Mechanisms of photoinduced Calpha[Single Bond]Cbeta bond breakage in protonated aromatic amino acids.
Photoexcitation of protonated aromatic amino acids leads to C(alpha)[Single Bond]C(beta) bond breakage among other channels. There are two pathways for the C(alpha)[Single Bond]C(beta) bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak C(alpha)[Single Bond]C(beta) bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed.
Lucas
B
B
Université Paris-Sud 91405, LCAM, Bât. 351, Orsay F-9140, France.
Barat
M
M
Fayeton
J A
JA
Perot
M
M
Jouvet
C
C
Grégoire
G
G
Brondsted Nielsen
S
S
D024322:Amino Acids, Aromatic / Q000737:chemistry* / Q000528:radiation effects; D001665:Binding Sites; D002244:Carbon / Q000737:chemistry*; D003198:Computer Simulation; D008027:Light; D008956:Models, Chemical; D008958:Models, Molecular; D010777:Photochemistry / Q000379:methods*; D017785:Photons
D024322:Amino Acids, Aromatic; D002244:Carbon