Why water makes 2-aminopurine fluorescent?

Area of Science:

• Biophysical Chemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• 2-Aminopurine (2AP) is a vital fluorescent probe in nucleic acid research, serving as a surrogate for purine bases.
• The luminescence of 2AP is highly sensitive to its surrounding environment, particularly the presence of water molecules.
• Understanding the photophysical mechanisms governing 2AP's fluorescence is crucial for its effective application in biological studies.

Purpose of the Study:

• To elucidate the molecular mechanisms behind the environmentally-dependent luminescence of 2-aminopurine (2AP).
• To investigate the role of water molecules in modulating the excited-state lifetime and fluorescence of 2AP.
• To identify the specific interactions and pathways responsible for the observed changes in 2AP's photophysical properties.

Main Methods:

• Computational simulations were employed to study isolated 2AP and various 2AP-water clusters.
• Quantum chemical calculations were performed to analyze potential energy surfaces and excited-state dynamics.
• Investigated competing photophysical pathways including internal conversion, proton transfer, and intersystem crossing.

Main Results:

• The excited-state lifetime of 2AP is directly influenced by the number and arrangement of water molecules.
• A significant energy barrier for internal conversion between the S1 minimum and a conical intersection controls 2AP's luminescence.
• Hydrogen bonding of a single water molecule to the amino group of 2AP increases this energy barrier, enhancing fluorescence.

Conclusions:

• The observed environmental tuning of 2AP luminescence is attributed to the ordering of nπ* and ππ* states.
• A non-adiabatic change in the pathway from the S1 minimum to the conical intersection, induced by water interaction, elevates the internal conversion barrier.
• Even a single water molecule hydrogen-bonded to the amino group is sufficient to render 2AP fluorescent, highlighting a key mechanism for its photophysical modulation.