EPR non-detectable copper in pyridoxal biogenic amine Schiff base complexes.

Schiff bases formed with octopamine, pyridoxal and pyridoxal phosphate react with copper ions to give various pH-dependent species. The outstanding feature of these complexes is their absence of EPR spectra at physiological pH values. We propose dimeric dipolar coupled structures for the EPR non-detectable copper complexes, involving hydroxyde anions and vitamin B-6 Schiff bases. These results establish that EPR non-detectable copper in enzymes may arise from dipolar coupling between metal ions involved in Schiff base type complexes.