Analytical optimization of achiral and chiral amino acids forcompound-specific and enantiomer-specific isotope studies.
Y. Takano1, Y. Chikaraishi1, H.-S. Chan1,2, and N. Ohkouchi1
1
JAMSTEC. 2 Imperial College London.
E-mail: takano@jamstec.go.jp
Since the time of Pasteur, the development of biomolecularchirality has remained one of the most important problems withregard to our knowledge concerning chemical evolution. The
homochiral amino acids and sugars are essential to the formation,
structure, and function of biopolymers and are a defining
molecular trait of life on the Earth [1] (cf. asymmetricphotochemical processes [2]). Meteorites, specificallycarbonaceous chondrites, carry pristine abiotic signatures ofmolecular chirality in the solar system. Enantiomeric excesses of
L-form α-methyl amino acids are found in the CM meteorite
Murchison and the CI meteorite Orgueil with the correlation ofhydro-alteration processes in the parent body [3]. For further
information of compound-specific and enantiomer-specificnitrogen isotopic composition, here we developed the analyticaloptimization for achiral and chiral amino acids.
The precise determination of amino acids in calcareous,siliceous, geological samples (including mineral matrix in C-type
meteorites) is troublesome, since extracts of these samples often
contain a significant amount of interfering organic and inorganic
substances. To overcome this critical issues, at first we employedcation-exchange chromatography of protein and non-proteinamino acids prior to derivatization for gas chromatographic
separation, including gas chromatograph/ combustion/ isotope
ratio mass spectrometry (GC/C/IRMS) [4,5]. Among the wet
chemical treatment, we confirmed that the average recovery ofamino acids was better than 94% and there was no nitrogenisotopic fractionation [5]. Pre-treatment of an extract has two
advantages: (i) Separation from complex hydrophilic compounds,
including sugars and organic acids, which consume derivatization
reagents during esterification, (ii) desalting of inorganic
compounds derived from matrix minerals, thereby preventing
damage to the combustion and reduction furnaces in the system.
We occasionally use ion-pair liquid chromatography combinedwith electro-spray ionization mass spectrometry (LC/ESI-MS) for
further compound isolation [unpulished].
A remarkable feature of the nitrogen isotopic compositions ofbulk organics from various carbonaceous chondrites is that they
are significantly higher than those of terrestrial materials, up to+3200‰ (vs. Air) [6]. We successfully applied the present
optimized procedure to representative carbonaceous meteorites[7], including wide variety of geological samples. We believethat application of compound-specific nitrogen isotopic analysis
will reveal the role of indivisual amino acids and open up ourknowledge of the abiotic chemical processes [8].
References: [1] Meierhenrich, 2008. Amino acids and theasymmetry of life, Springer, pp. 1-241. [2] e.g., Takano et al.,2007. Earth Planetary Science Letters, 254, 106-114; Meinert etal., 2011. Physics Life Reviews, 8, 307-330. [3] Glavin andDworkin, 2009. PNAS, 106, 5487-5492. [4] Chikaraishi et al.
2010. Earth, Life, Isotopes, Kyoto Univ Press., pp. 367-386. [5]Takano et al. 2010. Rapid Communications Mass Spectrometry,24, 2317-2323. [6] Busemann et al. 2006. Science, 312, 727-730.
[7] Chan et al., 2013. Abstract of Hayabusa Symposium in thisissue. [8] Ohkouchi and Takano, 2013. Organic nitrogen: sources,fates, and chemistry. Treatise on Geochemistry, in press.