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-{\alpha}-alanine and L-{\alpha}-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 {\Delta}15ND-L (defined as {\delta}15ND-Ala - {\delta}15NL-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 ({\Delta}15ND-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 {\Delta}15ND-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)