I have often noticed red shift after moving from high to low temperature.
The major contribution of fluorescence from most proteins arises from the indole moiety of tryptophan. The excited state of indole has a dipole moment that differs from that of the ground state. Following photo-excitation there is a rapid shift in the dipole moment, and if the indole is in a polar solvent (such as water) solvent molecules must reorganize to stabilize the excited state . This stabilization of the excited state results in a red shift of the fluorescence(referred to as a fluorescence Stoke's shift) . This stabilization will be far less in a more nonpolar environment such as the interior of a protein, and thus result in less of a red shift.
From what is suggested by your observation is that the protein undergoes a conformational change as the temperature is lowered. This conformational change exposes the tryptophan to the aqueous environment at the lower temperature resulting in a fluorescence red shift.
The major contribution of fluorescence from most proteins arises from the indole moiety of tryptophan. The excited state of indole has a dipole moment that differs from that of the ground state. Following photo-excitation there is a rapid shift in the dipole moment, and if the indole is in a polar solvent (such as water) solvent molecules must reorganize to stabilize the excited state . This stabilization of the excited state results in a red shift of the fluorescence(referred to as a fluorescence Stoke's shift) . This stabilization will be far less in a more nonpolar environment such as the interior of a protein, and thus result in less of a red shift.
From what is suggested by your observation is that the protein undergoes a conformational change as the temperature is lowered. This conformational change exposes the tryptophan to the aqueous environment at the lower temperature resulting in a fluorescence red shift.
Let me provide you a reference for the fluorescence stokes shift resulting from solvent reorganization.
Text: Principles of Fluorescence Spectroscopy, Second Ed., page 190
Author: Joseph Lakowicz
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