizable calcite crystal collection. Now to design an ex-
periment.
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 previ-
ous 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
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GENESIS
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 Geo-
physical 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 prac-
tice, we got the little plastic pieces to stick to the smooth calcite sur-
faces. 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 solu-
tion 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 care-
fully 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 par-
allel to the bench top. With my right hand, I used a long dropper to
cover that face with a thin puddle of acid. We were fortunate that water
beads strongly on calcite, so it was relatively easy to keep the acid from
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spilling over the sides. After about 20 seconds, I sucked the acid back
up into the dropper and emptied it into a waiting vial. We knew that
any attached molecules would have been stripped off along with the
outer layers of calcite.
Face by face, I applied the acid wash to each of four crystals—
more than 20 surfaces in all. Most of these faces were left- or right-
handed, but a few were flat, fracture surfaces—the natural breakage
planes of the crystal—that are not chiral at all. Those faces would serve
as a control on our methods, for they should show no preference for D-
or L-amino acids. After an hour and a half of concentrated effort, we
had 23 vials in hand. Would they show the desired effect?
In spite of the hassles with contamination, generating the amino
acid samples turned out to be the easiest experimental task by far. The
real trick, mastered by only a handful of analytical chemists in the
world, was to determine whether the calcite crystal faces had preferen-
tially selected D- or L-aspartic acid.
Tim Filley is an expert at organic chemical analysis, so we were
confident that we could do the work ourselves. We
had an adequate gas
chromatograph/mass spectrometer, the analytical machine of choice;
we had a chiral chromatographic column that should separate D- from
L-aspartic acid; and we had a well-documented procedure for prepar-
ing our samples prior to injecting them into the GCMS. The setup
took some time, but the methods seemed straightforward.
After a lot of practice with aspartic acid standards in different D:L
ratios and different concentrations, we were ready to try the real thing.
Our first analyses took place just before Christmas 1999.
GCMS affords a kind of rapid (if not instantaneous) gratification,
because the ratio of two compounds, such as D- and L-aspartic acid,
corresponds to the areas of two peaks that appear side by side on a
graph. Joined by Tim’s wife, Rose, who is also a skilled analytical chem-
ist, we injected our first sample and waited. It took the better part of
20 minutes for the molecules to work their way through the 50-meter-
long coiled chiral column, which was packed with chemicals that slow
down L-amino acids slightly more than D. D-aspartic acid passed
through the column about 20 seconds faster than L-aspartic acid. The
first peak showed up sharp and clear—plenty of D-aspartic acid mol-
ecules there. The next 20 seconds seemed to take a lot longer than any
20 normal seconds should, but the second peak, representing L-aspar-
tic acid, appeared on schedule. And it was clearly bigger—at a guess,
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GENESIS
more than 10 percent bigger. We allowed the machine to run just a few
minutes longer, to make sure there weren’t any obvious contaminants,
and then stopped to let the computer calculate the peak areas.
The actual ratio turned out to be closer to a 5-percent excess of L,
but it was a tantalizing result that clearly pointed to chiral selection.
We decided to run the same sample a second time, just to be sure.
Twenty minutes later, the pair of peaks appeared again, but this time
we saw no more than a 1-percent excess of L, not enough to be sure the
effect was real. The average value was a 3-percent excess, while our
experimental error was evidently close to ±2 percent. Tim suggested
that we run a couple of more standards to be sure the machine was
giving us clean peaks.
We were off and running, with 22 more samples, each to be pre-
pared and run in duplicate, plus standards after every few sample in-
jections. Given the prep time, it took more than two hours to test each
sample or standard. We started running in three overlapping shifts.
With two small girls, Rose was always up early and did the first few
injections. Tim and I usually couldn’t stay away, so we’d take over in
the morning and keep things running through the evening and into
the wee hours. For the first couple of days, we were there 18 or more
hours at a stretch, running sample after sample, eagerly watching the
screen as the pair of peaks appeared 20 minutes into each run. It was
addictive and exhausting, like a slow-motion video game.
So it went: Run a few samples with tantalizing results, then more
standards. One encouraging sample suggested a 4-percent excess of D-
aspartic acid—a critical result, since L contamination, the life sign, was
everywhere, but an excess of D could come about only through crystal
selection. Still, the results weren’t consistent. What was even worse, the
peak shapes began to broaden and degrade after only about 25 injec-
tions. Broad peaks overlap, making analysis of the D:L ratio all but
impossible.
We called the supplier of the chiral column to complain—a new
$500 column like ours should have lasted for hundreds of runs, not
just 25. They suggested that we bake out the column to make sure that
large, unwanted molecules hadn’t contaminated it. We had been turn-
ing off the machine a few minutes after we saw the aspartic acid peaks,
because we were eager to do the analyses as fast as possible. Perhaps
other slower molecules hadn’t made it all the way through the column
and were gumming up the system.
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After the prescribed treatment, the column worked better for a few
injections, but soon the peak shapes were worse than ever. We called
the company again.
“What kind of samples are you running?” they asked.
“Just aspartic acid,” Tim said.
“Are you sure there’s nothing else in the samples?” they responded.
“Well, maybe a little bit of dissolved calcite; it’s just calcium
carbonate.”
That was it. The company rep told us that the mineral residue had
quickly degraded our column, making it useless for further work. They
generously offered to send us another for free (probably suspecting
that we’d be going through a great many $500 columns in the coming
months).
So now we’d have to execute an additional chemical step to re-
move the calcium carbonate with acid before injection. We weren’t
happy about that, because each additional treatment of the samples
increased the chance of contamination.
We kept trying, but those first experiments were just too flaky. Lots
of L excesses of a few percent, one or two D excesses, but nothing really
systematic. What’s more, some of the L excesses came from the fracture
surfaces, which should have had no effect, while the right- and left-
handed calcite faces gave inconsistent results at best.
We had spent months at the project with nothing to show for it
but a lot of experience under our belts. In the spring of 2000, Tim’s
postdoc was nearly up. He and Rose were looking forward to setting up
a new home in Indiana. I had to make a decision whether, and if so
how, to continue.
I decided to give it another shot. One thing was absolutely clear: The
experiments had to be performed in ultraclean facilities, with every
possible precaution against contamination. My colleagues at our sister
lab, the Department of Terrestrial Magnetism (DTM for short), main-
tain a chemical clean lab at our Northwest Washington campus for
isotope and trace-element work, and they generously let me use one of
their high-tech chemical hoods for a few months.
In May of 2000, I hauled box upon box of chemicals, glassware,
and crystals to the second-floor clean lab of DTM’s venerable experi-
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ment building. The sparkling, well-lit labs could be entered only
through a small glassed-in vestibule, where the transition to clean-
room protocols took place. I removed my shoes and slipped into a pair
of pale-blue booties, white coveralls, hood, respirator mask, and latex
gloves. The protective gear wasn’t uncomfortable, but it took a bit of
getting used to.
After organizing my supplies, I began the experiment. Leaving
nothing to chance, I treated the thrice-washed crystals with extra care,
used the highest-purity amino acids, ultra-pure water (at $40 per gal-
lon!), and freshly baked glassware. Every step was carried out in a
chemical
hood, a glass-enclosed volume about 4 feet wide, 2 feet deep,
and 3 feet high—plenty of room for my beakers. My hood was main-
tained with positive pressure to prevent outside air from entering the
enclosed area.
The experiments were conducted almost exactly as before. First,
soak four crystals for 24 hours in the 50:50 amino acid bath. This time
I made sure that the pH of the solution remained close to 8, a value
ensuring that the calcite would not start to dissolve. Then, after a day
of soaking, I repeated the now-familiar wash process, applying acid to
each crystal, face by face. The day’s work produced 23 small glass vials
of acid extract. Over the next two days, I repeated the entire experi-
ment two more times to be absolutely certain.
A week’s work yielded 69 vials of aspartic acid solution washed
from crystal faces, plus several vials with individual samples of each
day’s aspartic acid solution, of the pure water, and of the acid wash. I
wanted samples of everything, in case I had to track down contami-
nants. More than 75 sample vials needed to be processed, each to be
analyzed at least 3 times. Two-hundred-plus amino acid analyses is a
huge job, and the Geophysical Lab facilities were not up to it. So for
that crucial, final step I headed downtown, hat in hand, to George
Washington University, to see geochemist Glenn Goodfriend, a former
colleague at the Geophysical Laboratory.
Success in a scientific career can be measured in many ways. Some
scientists crave admiration and respect from their peers. Others prize a
flexible job that gives them the freedom to pursue any line of research.
And for many researchers, the quiet exuberance of doing good science
is the prime measure of a happy and successful career. By all these
measures, Goodfriend was one of the most successful scientists I’d ever
known.
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At the agreed-upon late-morning hour, I arrived at his modest of-
fice in the basement of Lisner Hall, home of the Geology Department.
Glenn was a research professor, his work sustained almost entirely by a
succession of two- and three-year grants from the National Science
Foundation. Few scientists have the stature and stamina to survive like
that for long, but Glenn was a master with more than a decade of steady
funding.