Interaction of apocytochrome c and derived polypeptide fragments with sodium dodecyl sulfate micelles monitored by photochemically induced dynamic nuclear polarization proton NMR and fluorescence spectroscopy

Abstract

The topology of apocytochrome c, the heme-free precursor of the mitochondrial protein cytochrome c, was investigated in a lipid-associated form. For this purpose photochemically induced dynamic nuclear polarization 1H nuclear magnetic resonance (CIDNP 1H NMR) spectroscopy and quenching of tryptophan and tyrosine fluorescence by acrylamide were applied to an apocytochrome c-sodium dodecyl sulfate (SDS) micellar system. A pH titration of the chemical shifts of the histidine C2 proton resonances of apocytochrome c, using conventional 1H NMR, yielded pK(a)'s of 5.9 +/- 0.1 and 6.2 +/- 0.1, which were assigned to histidine-18 and -33 and histidine-26, respectively. In the presence of SDS micelles an average pK(a) of 8.1 +/- 0.1 was obtained for all histidine C2 protons. Photo-CIDNP enhancements of the histidine, tryptophan, and tyrosine residues, contained in the intact apocytochrome c and in chemically and enzymatically prepared fragments of the precursor, were reduced in the presence of SDS micelles. Similarly, the quenching of the tryptophan fluorescence of the polypeptides by acrylamide was diminished in the presence of SDS. These results indicate the aromatic residues studied are localized in the interface of the SDS micelle.