Ligand‐Protein Interactions in Imidazole and 1,2,4‐Triazole Complexes of Methaemoglobin from <i>Chironomus plumosus</i>

Abstract

Imidazole, 4(5)‐methylimidazole, N ‐methylimidazole, N ‐vinylimidazole, 1,2,4‐triazole, 3‐amino‐1,2,4‐triazole and N ‐methyl‐1,2,4‐triazole interact with methaemoglobin from Chironomus plumosus. In a weakly acidic environment all complexes show light absorption spectra of the parahaematin type with maxima near 535 and 415 mμ. The affinity of these ligands to Chironomus ‐methaemoglobin rises with increasing basic strength, with the exception of 3‐amino‐1,2,4‐triazole. The spectra of the N ‐alkylated imidazole‐ or triazole‐complexes are pH‐invariant, while those of the complexes with ligands containing a free NH‐group shift their maxima to longer wavelengths in alkaline medium (about 542 and 417 mμ). This spectral change is attributed to the deprotonation of the NH‐group of the bound ligands. The ionization constants of this reaction have been estimated. They demostrate a strong increase of the acidity of these ligands as a result of their interaction with the haemoprotein. Furthermore, the ligands influence the ionization constants of some haemin‐linked protein groups. The results are an example of the changes of the electronic properties of low‐molecular compounds (drugs etc. ) interacting with specific receptors of biomacromolecules and vice versa.