There is no simple answer to this question as the quantum yield of a fluorescent dye can vary widely, depending on the dye’s micro-environment. For example, the quantum yield of a dye attached to a protein may be very different from the quantum yield of the free dye. For dyes attached to a protein, the quantum yield is highly dependent on how many molecules of the dye are attached to the protein (i.e. degree of protein labeling). In general, a higher degree of protein labeling leads to a lower dye quantum yield due to fluorescence quenching via dye-to-dye interaction. For this reason, as the degree of labeling increases, fluorescence intensity of the labeled protein will eventually reach a maximum and start to decline thereafter. In fact, one of the best ways to compare the relative quantum yields of different dyes is to plot the total fluorescence of the labeled proteins as a function of degree of labeling by the dyes as we have done with CF® dyes and other commercial dyes. CF® dyes generally give higher slopes than other commercial dyes in the plots, suggesting less quantum yield decline with increasing degree of protein labeling.
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