[meteorite-list] Wet, Carbonaceous Asteroids: Altering Minerals, Changing Amino Acids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 7 Apr 2011 17:27:17 -0700 (PDT)
Message-ID: <201104080027.p380RHSd007875_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/April11/amino_acids.html

Wet, Carbonaceous Asteroids: Altering Minerals, Changing Amino Acids
Planetary Science Research Discoveries
April 7, 2011

--- Aqueous alteration in asteroids containing organic compounds leads
to formation of hydrous minerals and changes in the mix of amino acids.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Many carbonaceous chondrites contain alteration products from water-rock
interactions at low temperature and organic compounds. A fascinating fact
known for decades is the presence in some of them of an assortment of
organic compounds, including amino acids, sometimes called the building
blocks of life. Murchison and other CM carbonaceous chondrites contain
hundreds of amino acids. Early measurements indicated that the amino
acids in carbonaceous chondrites had equal proportions of L- and
D-structures, a situation called racemic. This was in sharp contrast to life on
Earth, which heavily favors L-forms. However, beginning in 1997, John
Cronin and Sandra Pizzarello (Arizona State University) found L-excesses
in isovaline and several other amino acids in the Murchison
carbonaceous chondrite. In 2009, Daniel Glavin and Jason Dworkin
(Astrobiology Analytical Lab, Goddard Space Flight Center) reported the
first independent confirmation of L-isovaline excesses in Murchison
using a different analytical technique than employed by Cronin and
Pizzarello.

Inspired by this work, Daniel Glavin, Michael Callahan, Jason Dworkin,
and Jamie Elsila (Astrobiology Analytical Lab, Goddard Space Flight
Center), have done an extensive study of the abundance and symmetry of
amino acids in carbonaceous chondrites that experienced a range of
alteration by water in their parent asteroids. The results show that
amino acids are more abundant in the less altered meteorites, implying
that aqueous processing changes the mix of amino acids. They also
confirmed the enrichment in L-structures of some amino acids,
especially isovaline, confirming earlier work. The authors suggest that
aqueously-altered planetesimals might have seeded the early Earth with
nonracemic amino acids, perhaps explaining why life from microorganisms
to people use only L-forms to make proteins. The initial imbalance
caused by non-biologic processes in wet asteroids might have been
amplified by life on Earth. Alternatively, the same processes that
produced the L-amino acid excesses in carbonaceous asteroids also
operated on the early Earth.


Reference:

    * Glavin, D. P., Callahan, M. P., Dworkin, J. P., and Elsila, J. E.
      (2011) The Effects of Parent Body Processes on Amino Acids in
      Carbonaceous Chondrites. /Meteoritics and Planetary Science,/ v.
      45(12), p. 1948-1972, doi: 10.1111/j.1945-5100.2010.01132.x
    * *PSRDpresents:* Wet, Carbonaceous Asteroids: Altering Minerals,
      Changing Amino Acids --Short Slide Summary <PSRD-amino_acids.ppt>
      (with accompanying notes).


Wet and Gunky Meteorites

Carbonaceous chondrites have a range of properties. Some are wetter than
others, some have more carbon than others, some have much higher
concentrations of organic compounds than others. Daniel Glavin and his
co-authors focused on those with significant amounts of organic
compounds--up to only 2-3%, but that's enough to give the meteorites a
distinctive earthy smell, a bit like tar. Their set of meteorites also
included a range in the amount to which water affected their primary
anhydrous minerals to produce a host of complicated hydrous minerals.
Examples of the effects of aqueous alteration are shown in the electron
microscope images below.

BSE image of a typical chondrule in a meteorite that did not experience
aqueous alteration. BSE image of a type of chondrule that was aqueously
altered.

[Left] Image taken in a scanning electron microscope using
backscattered electrons of a typical chondrule in a meteorite that has
not experienced aqueous alteration. The chondrule, which fills the field
of view, is composed mostly of olivine (ol) with inclusions of iron
sulfide (sf). None of the olivine crystals are altered. Even the
mesostasis (mes), the last of the molten chondrule to crystallize, is
unaffected. [Right] Backscattered scanning electron micrograph of a
typical chondrule (specifically what cosmochemists call a Type IAB
chondrule) in the CM chondrite ALH 81002. The high-temperature primary
mineral enstatite has been partially altered to serpentine (ragged,
darkest gray crystals). In the outer parts of the chondrule the
enstatite crystals have been almost completely replaced by
Mg-serpentine, though the outlines of the enstatities are preserved. The
brighter interstitial regions are mainly altered mesostasis glass that
has been replaced by an early generation of Fe-serpentine. This
chondrule is surrounded by a broad, fine-grained rim.


Chondritic meteorites vary substantially in their properties, although
all but one type contain chondrules,
rounded, millimeter-sized objects that were at one time molten droplets.
They are further classified by the amount of alteration they
experienced. For historical reasons, the most unaltered ones fall into
Type 3. Thermal metamorphism caused some to be heated to several hundred
degrees, producing Types 4 through 6, with Type 6 being heated to the
highest temperature (about 900 ^o C). In chondritic asteroids that
contained water (almost certainly ice to begin with), heating was less
severe, but alteration was considerably more as water flowed in cracks,
permeating asteroid interiors. The water reacted with the original
chondrule minerals to form hydrous alteration products.
Aqueously-altered chondrites are classified as Type 1 and 2, with Type 1
being the most altered, so altered in fact, that chondrules are not even
visible.

Chondrites are also divided into groups based on chemical and isotopic
compositions, and then named after a typical chondrite in the group
(usually one that was observed to fall). For carbonaceous chondrites,
the groups include CI (Ivuna-like), CM (Mighei-like), CR (Renazzo-like),
CO (Ornans-like), CV (Vigarano-like), and CK (Karoonda-like).
Carbonaceous chondrites also contain carbon, including organic
compounds, which we discuss below. Combined with the alteration
classification, meteorites are classified as, for example, CM1 or CM2,
CR1 and CR2, and so on. A summary of the classification scheme appears
below; colored boxes show the categories in which we have identified
carbonaceous chondrites.
.
Carbonaceous chondrites are classified by their bulk chemical
composition, which places them into groups (CI, CM, etc.), and by the
amount of aqueous alteration (petrographic types 1 and 2) and heating
without much water (3 through 6). Areas in the table are colored to
signify which categories have been seen. Darker blue colors represent
more aqueous alteration and redder colors represent increased thermal
metamorphism.

Daniel Glavin and his co-authors studied the amino acids in carbonaceous
chondrites that exhibited a range of aqueous alteration. Their goal is
to understand the role aqueous alteration plays in changing amino acids
(decomposing some, synthesizing others, changing their relative
abundance, and possibly enhancing the ratio of L- to D-amino acids).

Amino Acids

Amino acids are the foldable structures that make up proteins. DNA is
coded to use only twenty of them in protein construction, but organisms
use other amino acids for assorted biological functions. The Murchison
CM2 chondrite [Data link
<http://www.lpi.usra.edu/meteor/metbull.php?code=16875> from the
Meteoritical Bulletin] contains over 100 different amino acids, only
eight of which are used in the proteins in terrestrial life. Some appear
to be presolar grains (*PSRD* article: Interstellar Organic Matter in
Meteorites <../May06/meteoriteOrganics.html>). The essential components
of amino acids are an amine group (NH_2 ), which contains the nitrogen,
and a carboxylic acid group, COOH. These are joined to numerous types of
other functional groups, called side chains, to make up a menagerie of
amino acids. Amino acids can also contain extra carbon atoms, designated
as, for example, ??-alanine, ??-alanine, etc., depending on which carbon
atom the amine group is attached to.

Structures of amino acids.

Amino acids are composed of two main components, an amine group
(containing nitrogen) and a carboxylic acid group. These join with other
molecular groups (called side chains) to make different amino acids, as
depicted by the R in the diagram on the left. The differences in the
nature of the side chains lead to different chemical compositions and
uses of each amino acid, such as alanine (center) and valine (right).
Most amino acids are more complicated than those shown.

Due to the geometry of many amino acids they often come in two
structural shapes, one a mirror image of the other, but with the same
chemical formula (a property called chirality). Either one of the pair
is called an enantiomer. These mirror images are designated L- and D-.
Biology prefers L, and all the amino acids in proteins are in the L-
configuration (designated by names such as L-alanine or L-valine).

This artist's view uses hands to illustrate the left and right-handed
versions of the amino acid isovaline. Credit: NASA/Mary Pat
Hrybyk-Keith. Almost all amino acids have symmetrical forms, designated
L and D. The one shown in this artist's illustration is isovaline,
which is one of the most abundant amino acids in carbonaceous chondrite
meteorites.

Assaying Amino Acids

The Astrobiology Analytical Lab at Goddard Space Flight Center teems
with state-of-the-art instruments to measure the types of organic
compounds and their abundances in almost any type of sample. For this
study they used a high performance liquid chromatograph coupled to a
time-of-flight mass spectrometer. These tools allow measurement of amino
acids in solutions from chemically-treated samples. The use of a special
detector in the chromatograph and coupling to the time-of-flight mass
spectrometer vastly improves the detection limit of amino acids compared
to current state-of-the-art gas chromatography-mass spectrometer
measurements. This vast improvement in performance is essential because
of the extremely low abundances of individual amino acids even in
carbonaceous chondrites and because only limited amounts of these rare
meteorites are available.

Before the high-tech gizmos can be used, the amino acids have to be
extracted from the meteorite samples. This crucial procedure required
use of ultra-clean glassware, ceramic containers, and sample-handling
tools. All reagents and water were of the highest purity available.
Extraction steps included heating, exposure to hydrochloric acid,
cation-exchange resins, and chemical modification (derivatization) with
o-phthaldialdehyde/N-aceteyl-L-cysteine. (The use of these toxic
chemicals explains why chemistry laboratories have fume hoods and
emergency showers.) Analyses indicate that even after all that
treatment, the amino acids in the samples were not degraded.

As a testament to how complicated the entire wet chemical and
instrumental procedure is, just glance at the wonderfully technical list
of ingredients in the standards Glavin and his co-workers prepared for
this research:

    "Individual d- and l-2-amino-2-methylbutanoic acid (2-a-2-mba,
    isovaline) were purchased from Acros Organics (>99% purity), and a
    racemic mixture (*d* = *l*) was prepared by mixing the appropriate
    volumes of each individual solution. dl-2-aminopentanoic acid
    (2-apa, norvaline), dl-2-amino-3-methylbutanoic acid (2-a-3-mba,
    valine), and 5-aminopentanoic acid (5-apa) standards were purchased
    from Sigma-Aldrich (97-99% purity). dl-3-aminopentanoic acid (3-apa)
    was from AzaN (97% purity). 3-amino-3-methylbutanoic acid
    (3-a-3-mba), 3-amino-2,2-dimethylpropanoic acid (3-a-2,2-dmpa),
    dl-3-amino-2-methylbutanoic acid (3-a-2-mba and allo- 3-a-2-mba, 4
    stereoisomers), dl-4-aminopentanoic acid (4-apa),
    dl-4-amino-3-methylbutanoic acid (4-a-3-mba),
    l-3-amino-2-ethylpropanoic acid (3-a-2-epa), dl-4-amino-
    2-methylbutanoic acid (4-a-2-mba) were individually synthesized and
    provided by S. Pizzarello (ASU), S. Miller (UCSD), S. Davies
    (Oxford), and R. Duke (U. Sydney). The l-3-a-2-epa was mostly
    racemized by 6 M HCl acid vapor hydrolysis (150 _C for 1 week)."

Changes in the Amino Acid Set as Asteroid Water Flowed

Daniel Glavin and his team present the first large database of amino
acids in a broad suite of carbonaceous chondrites representing
petrographic types 1 through 3, hence a range in the extent of aqueous
processing, and all measured with the same experimental protocol and
instruments. This allows them to take a close look at how aqueous
alteration affected the primary amino acids in carbonaceous asteroids.
The graph below shows their results in simplified summary form, with the
degree of aqueous alteration increasing from right to left. A striking
change is the increase in the relative abundance of ??-alanine with more
aqueous alteration. Orgueil, the most altered petrographic type 1
chondrite, has the most ??-alanine, and some type 2 chondrites have no
detectable ??-alanine. Although it is possible that the increase in
??-alanine is due to differences among the parent asteroids of the
meteorites studied, Glavin and his coworkers suggest that it is more
likely that the extent of aqueous processing is the key difference.

Comparison chart of the relative abundances of four amino acids in
carbonaceous chondrites.

Comparison of the relative abundances of four amino acids in
carbonaceous chondrites. All abundances are normalized to the amount of
glycine present (which if plotted would always have a value of 1). The
error bars represent the uncertainties derived from the standard
deviations of 4 to 8 analyses for each meteorite. ??-alanine increases
with increasing amount of aqueous alteration. The abundance of isovaline
appears to decrease with alteration.

Excess of L over D Structures

Researchers had previously reported small excesses of L over D
structures in amino acids in carbonaceous chondrites. Glavin and
colleagues confirm these excesses (see graph below). It appears that the
percentage of L-isovaline is greater in the more aqueously altered
chondrites. Orgueil, SCO 06043, GRO 95577, and Murchison all have L
excesses well outside experimental uncertainties (shown by the bars in
the figure). In the other, less altered samples, the L excess is zero
within experimental uncertainty. A concern with an L excess is that it
is caused by contamination once the meteorite landed on Earth, where
amino acids are almost entirely L. However, in a 2009 paper, Daniel
Glavin and Jason Dworkin address the analytical and contamination issues
in detail. They conclude that contamination of the interiors of the
meteorite samples is unlikely; the L-isovaline excess is of
extraterrestrial origin and not an analytical artifact. They also point
out that the concentrations of isovaline on Earth are very small.
Contamination of the other amino acids is thus more likely, and those
that make up proteins have L/D of 1, indicating no contamination.


Glavin and coworkers conclude that the processes involved in aqueous
alteration cause the L-isovaline excesses. Researchers had thought
that the small excesses measured previously were caused by pre-accretion
processes in the solar nebula, such as irradiation with ultraviolet
light. While possible, the correlation between aqueous alteration and
L excess indicates a role for alteration by water in the parent
asteroids of carbonaceous chondrites.


Chart showing the L-isovaline excesses in carbonaceous chondrite
meteorites determined by Glavin et al, 2011.
This chart shows L excess [L/(D+L) isovaline, expressed as
percent] for each chondrite studied, listed in decreasing amount of
aqueous alteration from left to right. Experimental uncertainties are
shown by the bars and are based on standard deviations of between 8 and
23 analyses for each chondrite. The four samples on the right are not
enriched in L-isovaline within experimental uncertainties. The four on
the left are clearly enriched.

Organics on the Early Earth

In a paper published in 1992, Chris Chyba (now at Princeton University)
and Carl Sagan suggested that asteroids similar to carbonaceous
chondrites may have delivered a significant complement of organic
compounds to Earth very early in its history. Following an idea by
Sandra Pizzarello (Arizona State University) and Art Weber (SETI/NASA
Ames Research Center), Daniel Glavin and his co-authors suggest that
although not abundant, amino acids such as isovaline might have served
as catalysts for organic reactions, hence transferring their asymmetry
to other amino acids. Perhaps, they suggest, life on Earth was biased
towards L-amino acid asymmetry from the beginning. This does not rule
out the possibility that aqueous alteration of mixtures of minerals and
organic compounds on Earth led to the L enrichment, just as it did in
carbonaceous asteroids. Perhaps aqueous alteration led to the L-amino
acid bias no matter where the alteration occurred.


Additional Resources Links open in a new window.

    * *PSRDpresents:* Wet, Carbonaceous Asteroids: Altering Minerals,
      Changing Amino Acids --Short Slide Summary <PSRD-amino_acids.ppt>
      (with accompanying notes).

    * Astrobiology Analytical Lab at Goddard Space Flight Center
      <http://science.gsfc.nasa.gov/691/analytical/>
    * Chyba, C. and Sagan, C. (1992) Endogenous Production, Exogenous
      Delivery and Impact-Shock Synthesis of Organic Molecules: An
      Inventory for the Origins of Life, /Nature,/ v. 355, p. 125-132.
    * Cronin, J. R. and Pizzarello, S. (1997) Enantiomeric Excesses in
      Meteoritic Amino Acids, /Science,/ v. 275, p. 951-955.
    * Glavin, D. P., Callahan, M. P., Dworkin, J. P., and Elsila, J. E.
      (2011) The Effects of Parent Body Processes on Amino Acids in
      Carbonaceous Chondrites. /Meteoritics and Planetary Science,/ v.
      45(12), p. 1948-1972, doi: 10.1111/j.1945-5100.2010.01132.x
    * Glavin, D. P. and Dworkin, J. P. (2009) Enrighment of the Amino
      Acid L-isovaline by Aqueous Alteration on CI and CM Meteorite
      Parent Bodies, /Proceedings of the National Academy of Sciences,/
      v. 106(14), p. 5487-5492, doi: 10.1073/pnas.0811618106.
    * Pizzarello, S. and Weber, A. L. (2004) Prebiotic Amino Acids as
      Asymmetric Catalysts, /Science,/ v. 303, p. 1151.
    * Sephton, M. A. (2002) Organic Compounds in Carbonaceous
      Meteorites, /Natural Product Reports,/ v. 19, p. 292-311, doi:
      10.1039/b103775g.
    * Taylor, G. J. (May 2006) Interstellar Organic Matter in
      Meteorites. /Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/May06/meteoriteOrganics.html
      <../May06/meteoriteOrganics.html>. /
Received on Thu 07 Apr 2011 08:27:17 PM PDT