This question illustrates the effect of a fluorophores environment on its spectrum. Four fluorescence emission spectra are shown in the following picture. Spectra A and B are for a protein in aqueous solution, labelled with a fluorescent group X on the side chain of a specific lysine residue. This protein can exist in solution as a monomer or a dimer. Assume that the two labelled lysine residues in the dimeric form of the protein do not affect each others spectra. Spectra C and D are for an X-labeled lysine derivative (-N-acetyl--X-lysine amide), which serves as a simple reference compound, in two different solvents. Propose a plausible explanation for the relative positions of the emission spectra for the monomeric vs. dimeric form of the labeled protein, based on the spectra for these forms of the protein and for the labelled reference compound.

To answer, first address these questions:

i) Is 70:30 EtOH:water more or less polar than water?

ii) To discuss the relative changes in position of the emission spectra, first measure the wavelength at which the maximum emission occurs, max. (Hint: refer to posted fluorescence notes)

Fig.1 Spectra A and B: from a fluorescently labeled Lysine residue in a protein in aqueous solution. The protein can exist either in monomer or dimer form. Spectr C and D: control spectra, where the fluorescently labeled Lysine amino acid is dissolved in two separate solvents