Burton, A. S.

Charnley, S. B.

Dworking, J. P.

Elsila, J. E.

Glavin, D. P.

NASA Technical Reports Server

Compound-specific isotopic ratios of amino acids in CM and CR chondrites.  J. E. Elsila1, S. B. Charnley1, A. S. 
Burton2, D. P. Glavin1, and J. P. Dworkin1, 1NASA Goddard Space Flight Center, Greenbelt, MD 20771, email:  
Jamie.Elsila@nasa.gov, 2Catholic University of America, 8800 Greenbelt Rd., Greenbelt MD 20771. 
 
 
Introduction:  Carbonaceous chondrites contain a 
wealth of organic compounds, including a variety of 
amino acids.  The abundance, isotopic composition and 
structural distributions of these compounds reflect their 
synthesis and evolution, providing information on the 
original composition and processing of the molecular 
cloud and solar nebula from which the solar system 
formed and on subsequent processing of meteorite par-
ent bodies. 
Many formation mechanisms have been proposed 
for the amino acids detected in carbonaceous chon-
drites. These include the Strecker-cyanohydrin synthe-
sis, which postulates the formation of α-amino acids 
from the reaction of ketones or aldehydes with hydro-
gen cyanide, ammonia, and water [1], as well as Mi-
chael addition, in which β-amino acids are formed from 
addition of ammonia to an α,β-unsaturated nitrile [2]. 
Other proposed mechanisms include addition of carbon  
dioxide to amines [3] or the reductive amination of 
keto acids [4]. 
As each of these proposed mechanisms utilizes dif-
ferent precursor molecules, the stable carbon, nitrogen, 
and hydrogen isotopic ratios (δ13C , δ15N, and δD) of 
the resulting individual amino acids may point toward 
the most likely of these proposed pathways. Here we 
report on the compound-specific stable isotopic ratios 
of amino acids from four CM and three CR carbona-
ceous chondrites.  This data both confirms previous 
measurements and greatly extends the available set of 
compound-specific meteoritic amino acid isotopic data. 
We compare the trends observed in these ratios with 
predictions of expected isotopic enrichments from po-
tential formation pathways and environments. 
Analytical techniques and samples: The follow-
ing carbonaceous chondrites were analyzed: ALH 
83100 (CM1/2), Murchison (CM2), LEW 90500 
(CM2), LON 94101 (CM2), GRA 95229 (CR2), EET 
92042 (CR2), and QUE 99177 (CR3).  Sample sizes 
from 0.5 to 8.2 g were crushed, extracted in water, hy-
drolyzed with HCl acid vapor, and desalted with cati-
on-exchange resin.  Amino acid abundances and distri-
butions were analyzed via ultra-performance liquid 
chromatography with fluorescence detection and time-
of-flight mass spectrometry (UPLC-FD/ToF-MS); 
measurements were made on ~1% of each extract fol-
lowing derivatization with o-phthaldialdehyde/N-
acetyl-L-cysteine (OPA/NAC).  Compound-specific 
isotopic measurements were performed on the bulk of 
the extract, with ~10% used for carbon measurements, 
~30% for nitrogen, and ~60% for hydrogen. Extracts 
were derivatized with trifluoroacetic anhydride 
(TFAA) and isopropanol prior to analysis via gas 
chromatography and combustion coupled with mass 
spectrometry and isotope ratio mass spectrometry (GC-
MS/IRMS). Experimental details have been provided 
elsewhere [5]. 
Results and Discussion: Complete isotopic data 
for the individual amino acids detected in the seven 
carbonaceous chondrites is presented elsewhere [5].  
Figure 1 shows the isotopic relationships (δD versus 
δ13C , δ15N versus δ13C , and δD versus δ15N) for amino 
acids when more than one isotope was measured in a 
single amino acid in a given meteorite. Areas of the 
plots in which structurally similar amino acids grouped 
together are outlined. 
We compared the isotopic data with predictions for 
four formation mechanisms (details provided in [5]).  
The Strecker-cyanohydrin synthesis should produce α-
amino acids, with a decrease in 13C enrichment as chain 
length increases and a corresponding increase in D 
enrichment.  Our measurements of the two-to-five car-
bon α-amino, α-H amino acids in five of the carbona-
ceous chondrites followed these trends and thus sup-
port a Strecker origin; similar measurements were not 
possible due to low abundances of these amino acids in 
the other two meteorites (ALH 83100 and LEW 
90500). 
We investigated the effects of amine position, and 
observed a lower δ13C  value for β-alanine than for its 
corresponding α-amino acids (D- and L-alanine) in the 
six meteorites with measurable levels of these amino 
acids. This observation could reflect formation of β-
alanine via Michael addition, because the precursor 
nitriles involved in this reaction are expected to be less 
enriched in 13C than the carbonyl-containing precursors 
that form α-amino acids through Strecker synthesis. 
We observed a correlation of isotopic enrichment 
with alpha-subsituent.  Amino acids with an α-methyl 
group were consistently more enriched in D than the 
corresponding α-hydrogen amino acids.  No consistent 
δ13C  correlation was observed.  The α-methyl amino 
acid isovaline was less enriched in 15N than other ami-
no acids. The δD and δ15N trends may be caused by the 
formation of precursor molecules in cold interstellar or 
presolar environments where the spin-state of hydrogen 
molecules was important in fractionation chemistry. 
https://ntrs.nasa.gov/search.jsp?R=20130013629 2019-08-29T16:21:10+00:00Z
 Finally, we observed that individual amino acids in 
the CR chondrites are consistently more enriched in D 
than those in the CM chondrites, with no significant 
difference in δ13C  enrichment.  This may reflect dif-
ferences in reactions between amino acids and water 
on their respective parent bodies. 
In summary, this compound-specific isotopic data 
of meteoritic amino acids highlights differences in 
formation mechanisms, precursors, and environments 
between different structural classes of amino acids and 
different carbonaceous chondrite groups and petro-
graphic types. 
 
References: [1] Peltzer E. T. et al. (1984) Adv. 
Space Res., 4, 69-74. [2] Miller S. L. (1957) Biochim. 
Biophys. Acta, 23, 480-489. [3]Hudson R. L. et al. 
(2009) Bioastronomy, 420, 157-162. [4] Huber C. and 
Wächtershäuser G. (2003), Tetr. Lett., 44, 1695-1697. 
[5] Elsila J. E. et al. (2012), Met.& Planet. Sci., 47, 
1517-1536. [6] Burton A. S. et al. (2012) Chem. Soc. 
Rev., 41, 5459-5472. 
 
 
8000
6000
4000
2000
0
δD
 (‰
 V
SM
O
W
)
40200-20
δ
13C (‰ VPDB)
α-CH3 
amino acids
 ALH 83100 (CM1/2)
 Murchison (CM2)
 LEW 90500 (CM2)
 LON 94101 (CM2)
 GRA 95229 (CR2)
 EET 92042 (CR2)
 QUE 99177 (CR3)
300
200
100
0
δ1
5 N
 (‰
 A
IR
)
40200-20
δ
13C (‰ VPDB)
n-ω-amino acids
8000
6000
4000
2000
0
δD
 (‰
 V
SM
O
W
)
3002001000
δ
15N (‰ AIR)
α-CH3 
amino acids
 
Figure 1. Isotopic data for the amino acids in the seven 
analyzed carbonaceous chondrites. Outlined areas high-
light subsets of structurally similar compounds. Adapted 
from [5].  For comparison, terrestrial amino acids have 
δ13C  values ranging from -70 to +11‰  and δ15N values 
ranging from -20 to +30 ‰, while organic hydrogen in a 
range of terrestrial materials has values of -270 to +66 ‰ 
(see [6] and references therein). 
 


Compound-Specific Isotopic Ratios of Amino Acids in CM and CR Chondrites

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server