Lifetime evidence for a weak lowest electronic transition in adenosine

Area of Science:

• Photophysics
• Biophysical Chemistry
• Molecular Spectroscopy

Background:

• Adenosine is a fundamental component of nucleic acids.
• Understanding its photophysical properties is crucial for various biological and chemical applications.
• Previous studies have explored adenosine's spectral characteristics, but its fluorescence decay dynamics require further elucidation.

Purpose of the Study:

• To determine the fluorescence decay kinetics of adenosine in a 1:1 glycol/water glass matrix at 77 K.
• To analyze the decay data to obtain intrinsic radiative lifetimes and oscillator strengths.
• To compare experimental findings with current theoretical models for electronic transitions.

Main Methods:

• Utilized narrow pulse (700 ps) laser excitation at 290 nm.
• Employed fluorescence detection with a scanned narrow-gate (100 ps) fast sampler.
• Applied digital averaging and re-iterative non-linear least squares convolution for data analysis.

Main Results:

• The fluorescence decay of adenosine was best described by a bi-exponential function: I(t) = 0.59exp(-t/1.2ns) + 0.41exp(-t/7.0ns).
• Derived intrinsic radiative lifetimes of 150 ns and 220 ns.
• Calculated a combined oscillator strength of 1.5 x 10^-2, significantly lower than the 0.29 for the entire first absorption band.

Conclusions:

• The fluorescence decay data indicate that transitions to and from the lowest-lying state in adenosine's first absorption band are significantly forbidden.
• Current theoretical models do not fully account for these experimental observations.
• Further theoretical investigations are needed to explain the observed forbidden transitions in adenosine.