Meteoritic Isoleucine Epimerization in the Chronology of Asteroidal Parent Body Fluids

Introduction: Meteoritic amino acid abundances and abundance ratios have been diversely applied in cosmochemistry. Abundances have gauged the duration of experimental heating of Murchison [1], and abundance ratios have exhibited differences according to meteorite classification group and the extent of asteroidal parent body aqueous and thermal alteration [25]. Enantiomeric excesses (ees), expressable as amino acid enantiomer ratios ≠ 1, also have been shown to correlate with aqueous alteration. The ee of Lisovaline, a non-protein, α-methyl amino acid, increased with aqueous alteration in several meteorite groups [6,7], and the ees of L-isoleucine (L-Ile) and Dallo-isoleucine (D-aIle), α-H amino acids, decreased with aqueous alteration in a suite of CR chondrites [8]. Meteorite Hills (MET) 00426, a minimally altered CR 3.0 chondrite [9], contained the largest ee of a nonprotein amino acid to date [8,10]. The capability of amino acids to racemize (or epimerize) and undergo a reversal of configurational symmetry is fundamental to discussions about the effects of aqueous alteration on meteoritic amino acids, but this same capability also allows applications to sedimentary geochronology. Geochronometers based on amino acid destruction or racemization use naturally occurring amino acids in fossils [11] and in some cases can approximate ages of fossilized sediments younger than several 10 yr [12]. Two amino acids used for this purpose, L-Ile and DaIle, interconvert via α-epimerization following loss and acquisition of protons on opposite sides of the molecule and undergo a chiral reversal at the α-carbon, known also as C2. D-Ile and L-aIle, two other isoleucine stereoisomers of four total, also interconvert via α-epimerization. The kinetics of this process are described by isoleucine epimerization rate constants, experimentally shown to vary with changes in pH, temperature, and whether isoleucine is “free” in solution or chemically bound in peptides or proteins [1321]. Published rate constants have been combined here to establish the average and range of rate constants to estimate possible timescales for isoleucine epimerization in asteroidal parent body fluids at constant temperatures. We present kinetics calculations using stereoisomeric amino acid disequilibria (abundance ratios) to constrain the total residence time of these amino acids in their parent body fluids. Mineral and isotope analyses provide the chronological context to which these model calculations are compared. Methods: 10-10 amino acid concentration changes for each reversible, first order reaction were calculated by the finite difference method with Euler’s approximation, a standard approach for modeling reactions governed by simultaneous, differential rate equations [22]. In any given time step, concentration changes due to αand β-epimerization are calculable from rate constants k1 and k2 (α-epimerization) and k3 and k4 (β-epimerization) and the initial concentrations of amino acids during that step: