Porphyrin fluorescence dominates UV photoemission of folded cytochrome c

In this article we reinvestigate the bimodal fluorescence of cytochrome c (Cyt c) by using excitation-wavelength-dependent fluorescence spectroscopy. We show that its major contributions at pH 3-7 do not arise from tryptophan (Trp-59) fluorescence as hitherto assumed. Instead, different chromophores of Cyt c contribute at different pH values. At pH 3-7, the porphyrin system contributes about 80% and tryptophan about 20% to the total fluorescence upon excitation of Cyt c at 280 nm. At pH 2, the fluorescence originates nearly completely from the tryptophan residue. Porphyrin fluorescence is still present at pH 2 but its contribution is too small for quantitative deconvolution. Our results show that the UV fluorescence of Cyt c has to be deconvoluted before it can be used to perform time-resolved measurements of the folding of this small protein.