RESUMO
Studying the ultrafast dynamics of ionized aqueous biomolecules is important for gaining an understanding of the interaction of ionizing radiation with biological matter. Guanine plays an essential role in biological systems as one of the four nucleobases that form the building blocks of deoxyribonucleic acid (DNA). Guanine radicals can induce oxidative damage to DNA, particularly due to the lower ionization potential of guanine compared to the other nucleobases, sugars, and phosphate groups that are constituents of DNA. This study utilizes femtosecond optical pump-probe spectroscopy to observe the ultrafast vibrational wave packet dynamics of the guanine radical anion launched by photodetachment of the aqueous guanine dianion. The vibrational wave packet motion is resolved into 11 vibrational modes along which structural reorganization occurs upon photodetachment. These vibrational modes are assigned with the aid of density functional theory (DFT) calculations. Our work sheds light on the ultrafast vibrational dynamics following the ionization of nucleobases in an aqueous medium.
RESUMO
The phenylalanine radical (PheË) has been proposed to mediate biological electron transport (ET) and exhibit long-lived electronic coherences following attosecond photoionization. However, the coupling of ultrafast structural reorganization to the oxidation/ionization of biomolecules such as phenylalanine remains unexplored. Moreover, studies of ET involving PheË are hindered by its hitherto unobserved electronic spectrum. Here, we report the spectroscopic observation and coherent vibrational dynamics of aqueous PheË, prepared by sub-6 fs photodetachment of phenylalaninate anions. Sub-picosecond transient absorption spectroscopy reveals the ultraviolet absorption signature of PheË. Ultrafast structural reorganization drives coherent vibrational motion involving nine fundamental frequencies and one overtone. DFT calculations rationalize the absence of the decarboxylation reaction, a photodegradation pathway previously identified for PheË. Our findings guide the interpretation of future attosecond experiments aimed at elucidating coherent electron motion in photoionized aqueous biomolecules and pave way for the spectroscopic identification of PheË in studies of biological ET.