UV-Induced Proton-Coupled Electron Transfer in Cyclic DNA Miniduplexes

Area of Science:

• Photochemistry
• Molecular Biophysics
• DNA Dynamics

Background:

• Understanding DNA excited-state dynamics is crucial for photochemistry and photobiology.
• Previous studies on longer DNA duplexes suggest complex decay pathways involving electron and proton transfer.
• Investigating minimal DNA structures can elucidate fundamental photophysical processes.

Purpose of the Study:

• To investigate the excited-state dynamics of minimal DNA miniduplexes.
• To elucidate the role of base stacking and pairing in DNA excited-state decay.
• To determine if simple DNA structures can model decay channels in longer sequences.

Main Methods:

• Time-resolved infrared spectroscopy was employed to monitor ultrafast processes.
• Two cyclic DNA miniduplexes, each with two base pairs, were synthesized and studied.
• A modified guanine (8-oxo-7,8-dihydroguanine) was introduced to probe the effect of disrupted base stacking.

Main Results:

• UV excitation induced intrastrand electron transfer followed by interstrand proton transfer in a G·C miniduplex.
• The excited state associated with this transfer decayed within tens of picoseconds.
• Substitution of guanine with 8-oxo-7,8-dihydroguanine abolished this excited state, indicating the importance of base stacking.

Conclusions:

• A two-stacked-base-pair DNA miniduplex effectively models the interplay of intrastrand and interstrand decay channels.
• Excited states in longer DNA duplexes likely localize on two adjacent base pairs.
• Base stacking plays a critical role in DNA excited-state dynamics and decay pathways.