Paramagnetism in melanins: pH dependence

Changes in the paramagnetic properties of aqueous suspensions of melanin polymers have been monitored over a pH range from 1 to 12. Distinct changes in spin concentration and electron spin resonance spectral parameters (effective g value and line shape) are shown to occur. These data are interpreted in terms of pH- and temperaturedependent equilibria between diamagnetic and paramagnetic units on the melanin polymer, which can be partly or completely quenched if the melanin is precipitated by lowering the pH or by increasing the salt concentration. The heterogeneity of these units and possible chemical structures are discussed.