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NASA Astrobiology Institute Goddard Center for Astrobiology. Investigating amino acid enantiomeric excesses in CM meteorites with liquid chromatography time of flight mass spectrometry. Daniel Glavin and Jason Dworkin NASA Goddard Space Flight Center, Greenbelt MD GCA Team Meeting

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slide1

NASA Astrobiology Institute

Goddard Center for Astrobiology

Investigating amino acid enantiomeric excesses in CM meteorites with liquid chromatography time of flight mass spectrometry

Daniel Glavin and Jason Dworkin

NASA Goddard Space Flight Center, Greenbelt MD

GCA Team Meeting

March 23, 2006

background
Background
  • Over 80 amino acids identified in the CM carbonaceous meteorite Murchison
  • Most amino acids nearly racemic (D/L ~ 1) indicating abiotic origin (Kvenvolden et al. 1970; 1971)
  • Several a-dialkyl amino acids in Murchison have small L-enantiomeric excesses (ee’s) (Cronin and Pizzarello, 1997; Pizzarello and Cronin, 2000)
  • L-isovaline excesses in Murchison range from 0 to 15% (Pizzarello et al. 2003)
  • EE’s inconsistent with Strecker synthesis (Peltzer et al. 1984; Bada 1997)
  • Meteoritic contribution to origin of biologic homochirality on Earth?

Left handed (L) Right handed (D)

D,L-Isovaline (Iva)

goals of this study
Goals of this Study
  • Demonstrate that UV fluorescence liquid chromatography-time of flight-mass spectrometry (LC-ToF-MS) technique can be used for amino acid analyses of meteorites
  • Measure D- and L-isovaline abundances in CM meteorites Murchison and LEW 90500 using LC-ToF-MS
  • Determine D/L ratios for both free and bound isovaline
  • Investigate all possibilities for reported L-isovaline enantiomeric excesses (ee’s)
protocol
Protocol

LC-ToF-MS Instrument at Goddard

Meteorite powder (~5-6 g)

Water extraction (100ºC 24 h)

50%

Acid hydrolysis (6 M HCl 150ºC 3 h)

HPLC with UV fluorescence

+ ToF-MS detection

50%

Desalting (AG50W-X8 resin)

Derivatization (OPA/NAC primary amines)

slide5

UV Fluorescence LC-ToF-MS

val

C6

iva

C5

m/z=379.13

C4

C3

C2

c 5 amino acid isomers
C5 Amino Acid Isomers

13 possible amino acid isomers of isovaline (C5H11NO2)

* = chiral carbon

lc tof ms data
LC-ToF-MS Data

L-iva

m/z = 379.13 ± 0.02 (ES+ single ion traces)

% Lee =

[(L-D)/(L+D)] x 100

D-iva

3-a-3-mba

3-a,2,2-dmpa

5-apa

L-val

Murchison

D-val

D,L-Nor

LEW 90500

Serpentine Blank

enantiomeric excess data
Enantiomeric Excess Data

Total = free + bound amino acids in sample hydrolyzed by 6 M HCl

Free = amino acids in unhydrolyzed water extract

Free isovaline has lower Lee than bound isovaline

other explanations
Other Explanations…..

Iva

  • Co-eluting C5 amino acid that is not L-isovaline?
    • D/L ratios after 1 and 15 min derivatization are identical
    • Only two a-dialkyl C5 amino acids increase in area after 15 min (3-a-3-mba and isovaline)
    • 3-a-3-mba does not interfere

3-a-3-mba

D/L = 0.66 ± 0.02

15 min.

D/L = 0.65 ± 0.04

1 min.

  • Interfering non OPA/NAC labeled compound with m/z = 379.13?
    • No peaks observed at this mass in unlabeled Murchison extract
  • Co-eluting a-dialkyl C5 primary amine compound that is not an acid?
    • Need to test for this possibility

e.g. 3-Amino-4-hydroxy-3-methyl-2-butanone

(C5H11NO2: m/z = 379.13)

slide10

L-Isovaline Excess in Meteorites

  • Isovaline resistant to racemization
    • a-Hydrogen analogs (e.g. norvaline) in Murchison racemic (Cronin and Pizzarello, 1997)
    • No significant racemization at 100ºC in 6M NaOH (Pollock et al. 1975)
    • Up to 6% radioracemization by g-irradiation (Bonner et al. 1979)
    • Photostability of bound amino acids greater than free amino acids (Takano et al. 2004)

slow

L-isovaline D-isovaline

  • Asymmetric decomposition due to UV CPL
    • Bonner-Rubenstein Hypothesis: Degradation of amino acids in molecular cloud
    • Excesses up to 2.5% achieved for UV CPL photolysis of racemic leucine (Flores et al. 1977; Nishino et al. 2002)
  • Strecker synthesis of isovaline on CM parent body?
    • Formation of racemic isovaline from 2-butanone, HCN, NH3 and H2O
    • Mechanism inconsistent with ~20% L-isovaline excess found in Murchison
amplification of isovaline ee s
Amplification of Isovaline EE’s?
  • E.g. amplification of small ee of 5-pyrimidyl alkanol via enantioselective reactions with an asymmetric catalyst has been demonstrated (Soai et al. 1995)

+

Low ee (~2%)

High ee (~90%)

Asymmetric autocatalysis

Soai autocatalytic reaction

implications for homochirality
Implications for Homochirality
  • Contribution of meteoritic a-dialkyl amino acids with L-enantiomeric enrichment to prebiotic Earth
  • Amplification of amino acid ee’s via asymmetric autocatalysis to generate homochirality?
  • a-Dialkyl amino acids readily form helical structures
  • Transfer a-dialkyl asymmetry to a-hydrogen amino acids common to modern terrestrial life
  • If extinct or extant life in the solar system is based on same handedness, then search for evidence of independent origin of life could be difficult (Bada, 1997)
summary
Summary
  • Largest L-enantiomeric excesses for isovaline in Murchison reported to date (+18 to +20%)
  • Smaller ee’s detected in LEW 90500 (-1 to +3%)
  • These ee’s are not due to interfering C5 amino acid isomers
  • Lower ee of free compared to bound isovaline is consistent with photolysis and radioracemization stability
  • Mechanism(s) for the formation of large ee’s of a-dialkyl amino acids in Murchison remains unclear.
  • Future work: synthesis of isovaline from irradiation of interstellar ice analogs?
acknowledgments
This research was supported by the NASA Astrobiology Institute and the Goddard Center for Astrobiology.

We thank K. Righter, T. McCoy, and L. Welzenbach for providing the meteorites used in this study, and S. Pizzarello for providing several amino acid standards.

Meteorite extracts were obtained with the support of A. Aubrey and J. L. Bada at the Scripps Institution of Oceanography.

Acknowledgments