Ultraviolet Photodissociation of Proteinogenic Amino Acids

Theultraviolet photochemistry of the amino acids glycine, leucine,proline, and serine in their neutral forms was investigated usingparahydrogen matrix-isolation spectroscopy. Irradiation by 213 nmlight destroys the chirality of all three chiral amino acids as aresult of the alpha-carbonyl C-C bond cleavage and hydrocarboxyl(HOCO) radical production. The temporal behavior of the Fourier-transforminfrared spectra revealed that HOCO radicals rapidly reach a steadystate, which occurs predominantly due to photodissociation of HOCOinto CO + OH or CO2 + H. In glycine and leucine, the amineradicals generated by the alpha-carbonyl C-C bond cleavagerapidly undergo hydrogen elimination to yield methanimine and 3-methylbutane-1-imine,respectively. Breaking of the alpha-carbonyl C-C bond inproline appeared to yield 1-pyrroline, although due to its weak absorptionit remains unconfirmed. In serine, additional products were formaldehydeand E/Z ethanimine. The presentstudy shows that the direct production of HOCO previously observedin alpha-alanine generalizes to other amino acids of varying structure.It also revealed a tendency for amino acid photolysis to form iminesrather than amine radicals. HOCO should be useful in the search foramino acids in interstellar space, particularly in combination withsimple imine molecules.