Gas-phase ionic syntheses of amino acids: beta versus alpha

Area of Science:

• Astrochemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Amino acids like glycine (Gly) and alanine (Ala) are fundamental biomolecules.
• Understanding their formation pathways, especially in extraterrestrial environments, is crucial.

Purpose of the Study:

• To investigate the gas-phase reaction mechanisms for synthesizing protonated glycine (GlyH+) and alanine (AlaH+) from protonated hydroxylamine and small carboxylic acids.
• To determine the factors governing the stereochemical outcome (alpha- vs. beta-isomer) of alanine formation.

Main Methods:

• Theoretical calculations (density functional theory) to determine reaction pathways, transition states, and energy profiles.
• Experimental validation using selected ion flow tube mass spectrometry (SIFT-MS) and multi-collision-induced dissociation (MCID).

Main Results:

• The reaction proceeds via amino group insertion into a C-H bond.
• Formation of beta-alanine (beta-AlaH+) is kinetically and thermodynamically favored over alpha-alanine (alpha-AlaH+).
• Lower activation energy and more stable hydrogen bonding in transition states and products explain the preference for the beta-isomer.

Conclusions:

• Gas-phase synthesis of amino acids shows a strong preference for the beta-isomer of alanine.
• Interstellar chemical processes may be responsible for the presence of glycine and beta-alanine in carbonaceous chondrite meteorites.