Quantifying hydrogen-deuterium exchange of meteoritic dicarboxylic acids during aqueous extraction Item Type Article; text Authors Fuller, M.; Huang, Y. Citation Fuller, M., & Huang, Y. (2003). Quantifying hydrogendeuterium exchange of meteoritic dicarboxylic acids during aqueous extraction. Meteoritics & Planetary Science, 38(3), 357-363. DOI 10.1111/j.1945-5100.2003.tb00271.x Publisher The Meteoritical Society Journal Meteoritics & Planetary Science Rights Copyright © The Meteoritical Society Download date 29/09/2021 19:01:11 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version Link to Item http://hdl.handle.net/10150/655665 Meteoritics & Planetary Science 38, Nr 3, 357–363 (2003) Abstract available online at http://meteoritics.org Quantifying hydrogen-deuterium exchange of meteoritic dicarboxylic acids during aqueous extraction Megan FULLER and Yongsong HUANG* Department of Geological Sciences, Brown University, Providence, Rhode Island 02912–1846, USA *Corresponding author. E-mail: [email protected] (Received 15 July 2002; revision accepted 8 January 2003) Abstract–Hydrogen isotope ratios of organic compounds in carbonaceous chondrites provide critical information about their origins and evolutionary history. However, because many of these compounds are obtained by aqueous extraction, the degree of hydrogen-deuterium (H/D) exchange that occurs during the process needs to be quantitatively evaluated. This study uses compound- specific hydrogen isotopic analysis to quantify the H/D exchange during aqueous extraction. Three common meteoritic dicarboxylic acids (succinic, glutaric, and 2-methyl glutaric acids) were refluxed under conditions simulating the extraction process. Changes in δD values of the dicarboxylic acids were measured following the reflux experiments. A pseudo-first order rate law was used to model the H/D exchange rates which were then used to calculate the isotope exchange resulting from aqueous extraction. The degree of H/D exchange varies as a result of differences in molecular structure, the alkalinity of the extraction solution and presence/absence of meteorite powder. However, our model indicates that succinic, glutaric, and 2-methyl glutaric acids with a δD of 1800‰ would experience isotope changes of 38‰, 10‰, and 6‰, respectively during the extraction process. Therefore, the overall change in δD values of the dicarboxylic acids during the aqueous extraction process is negligible. We also demonstrate that H/D exchange occurs on the chiral α-carbon in 2-methyl glutaric acid. The results suggest that the racemic mixture of 2-methyl glutaric acid in the Tagish Lake meteorite could result from post-synthesis aqueous alteration. The approach employed in this study can also be used to quantify H/D exchange for other important meteoritic compounds such as amino acids. INTRODUCTION the stable isotope pairs, e.g., 13C/12C and 15N/14N (see Sanford, Bernstein, and Dworkin 2001 and references Dicarboxylic acids are abundant constituents of within). The δD values for succinic, 2-methyl succinic, carbonaceous chondrites and have been described in the glutaric, and 2-methyl glutaric acids from the Tagish Lake and Murchison (Lawless et al. 1974; Cronin et al. 1993) and, more Murchison meteorites were found to range from +389.5 to recently, the Tagish Lake meteorites (Pizzarello et al. 2001). +1550.7‰, while δ13C values varied from +5.5 to +28.1‰ In the Tagish Lake meteorite, they are the most abundant (Pizzarello and Huang 2002). These isotopic enrichments are soluble organic species, with a concentration of over 75 nmol/ comparable to those described previously for other classes of gram of meteorite, and show molecular and isotopic meteoritic compounds, such as amino acids. These distributions that are remarkably similar to those observed in compounds have been attributed, particularly because of their the Murchison meteorite (Pizzarello and Huang 2002). high δD values, to partially altered, highly deuterated Pizzarello and Huang (2002) reported compound specific interstellar precursors (see Cronin and Chang 1993, for a hydrogen and carbon isotopic measurements for several review). dicarboxylic acids found in the Tagish Lake and Murchison However, the δD values determined for several classes of meteorites. Hydrogen isotopic ratio analysis is a particularly meteoritic compounds may in fact be minimum values. sensitive approach for identifying organic compounds that Hydrogen atoms on meteoritic compounds could exchange originated in cold interstellar space. Because of the large mass with those in water used during the aqueous extraction difference (100%) between hydrogen (H) and deuterium (D), process. For example, the carboxyl hydrogens on the hydrogen isotope fractionation is the largest among all of dicarboxylic acids exchange completely with the hydrogen in 357 © Meteoritical Society, 2003. Printed in USA. 358 M. Fuller and Y. Huang the extraction water. Also, α-hydrogens, i.e., those bound to known δD values were placed into vials and dried using a the carbon adjacent to the carboxyl carbon, are slightly acidic rotary evaporator. Each type of reflux experiment we and could partially exchange with hydrogen in the extraction performed is described below: water (Epstein et al. 1987; Pizzarello and Huang 2002). The 1. D2O reflux: to measure the net exchange that occurred in potential for isotopic exchange has been investigated for the D2O-dicarboxylic acid system, the compounds were some amino acid compounds by Pereira et al. (1975), who refluxed in the presence of 2 ml D2O (99.9%, Aldrich). used stable isotope mass fragmentography to estimate the The pH of the solution was 3.4. amount of hydrogen-deuterium exchange on eight protein 2. D2O + meteorite powder: to assess the affect of amino acids and found that the amino acids showed variable meteorite powder on the exchange process, degrees of hydrogen exchange. Further work was done on the approximately 700 mg of Murchison meteorite powder hydrogen exchange of amino acids by Lerner (1995). Lerner was added to a dicarboxylic acid/D2O solution. The concluded that temperature and the amount of meteorite rock:water ratio of 350 mg:1 ml is approximately what powder controlled the rate at which hydrogen exchange takes was used to extract the Tagish Lake meteorite (Pizzarello place. However, an accurate assessment of hydrogen and Huang 2002). The powder was annealed at 550ºC exchange of individual compounds cannot be achieved before use to remove any organic compounds in the without accurate measurements of D/H ratios at the molecular powder. The pH of the diacid/D2O/meteorite solution level. was 7.1. The primary objective of this study is to use compound- 3. D2O + meteorite powder + pyridine: to assess the role of specific hydrogen isotope analysis to quantify hydrogen alkalinity on the exchange process, pyridine was added isotope exchange caused by aqueous extraction for three (6.8 µl pyridine/ml D2O) to the diacid/D2O/meteorite common meteoritic dicarboxylic acids. Specifically, our goals solution. Pyridine creates a buffer solution with a pH of are to: 1) quantify the hydrogen-deuterium exchange during 8.1, similar to the pH of typical meteorite extractions. extraction of dicarboxylic acids using deuterium oxide (D2O); Each vial was capped with a Teflon-lined lid and placed 2) obtain the exchange rate constant for each diacid and use on a heating block at 100–110ºC for 24 hr. These the rate constant to compute the isotopic exchange caused by experiments were run in duplicate. One set of samples were the aqueous extraction process; 3) assess changes in the not stirred (referred to as Case 1) while the other set of hydrogen-deuterium exchange in the diacids under different samples were stirred continuously (Case 2) over the 24 hr extraction conditions (e.g., presence/absence of stirring, reaction period. The samples were cooled and dried using a meteorite powder, and different alkalinities); and 4) compare rotary evaporator. The samples containing meteorite powder the rate of hydrogen exchange between glutaric acid and 2- were centrifuged, and the supernatant was decanted into a methyl glutaric acid and examine the role of hydrogen clean vial, along with subsequent rinses, and dried. Care was exchange in the possible racemization of 2-methyl glutaric taken to ensure that there was no cross-contamination of acid. This fourth goal is important for understanding the samples via the rotary evaporator by rinsing with solvents origin of the racemic 2-methyl glutaric acid in the Murchison between samples. and Tagish Lake meteorites (Pizzarello and Huang 2002). If hydrogen exchange occurs at a chiral center (e.g., the chiral Sample Preparation and GC Analysis center in 2-methyl glutaric acid), it may conceal any previous enantioselective process that could have acted on the The diacids were derivatized using acidified isopropyl meteoritic molecule. alcohol (IPA). Approximately 200 µl of acidified isopropanol were added to the vials. The vials were capped and heated at EXPERIMENTAL METHODS 100°C for 1 hr. The contents of the vials were dried and diluted in toluene for analysis. This procedure eliminates the Experimental Design carboxyl hydrogen which is fully exchanged with the extraction water. Therefore, it is unnecessary to include the Soluble organic compounds are recovered from carboxyl hydrogens in the calculation of exchange rates. We carbonaceous chondrites by powdering the stone, placing the only report the exchange rates