Speaker
Description
Exposure of human body to ultraviolet light can induce formation of dimeric crosslinks at bipyrimidine sites within deoxyribonucleic acid. Nucleotide excision repair enzymes normally recognize the crosslinks and remove them. In-born genetic mutations of the enzymes result in severe photosensitivity and high risk of skin cancer. Completely avoiding sun or consuming medication for pain-free light exposure are the only treatments available. Two main types of crosslinks are known to occur: cyclobutane pyrimidine dimer (CPD) and 6-4 pyrimidine-pyrimidone (64PP) adduct. Several diastereoisomers of CPD and a Dewar isomer of 64PP-adduct are known to form. Isomeric and isobaric nature of the system complicates unambiguous assignment of structures with traditional mass spectrometry based methods. Other traditional spectroscopy techniques, such as nuclear magnetic resonance spectroscopy, require large amount of purified samples to elucidate the structures. Here, we use cryogenic gas-phase infrared action spectroscopy technique coupled with nano-electrospray ionization instead. A custom-built instrument is used for spectroscopic analysis of mass-selected ions trapped in helium nanodroplets cooled down to 0.37 K.
Crosslinked nucleotides produced by exposing a solution of mono- or di-nucleotides to 254 nm ultraviolet radiation was used to test the feasibility of the technique for analyzing such molecules. Mass-selected ions of the crosslinked nucleotides trapped in helium nanodroplets are irradiated with infrared laser to vibrationally excite the ions. The ions released post evaporation of the helium nanodroplets due to the absorption of energy from the photons are detected to plot a highly resolved spectrum. The experimental spectrum is compared with theoretical spectra, of different possible isomers, calculated using density-functional theory. Clearly cis isomers of CPD and non-Dewar isomer of 64PP-adduct are the main products. The presence of other isomers of CPD cannot be ruled out. Cryogenic infrared action spectroscopy of mobility-separated and mass-selected ions will likely provide a conclusive answer.
