Theoretical and experimental investigations on the reactions of positively charged phenyl radicals with aromatic amino acids.
The chemical behavior of positively charged phenyl radicals 3-dehydro-N-phenylpyridinium (a), N-(3-dehydro-5-chlorophenyl)pyridinium (b), and N-(3-dehydrophenyl)pyridinium (c) toward L-tyrosine, phenylalanine, and tryptophan was investigated in the gas phase both theoretically by performing molecular orbital calculations and experimentally by using FT/ICR mass spectrometry. All radicals react with phenylalanine and tryptophan nearly at the collision rate. The overall reactivity of the radicals toward tyrosine follows the order a > b > c, which is consistent with the electron affinity (EA) ordering of the radicals. The higher the electrophilicity (or EA) of the radical, the greater the reactivity. As expected, all radicals abstract a hydrogen atom from all of the amino acids. However, the most electrophilic radical a was also found to react with these amino acids via NH2 abstraction. A new reaction observed between radicals a-c and aromatic amino acids is the addition of the radical to the aromatic ring of the amino acid followed by Calpha-Cbeta bond cleavage, which leads to side-chain abstraction by the radical.
Huang
Yiqun
Y
Brown Laboratories, Department of Chemistry, Purdue University, 425 Central Drive, West Lafayette, Indiana 47907-2018, USA.
Kenttämaa
Hilkka
H
D024322:Amino Acids, Aromatic / Q000737:chemistry*; D001555:Benzene Derivatives / Q000737:chemistry*; D003545:Cysteine / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D010649:Phenylalanine / Q000737:chemistry; D011726:Pyridinium Compounds / Q000737:chemistry; D013816:Thermodynamics; D014364:Tryptophan / Q000737:chemistry; D014443:Tyrosine / Q000737:chemistry
D024322:Amino Acids, Aromatic; D001555:Benzene Derivatives; D005609:Free Radicals; D011726:Pyridinium Compounds; D014443:Tyrosine; D010649:Phenylalanine; D014364:Tryptophan; D003545:Cysteine