Analysis of the local conformation of proteins with two-dimensional fluorescence techniques
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Two 2D fluorescence techniques are described which allow the study of conformational changes in proteins in their native form in μM solutions using aromatic amino acids (tryptophan, tyrosine) as intrinsic fluorescence markers. Simultaneous time- and wavelength-resolved fluorescence spectra are measured using a 80 ps laser source in conjunction with streak detection in the exit plane of an astigmatism-corrected spectrometer. This approach allows identification of different photophysical processes by their associated lifetime and spectral intensity distribution; errors due to the more common integration over a wider spectral range are avoided. Time-resolved spectra are sensitive to changes in the collisional environment (dynamic quenching) and can thus be used to monitor local conformation changes close to the respective fluorophors. This is demonstrated for the Ras protein which undergoes a drastic conformation change while binding to different nucleotides.
Excitation-emission spectra are two-dimensional fluorescence images with one axis corresponding to the excitation and the other to the emission wavelength. Thus, they contain all conventional excitation and fluorescence spectra of a given substance. The 2D structure facilitates the interpretation of these spectra and allows the direct identification of resonance effects, scattering and the isolation of the contribution of different fluorophors to the complete spectrum. This is demonstrated for mixtures of tyrosine and tryptophan. In this case, both wavelength-resolved spectra and temporal decays are affected by energy transfer processes between the two amino acids.
In a last example, both static and time-resolved spectral methods are combined to determine the respective contribution of static and dynamic quenching in calsequestrin. Evaluation of the fluorescence data is in good agreement with a recent crystallographic analysis which shows that all tryptophans are located in a conserved domain of the protein. Addition of Ca2+ ions leads to a more compact form of calsequestrin and to polymers. This information would not be obtainable from either of the two techniques alone.
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