Tryptophan Fluorescence as Probe of Protein Structure
Tryptophan in the ground-state energy level S0 has a dipole moment of approximately 2.0 debye (D)9; as it transitions to the first excited state S1, the dipole moment of the molecule changes to 6.0 D.9 Because of this large dipole moment, tryptophan in the S1 state is a highly polar molecule and interacts strongly with polar moieties existing in its environment. This strong interaction stabilizes the excited-state tryptophan relative to the ground state causing a reduction in the energy difference between the two states. Therefore, in highly polar environments, the excited tryptophan molecules are stabilized and emit lower energy light (larger wavelengths), whereas in low polarity environments the excited tryptophan molecules experience minimal stabilization and, therefore, emit higher energy light at shorter wavelengths. This can be seen by comparing the steady-state fluorescence emission spectrum of free tryptophan dissolved in water that has a peak emission wavelength of 355 nm with that of tryptophan (or rather indole) dissolved in cyclohexane that exhibits peak emission close to 300 nm. This polarity dependence can be used to probe protein structure.
Monitoring the effect of calcium upon the fluorescence spectrum of cod parvalbumin is a classical example that demonstrates how fluorescence can be used to probe changes in protein structure.7 The fluorescence spectrum of cod parvalbumin in the absence of calcium ion exhibits maximum emission close to 350 nm, indicating that the lone tryptophan residue of the protein is located in an environment that is highly exposed to water molecules. On the other hand, when the protein binds to calcium ion, the fluorescence maximum occurs at approximately 320 nm, showing that in the calcium-bound state the lone tryptophan is in a relatively nonpolar (i.e., water sequestered) environment. This shift in fluorescence emission maximum demonstrates that the binding of protein to calcium has led to a significant change in the local environment of the tryptophan, indicating that calcium binding induces structural changes in the parvalbumin protein.
The environmental sensitivity of tryptophan fluorescence makes this technique essential for probing the folding state of proteins. When a protein is unfolded, all residues (including the tryptophan) are exposed to water. All proteins that contain tryptophan exhibit maximum emission at 355 nm when they are unfolded. The tryptophan residues of most folded proteins are rarely fully hydrated; therefore, the emission maximum of native proteins occurs at wavelengths significantly shorter than 355 nm; typical values for the emission maximum of some single tryptophan proteins in the native state are: staphylococcal nuclease (335 nm), ribonuclease T1 (320 nm), and Azurin from Pseudomonas aeruginosa (305 nm).9 Measuring the fluorescence spectrum is a good diagnostic tool for determining if the protein is folded in the native state or not. The statements made above regarding single tryptophan proteins can also be generalized to proteins having multiple tryptophan residues, although a full quantitative analysis becomes difficult. The different tryptophan residues contribute differently to the measured fluorescence spectrum of multitryptophan proteins; luckily, it is usually the fluorescence of one particular tryptophan residue that becomes the major contributor to the fluorescence spectrum of the folded protein.
