Fluorescence of proteins induced by two-photon absorption

Area of Science:

• Biophysics
• Photochemistry
• Spectroscopy

Background:

• Proteins like trypsin, thyroglobulin, hemoglobin, and albumin exhibit UV fluorescence.
• Understanding protein photophysics is crucial for various biological and medical applications.

Purpose of the Study:

• To investigate the two-photon absorption and fluorescence properties of specific proteins.
• To determine the origin of fluorescence in these proteins under two-photon excitation.
• To analyze the influence of concentration on protein fluorescence.

Main Methods:

• Excitation of protein samples using a frequency-duplicated Q-switched Nd:YAG laser.
• Observation and analysis of UV fluorescence spectra.
• Comparison of one-photon and two-photon fluorescence properties.
• Estimation of two-photon absorption cross-sections.

Main Results:

• Proteins exhibit two-photon absorption and subsequent fluorescence at room temperature.
• The observed fluorescence primarily originates from tryptophan residues.
• Two-photon fluorescence shows a red shift of approximately 20 nm compared to one-photon fluorescence.
• Weak emission from tyrosine is attributed to forbidden two-photon transitions.
• Concentration-dependent effects on hemoglobin and tryptophan fluorescence were observed.

Conclusions:

• Tryptophan residues are the main source of two-photon excited fluorescence in the studied proteins.
• Two-photon spectroscopy provides insights into protein photophysics, including forbidden transitions.
• The findings contribute to the understanding of molecular interactions and fluorescence mechanisms in biological molecules.