Phase-resolved fluorescence study of mono-L-aspartyl chlorin E6

Area of Science:

• Photochemistry
• Photophysics
• Biophysical Chemistry

Background:

• Photodynamic therapy (PDT) utilizes photosensitizers to generate reactive oxygen species upon light activation.
• Understanding photosensitizer aggregation is crucial for optimizing PDT efficacy.
• Mono-L-aspartyl chlorin e6 (NPe6) is a photosensitizer with potential therapeutic applications.

Purpose of the Study:

• To investigate the photophysical properties and aggregation behavior of mono-L-aspartyl chlorin e6 (NPe6) in aqueous solutions.
• To determine the concentration-dependent behavior of NPe6 and its impact on fluorescence.
• To elucidate the structural factors influencing NPe6 dimerization.

Main Methods:

• Phase-resolved fluorescence spectroscopy was employed to analyze NPe6 in water across a concentration range (3.13x10(-7) to 8.00x10(-5) M).
• Experimental determination of photophysical parameters for the lowest singlet excited state.
• Semiempirical molecular orbital calculations were performed to assess molecular structure.

Main Results:

• NPe6 exists as isolated molecules below 1.00x10(-5) M, with fluorescence originating from the lowest singlet excited state.
• Above 1.00x10(-5) M, NPe6 forms dimers, leading to a red shift in fluorescence and enhanced emission (700-750 nm).
• Molecular orbital calculations indicated significant bending of the sodium aspartate group, hindering dimer formation at lower concentrations.

Conclusions:

• The concentration-dependent aggregation of NPe6 influences its photophysical properties in aqueous solution.
• The bent structure of the sodium aspartate moiety plays a key role in preventing early-stage dimerization.
• These findings provide insights into optimizing NPe6 formulation for photodynamic therapy.