Ultraviolet Photodissociation Spectroscopy of [dAMP-H]^- at Low Temperature

Area of Science:

• Physical Chemistry
• Molecular Biophysics
• Photochemistry

Background:

• Ultraviolet (UV) photoexcitation can induce nucleotide fragmentation, posing a risk to DNA strand integrity.
• Understanding the molecular-level stability of nucleotides under UV irradiation is crucial for assessing DNA damage mechanisms.

Purpose of the Study:

• To investigate the wavelength-dependent photofragmentation of deoxyadenosine monophosphate anions ([dAMP-H]⁻).
• To elucidate the fragmentation pathways and cross sections of [dAMP-H]⁻ following UV photoexcitation.
• To correlate spectral features with molecular structure and electronic transitions.

Main Methods:

• Wavelength-dependent relative photoabsorption cross-section measurements of [dAMP-H]⁻ were performed using a cryogenic 16-pole radiofrequency wire trap at 3 K.
• Quantum chemical calculations were employed to interpret the observed spectral features and electronic excitations.
• The yield of five distinct photofragments was studied as a function of photon energy.
• An absolute photofragmentation cross section was determined by calibrating against the photodetachment cross section of iodide ions (I⁻).

Main Results:

• Resolved spectral features were observed in the 240–270 nm absorption band, attributed to vibrational bands of specific conformers at low temperatures.
• Quantum chemical calculations indicated that the absorption arises from a ππ* excitation localized on the adenine moiety.
• The study identified five distinct photofragmentation channels, with varying fragment yields across the investigated photon energy range.
• No single photofragment channel was found to be dominant; instead, different fragments exhibited distinct energy-dependent trends.

Conclusions:

• The study provides detailed insights into the UV-induced photofragmentation dynamics of [dAMP-H]⁻.
• The observed spectral features and fragmentation patterns offer a molecular-level understanding of nucleotide stability and potential DNA damage.
• The determined absolute photofragmentation cross section is valuable for quantitative modeling of UV-induced DNA damage.