Optical characterization of Prodan aggregates in water medium

Area of Science:

• Biophysical Chemistry
• Fluorescent Probes
• Spectroscopy

Background:

• Prodan (2-dimethylamino-6-propionylnaphthalene) is a sensitive fluorescent probe widely used in biological systems.
• Its emission spectrum's sensitivity to medium polarity makes it valuable, though it partitions into water in lipid dispersions.
• Understanding Prodan's behavior in aqueous environments is crucial for accurate biological applications.

Purpose of the Study:

• To experimentally and theoretically investigate the optical properties of Prodan in aqueous media.
• To understand Prodan-water and Prodan-Prodan interactions and aggregation behavior.
• To assess the impact of aggregation on Prodan's fluorescence spectrum.

Main Methods:

• Absorption and emission spectroscopies were used to study Prodan in water and cyclohexane.
• Theoretical calculations included absorption spectra of monomeric and aggregated Prodan, and solvation free energies.
• Light scattering measurements were performed to detect aggregation.

Main Results:

• Prodan forms aggregates in water at very low concentrations (0.9 μM), confirmed by light scattering and supported by solvation energy calculations.
• The 358 nm absorption band is attributed to monomeric Prodan, saturating at higher concentrations as aggregation increases.
• Aggregation is further indicated by changes in absorption bands around 250 nm and 280 nm.
• Surprisingly, Prodan's fluorescence emission spectrum showed no sensitivity to aggregation up to 15 μM.

Conclusions:

• Prodan aggregation in aqueous solutions occurs at significantly lower concentrations than previously assumed.
• The insensitivity of Prodan's fluorescence emission to aggregation poses a challenge for its interpretation in biological systems.
• Researchers must account for Prodan aggregation, even at low concentrations, when utilizing it as a fluorescent probe in biological studies.