Effect of pH on the Release of Amino Acids from Microorganisms via Subcritical Water Extraction

Distributions of amino acids, based on either enantiomeric excess or relative abundances of amino acid type, are key chemical measurements in the search for life on potential future astrobiology missions. This study investigates the impact of subcritical water extraction (SCWE) at different pH values on the measurement of these amino acid distributions when the starting material contains microorganisms. Cells of Pseudoalteromonas haloplanktis at pH 2, 6, and 12 underwent SCWE at 200 °C for 30 min, and the released amino acids were analyzed by capillary electrophoresis with laser-induced fluorescence detection. The overall extraction yield was improved under both acidic and alkaline conditions relative to neutral ones. The characteristic biotic distributions of amino acid types present in these samples were preserved under all conditions, although some degradation was observed as a relative increase in the amount of simpler amino acids like glycine. Contrarily, while biotic enantiomeric excesses remained preserved under acidic conditions, racemization was observed under alkaline conditions. This work shows the power of SCWE for converting polypeptide biopolymers into amino acid monomers, independent of the initial pH of the sample. However, it also highlights the need to understand the physiochemical properties of the sample to allow the mitigation of undesirable changes to the native distributions of amino acids.