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02/07/2003

Our Left-handed Heritage

I’ve just spent five minutes gazing at the palms of my two hands.
I was struck by how similar they look, with the prominent lines
in the palms matching much more closely than I had expected.
Of course, those features that match are mirror images of each
other. You might be wondering, has Bortrum finally flipped,
wasting time looking at his two hands? Possibly, but mirror
images and right- and left-handedness are subjects that have
occupied many scientists over the years.

Obviously, right-handed individuals predominate in our general
population. Just look at the right-hand placement of the writing
surfaces on virtually all one type of school desks. How did most
of us humans come to be right-handed? I have no idea; my
concern here is not for human handedness. Molecules can also
be right- or left-handed. Last week we discussed how fatty acids
could be “cis” or “trans”, with the same chemical formula and
structure except for a couple of hydrogen atoms in different
positions. Other compounds can also have the same chemical
compositions and structures but still differ in that one is the
mirror image of the other. One is left-handed, the other right-
handed.

Important examples of such compounds are certain amino acids.
Let’s not worry ourselves about the chemical formulas of these
acids. However, we should recognize that amino acids are the
stuff of life, the compounds that we living critters use to make
proteins. Proteins are the wave of the future now that the DNA
has been mapped for ourselves and various other creatures. As
we’ve mentioned before, now comes the hard part, correlating all
those genes and other segments of DNA with the proteins they
produce or control. But that’s a problem for the future. Let’s go
back to the past. Regular readers will know that I can’t resist
anything that has to do with our origins.

An article titled “Astrochemistry” by Joe Alper in the Winter
2003 issue of the American Chemical Society publication
Chemistry prompted this column. Contrary to the preponderance
of right-handedness in our human population, amino acids that
Nature and our bodies use to synthesize proteins are
overwhelmingly left-handed. Why is Nature left-handed?
Where did the amino acids come from? When?

After the earth was formed some 4.6 billion years ago, it was a
hellish place, under bombardment from all kinds of meteors and
such. One impact apparently was sufficiently large so as to make
us a moon. Amazingly, evidence suggest that some form of
single-celled life was present around 4 billion years ago. It
seems that this primitive life appeared a mere hundred million
years or so after the bombardment of Earth from outer space
settled down to a calmer pace. In that geologically short time,
both the chemicals and the life had to be formed. But what if
those amino acids and other chemicals didn’t have to be formed
on Earth but were deposited from space? This is the subject of
Alper’s article.

The universe had been around for roughly 10 billion years when
Earth was formed. Huge numbers of galaxies had formed and
zillions of stars had blown up, spewing out heavy elements like
carbon, oxygen and nitrogen, not to mention iron and other
metallic elements. All the elements were there to form the
organic compounds needed for life. Indeed, with all our
powerful telescopes and sensitive detectors, astronomers have
identified scads of chemical compounds out there in dust clouds,
comet tails, etc. It doesn’t seem far-fetched that our earth might
have been the recipient of gobs of chemicals in those early days
of its existence.

But wait, some of you may recall the experiment back in 1953
that was the subject of an earlier column. This was the famous
experiment of Stanley Miller and Harold Urey that shook up the
biological and other scientific communities half a century ago.
Miller and Urey thought that early Earth was devoid of any
molecular oxygen, a “reducing” atmosphere, and that the
atmosphere contained compounds such as methane, water,
hydrogen and ammonia. Miller set up a closed vessel containing
these materials and electrodes. He then sparked the mixture
repeatedly to simulate lightning. Sure enough, amino acids were
formed. Later, ultraviolet light was shown to give the same
effect as the “lightning”. This ingenious experiment was taken
as proof that the necessary chemicals for life to arise could have
formed on the earth itself.

Since Miller’s experiment, the composition of the early earth’s
atmosphere has come into question. Instead of being “reducing”,
with no molecular oxygen, it appears that there could have been
molecular oxygen after all. The Alper article cites a NASA
chemist, Joseph Guth, as saying the Miller-Urey chemistry
would not have happened on an earth having no reducing
atmosphere. But there was another problem with that
experiment. It didn’t seem left-handed enough. The amino acids
that Miller produced were about equally divided between right-
and left-handed versions. If the amino acids were equally
handed, how did Nature become left-handed? Miller thinks that
in the course of evolution the left-handed amino acids were
selected more or less by chance in the early life forms. Once
formed, this left-handedness was propagated down the years.

But what if the amino acids came from space and they were
predominantly left-handed? There would be no need to invoke a
chance evolutionary origin of Nature’s lefty bent. Fortunately,
unless you’re hit by one, we’re still being bombarded by
meteorites, albeit much less frequently than in the early days.
Just three years ago, a meteorite broke up over British Columbia,
the pieces falling in a frozen environment and, fortuitously, some
were recovered within a week from frozen ice. The finder was
an amateur geologist who knew enough not to handle the ice
chunks with his bare hands. The find provided a rare opportunity
to look at pristine samples of a visitor from outer space.

Sandra Pizzarello of Arizona State University leads a team
studying this meteorite material and they found a bonanza of
biologically important compounds. They proved the compounds
were not of earthly origin by analyzing the ratios of different
carbon isotopes. And, what do you know, left-handed amino
acids predominate! One of the compounds found in the
meteorite was nicotinic acid. I have this vision of some
otherworldly creatures out there on another planet trying
desperately to give up smoking.

It seems that the left-handed molecules that give us life may
indeed have come from the stars. But, lacking absolutely
convincing evidence, the question remains - did lefty molecules
from space drive evolution or, as Miller thinks, did evolution
drive the lefty molecules? Personally, I prefer the space-driven
scenario but I’m sure it will be a controversial subject for many
years.

Sadly, something other than meteorites have fallen from the sky
this past week. The Columbia disaster shows us once more how
special are those individuals who enthusiastically and joyfully
ride those rockets into space. As President Bush told the
astronauts’ children, they can forever be proud of their parents.

Allen F. Bortrum



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-02/07/2003-      
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Dr. Bortrum

02/07/2003

Our Left-handed Heritage

I’ve just spent five minutes gazing at the palms of my two hands.
I was struck by how similar they look, with the prominent lines
in the palms matching much more closely than I had expected.
Of course, those features that match are mirror images of each
other. You might be wondering, has Bortrum finally flipped,
wasting time looking at his two hands? Possibly, but mirror
images and right- and left-handedness are subjects that have
occupied many scientists over the years.

Obviously, right-handed individuals predominate in our general
population. Just look at the right-hand placement of the writing
surfaces on virtually all one type of school desks. How did most
of us humans come to be right-handed? I have no idea; my
concern here is not for human handedness. Molecules can also
be right- or left-handed. Last week we discussed how fatty acids
could be “cis” or “trans”, with the same chemical formula and
structure except for a couple of hydrogen atoms in different
positions. Other compounds can also have the same chemical
compositions and structures but still differ in that one is the
mirror image of the other. One is left-handed, the other right-
handed.

Important examples of such compounds are certain amino acids.
Let’s not worry ourselves about the chemical formulas of these
acids. However, we should recognize that amino acids are the
stuff of life, the compounds that we living critters use to make
proteins. Proteins are the wave of the future now that the DNA
has been mapped for ourselves and various other creatures. As
we’ve mentioned before, now comes the hard part, correlating all
those genes and other segments of DNA with the proteins they
produce or control. But that’s a problem for the future. Let’s go
back to the past. Regular readers will know that I can’t resist
anything that has to do with our origins.

An article titled “Astrochemistry” by Joe Alper in the Winter
2003 issue of the American Chemical Society publication
Chemistry prompted this column. Contrary to the preponderance
of right-handedness in our human population, amino acids that
Nature and our bodies use to synthesize proteins are
overwhelmingly left-handed. Why is Nature left-handed?
Where did the amino acids come from? When?

After the earth was formed some 4.6 billion years ago, it was a
hellish place, under bombardment from all kinds of meteors and
such. One impact apparently was sufficiently large so as to make
us a moon. Amazingly, evidence suggest that some form of
single-celled life was present around 4 billion years ago. It
seems that this primitive life appeared a mere hundred million
years or so after the bombardment of Earth from outer space
settled down to a calmer pace. In that geologically short time,
both the chemicals and the life had to be formed. But what if
those amino acids and other chemicals didn’t have to be formed
on Earth but were deposited from space? This is the subject of
Alper’s article.

The universe had been around for roughly 10 billion years when
Earth was formed. Huge numbers of galaxies had formed and
zillions of stars had blown up, spewing out heavy elements like
carbon, oxygen and nitrogen, not to mention iron and other
metallic elements. All the elements were there to form the
organic compounds needed for life. Indeed, with all our
powerful telescopes and sensitive detectors, astronomers have
identified scads of chemical compounds out there in dust clouds,
comet tails, etc. It doesn’t seem far-fetched that our earth might
have been the recipient of gobs of chemicals in those early days
of its existence.

But wait, some of you may recall the experiment back in 1953
that was the subject of an earlier column. This was the famous
experiment of Stanley Miller and Harold Urey that shook up the
biological and other scientific communities half a century ago.
Miller and Urey thought that early Earth was devoid of any
molecular oxygen, a “reducing” atmosphere, and that the
atmosphere contained compounds such as methane, water,
hydrogen and ammonia. Miller set up a closed vessel containing
these materials and electrodes. He then sparked the mixture
repeatedly to simulate lightning. Sure enough, amino acids were
formed. Later, ultraviolet light was shown to give the same
effect as the “lightning”. This ingenious experiment was taken
as proof that the necessary chemicals for life to arise could have
formed on the earth itself.

Since Miller’s experiment, the composition of the early earth’s
atmosphere has come into question. Instead of being “reducing”,
with no molecular oxygen, it appears that there could have been
molecular oxygen after all. The Alper article cites a NASA
chemist, Joseph Guth, as saying the Miller-Urey chemistry
would not have happened on an earth having no reducing
atmosphere. But there was another problem with that
experiment. It didn’t seem left-handed enough. The amino acids
that Miller produced were about equally divided between right-
and left-handed versions. If the amino acids were equally
handed, how did Nature become left-handed? Miller thinks that
in the course of evolution the left-handed amino acids were
selected more or less by chance in the early life forms. Once
formed, this left-handedness was propagated down the years.

But what if the amino acids came from space and they were
predominantly left-handed? There would be no need to invoke a
chance evolutionary origin of Nature’s lefty bent. Fortunately,
unless you’re hit by one, we’re still being bombarded by
meteorites, albeit much less frequently than in the early days.
Just three years ago, a meteorite broke up over British Columbia,
the pieces falling in a frozen environment and, fortuitously, some
were recovered within a week from frozen ice. The finder was
an amateur geologist who knew enough not to handle the ice
chunks with his bare hands. The find provided a rare opportunity
to look at pristine samples of a visitor from outer space.

Sandra Pizzarello of Arizona State University leads a team
studying this meteorite material and they found a bonanza of
biologically important compounds. They proved the compounds
were not of earthly origin by analyzing the ratios of different
carbon isotopes. And, what do you know, left-handed amino
acids predominate! One of the compounds found in the
meteorite was nicotinic acid. I have this vision of some
otherworldly creatures out there on another planet trying
desperately to give up smoking.

It seems that the left-handed molecules that give us life may
indeed have come from the stars. But, lacking absolutely
convincing evidence, the question remains - did lefty molecules
from space drive evolution or, as Miller thinks, did evolution
drive the lefty molecules? Personally, I prefer the space-driven
scenario but I’m sure it will be a controversial subject for many
years.

Sadly, something other than meteorites have fallen from the sky
this past week. The Columbia disaster shows us once more how
special are those individuals who enthusiastically and joyfully
ride those rockets into space. As President Bush told the
astronauts’ children, they can forever be proud of their parents.

Allen F. Bortrum