Copper(II) complexes of Neobelliera Bullata Trypsin Modulating Oostatic Factor and its analogues

The stoichiometry, stability constants and solution structure of the complexes formed in the reaction of copper(II) with hexapeptide NPTNLH, i.e. the Neobelliera Bullata Trypsin Modulating Oostatic Factor (Neb-TMOF), and its analogues DPTNLH, Ac-NPTNLH and Ac-DPTNLH have been determined by potentiometric, UV–visible, CD and EPR spectroscopic methods. Upon raising pH for Ac-NPTNLH and Ac-DPTNLH peptides, copper(II) coordination starts from the imidazole nitrogen of the His6; afterwards three deprotonated amide nitrogens are progressively involved in metal ions coordination. In a wide pH range of 4.5–8.5 for the NPTNLH and DPTNLH ligands the CuL complex dominates with the imidazole nitrogen of His6 coordinated to form a macrochelate. The N-terminal amino group of the NPTNLH and DPTNLH peptides takes part in the coordination of the metal ion in the CuL, CuH−1L and CuH−2L complexes. However, at pH above 9 the CuH−3L complex with the {NIm, 3N−} coordination mode is formed. For the CuH−2L complex the spectroscopic data clearly indicate the 4N {NH2, CO or COO−, 2N−, NIm} bonding mode with the axial coordination of the N-terminal amine group to the metal ion.