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Enantiomer-specific isotope analysis of D- and L-alanine: Nitrogen isotopic hetero- and homogeneity in microbial and chemical processes

Nitrogen isotopic hetero- and homogeneity of D-α-alanine and L-α-alanine were investigated in microbial processes in the domain Bacteria and in chemical processes in symmetric organic synthesis. D-alanine is an enantiomer that is physiologically essential for microbial growth and metabolic maintenance. The nitrogen isotopic difference Δ15ND-L (defined as δ15ND-Ala - δ15NL-Ala) in peptidoglycan amino acids in bacteria such as the representative gram-positive phyla Firmicutes and Actinobacteria (Enterococcus faecalis, Staphylococcus aureus, Staphylococcus staphylolyticus, Lactobacillus acidophilus, Bacillus subtilis, Micrococcus luteus, and Streptomyces sp.) tended to be 15N-depleted in D-alanine (Δ15ND-L < -2.0 permil). These results suggest that the composition of isotopically heterogeneous components in these bacteria is primarily controlled by enzymatic pathways prior to formation of the bacterial cell wall. In contrast, the Δ15ND-L of racemic alanine in the chemical pathway during the nucleophilic substitution reaction (SN1 type) between 2-bromopropionic acid and ammonia identified fully homogeneous components for each enantiomer. The novel enantiomer-specific isotopic analysis (ESIA) method is useful in determining the origins of chirality in biogenic and abiogenic processes and is applicable to enantiomer studies. -- Keywords: D-alanine, microbial process, chemical process, nitrogen isotopic composition, enantiomer-specific isotope analysis (ESIA)