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Tryptophan as a probe for acid-base equilibria in peptides
Cássia Alessandra Marquezin1, Izaura Yoshico Hirata, Luiz Juliano
1Instituto de Física da Universidade de São Paulo, SP, Brasil.
Biopolymers
|November 25, 2003
Summary
Time-resolved fluorescence reveals how pH affects Tryptophan (Trp) and its peptides. Changes in protonation states influence fluorescence decay, allowing pK determination and understanding molecular behavior in solution.
Area of Science:
- Biophysical Chemistry
- Photochemistry
- Spectroscopy
Background:
- Tryptophan (Trp) fluorescence is sensitive to its microenvironment.
- Understanding Trp fluorescence dynamics is crucial for studying protein structure and function.
- Peptide conformation and ionization states significantly impact Trp fluorescence.
Purpose of the Study:
- To investigate the pH-dependent fluorescence decay of Tryptophan derivatives and peptides.
- To determine pK values associated with ionization events near the indole ring.
- To elucidate the role of rotameric interconversion in Trp fluorescence quenching and lifetime.
Main Methods:
- Time-resolved fluorescence spectroscopy.
- Global analysis of fluorescence decay profiles across a pH range (3.0-11.0).
- Fitting decay data to multi-exponential models to extract lifetime components and pre-exponential factors.
Main Results:
- Fluorescence decay components directly correlate with the ionization state of neighboring groups.
- Protonation of the amino terminus in Trp causes a shift in fluorescence lifetime.
- Peptide-bound Trp exhibits pH-dependent conformational changes (rotamer interconversion) affecting fluorescence.
Conclusions:
- Global analysis of time-resolved fluorescence is effective for monitoring species concentration changes with pH.
- pK values can be accurately determined from fluorescence decay parameters.
- pH control is critical for interpreting fluorescence data from Trp-containing peptides, especially those with ionizable side chains near Trp.