Radical-cationic gaseous amino acids: a theoretical study.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Biophysical Chemistry

Background:

• Radical-cationic amino acids (RCAAs) are key intermediates in biological processes.
• Understanding their stable forms and electronic properties is crucial for mechanistic studies.
• Previous studies have explored various aspects of RCAA stability.

Purpose of the Study:

• To investigate the stable structures and tautomers of various gaseous RCAAs.
• To elucidate the primary stabilizing factors governing RCAA electronic configurations.
• To analyze the role of spin delocalization and hydrogen transfer in RCAA isomerism.

Main Methods:

• Utilized the hybrid B3LYP exchange-correlation functional with diverse basis sets.
• Employed the highly correlated Coupled Cluster Singles Doubles and Triples (CCSD(T)) method.
• Examined aliphatic (Ala, Pro, Ser), sulfur-containing (Cys), aromatic (Trp, Tyr, Phe), and basic (His) RCAAs.

Main Results:

• Spin delocalization is the dominant stabilizing factor for most RCAAs.
• Hydrogen transfer from C(alpha) to the carboxyl group yields the lowest-energy enolic form for Ala, Pro, Ser, Cys, Tyr, and Phe.
• Histidine stabilizes via protonation and H-transfer to the side chain (keto form), while Tryptophan favors spin delocalization over aromaticity (keto form).

Conclusions:

• The captodative enolic form is energetically favored for most studied RCAAs due to efficient spin delocalization.
• Specific stabilization mechanisms, including protonation and unique spin delocalization pathways, are observed for His and Trp.
• These findings provide fundamental insights into the reactivity and stability of radical-cationic amino acid intermediates.