Ionization of tyrosine and lysine residues in native and modified horse cytochrome c.

1H-n.m.r. and 13C-n.m.r. spectroscopy of horse cytochrome c and 1H-n.m.r. spectroscopy of the lysine-modified proteins N epsilon-acetimidyl-, N epsilon-amidino-, N epsilon-trifluoroacetyl- and N epsilon-maleyl-cytochrome c have shown that, although the lysine modifications do not greatly perturb the protein structure at pH7 and 27 degrees C, at higher temperature or at alkaline pH some parts of the structure are markedly perturbed. At pH7 and 27 degrees C the region of the protein about Ile-57 is affected in all the modified proteins, though not all to the same degree. N epsilon-Maleylation most seriously affects the protein structure, and the fully maleylated protein is readily unfolded. At 27 degrees C all four of the tyrosine residues of native horse cytochrome c have pKa values above 11, but in N epsilon-acetimidyl-cytochrome c the pKa of one tyrosine residue is 10.2.