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https://hdl.handle.net/2142/21369
Description
Title
Metalloporphryin-oligopeptide complexes
Author(s)
Huffman, David Lee
Issue Date
1994
Doctoral Committee Chair(s)
Kenneth S. Suslick
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Biochemistry
Chemistry, Inorganic
Language
eng
Abstract
This work provides the first example of direct ligation of totally synthetic polypeptides to a metalloporphyrin to create heme-peptide complexes. Both the influence of the metalloporphyrin on the properties of the oligopeptide (e.g., structural perturbations) and the effect of the oligopeptide on the metalloporphyrin (e.g., ligand binding, redox potentials) were examined. Synthetic hydrophobic peptides in the range of 4 to 28 residues, possessing either one or two ligating residues situated near the ends of the sequence, were prepared and characterized. The alanine-rich hydrophobic peptides were expected to form helix and helix-turn-helix structural motifs upon binding to a metalloporphyrin. Instead, formation of 2:1 oligopeptide-metalloporphyrin complexes was found as shown by spectrophotometric titration. Ligand affinities for the metalloporphyrin were greatly enhanced by the presence of hydrophobic residues (with increases in equilibrium constants as high as $\approx$40,000 fold).
In a second class of compounds, synthetic heme-peptide complexes were prepared by coordinating simple, 15-mer amphiphilic peptides to a synthetic metalloporphyrin, iron(III) coproporphyrin-I or Fe(III)(Copro-I). The design of the peptide sequences utilized computer molecular modeling. Ligation from the oligopeptide was provided by the imidazole of histidine, located in the middle of the oligopeptide; the remaining residues of the sequences were chosen for their helix-forming propensity or for their side chain hydrophobicity.
Spectrophotometric titrations of the 15-mer peptides with Fe(III)(Copro-I) were performed and equilibrium constants were determined. The observed spectra and titration data analyses were consistent with the formation of 2:1 oligopeptide-metalloporphyrin complexes with ligation from the imidazole of the histidine. The equilibrium constants for binding the oligopeptides to Fe(III)(Copro-I) increased with the hydrophobicity of side chains in contact with the heme face. Binding constant enhancements of greater than 5000-fold were observed relative to the binding of histidine alone, and ranged from 0.225 to 47.0 mM$\sp{-2}$. Structural changes associated with binding of the oligopeptides to Fe(III)(Copro-I) were investigated by circular dichroism spectroscopy. A significant increase in helical content was observed for all of the peptides in the presence of Fe(III)(Copro-I). The reduction potentials for the metalloporphyrin-oligopeptide complexes were measured by cyclic voltammetry. The reduction potentials measured were more negative by 200-300 mV than cytochrome b, consistent with a solvent-exposed heme edge. The reduction potentials of the complexes were well-correlated with binding strength of the oligopeptide: the most tightly bound oligopeptides were the hardest to reduce.
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