Fluorescence quenching of tryptophan and tryptophanyl dipeptides in solution

Pedro L Muíño,Aaron P Osysko

doi:10.4236/jbpc.2011.23036

Abstract

We report measurements of fluorescence quantum yields of tryptophan, tryptophanylaspartate and tryptophanylarginine in several solvents as well as in aqueous solutions over a wide range of pH. We aim to test a computational model developed by Callis and coworkers of fluorescence quantum yield, which postulates that quenching in tryptophan arises from energy loss due to an electron transfer from the aromatic system of tryptophan to one of the amides in the protein backbone. Since the electron transfer state is expected to be high in energy, normally this would not be a possible outcome, but because of its large dipole, such a state should be more accessible in polar solvents. In addition, conditions of low (high) pH, which result in a net positive (negative) charge for the terminal amine (carboxyl) should result in an increase (decrease) of electron transfer rates and low (high) quantum yields. The observed results confirm the predictions of the model.

Highlights

The amino acid tryptophan is unique in that its large size makes its spectroscopic properties significantly different from those of the other amino acids [1]
The maximum value of the quantum yield depends on the nature of the chromophore, with tryptophan and tryptophanylaspartate showing strong fluorescence and tryptophanylarginine having maximum values near those of 3MI
We observed that high pH results in a high quantum yield because the negatively charged carboxyl results in a very low efficiency for the electron transfer events

Summary

Introduction

The amino acid tryptophan is unique in that its large size makes its spectroscopic properties significantly different from those of the other amino acids [1] This property, coupled to its relative scarcity and to its sensitivity to the nature of the microenvironment, has made tryptophan a good probe of protein structure, which is the key parameter to understand protein functionality [2]. To this end, there are two spectroscopic properties of interest: fluorescence wavelength and fluorescence quantum yield. There has been no clear way to make even qualitative predictions of fluorescence quantum yield for tryptophan in proteins

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