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Formation and Spectral Properties of Copper(II) Ion Complexes of the NH₂-terminal Pentapeptide from Sperm Whale Myoglobin and of Certain Tetra- and Pentapeptides

From the ¹ From the Department of Chemistry, Indiana University, Bloomington, Indiana 47401

1. The association and ionization constants of copper(II) complexes of glycylglycylglycylglycine, l-alanylglycylglycylglycine, glycylglycylglycylglycylglycine, l-alanylglycylglycylglycylglycine, glycylglycylglycylglycyl-l-alanine, l-valyl-l-leucyl-l-seryl-l-glutamyglycine, and l-valyl-l-leucyl-l-seryl-l-glutaminylglycine were computed from potentiometric measurements. The acid ionization constants of the free peptides were determined.

2. The visible absorption spectra of the reaction mixtures were resolved to show that identical species of the peptide complexes gave identical wave lengths of maximum absorbance with some variation in the molar extinction values.

3. Cotton effects were observed in the visible range for peptide complexes containing optically active amino acid residues. In each case the circular dichroism spectrum showed a single negative band or perhaps closely overlapping negative bands. The general form of the spectra was similar to that observed with tripeptide complexes containing l-alanyl residues, and indications of an additivity effect in substitutions in the chelate rings are presented. In all cases optically active transitions were observed near 300 mµ.

4. The contribution of the NH₂-terminal l-alanyl moiety in complexes of peptides containing 2,3, or 4 more subsequent glycyl residues was constant as to optical rotatory power, in the visible range, although the tripeptide complex differed in wave length of the extremum from the tetra- and pentapeptide complexes.

5. The optical rotatory properties of the fully formed complex with l-valyl-l-leucyl-l-seryl-l-glutamylglycine, corresponding to the NH₂-terminal sequence in sperm whale myoglobin, can be distinguished qualitatively from that of complexes with histidine-containing peptides.

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