Neuromelanins Isolated from Different Regions of the Human Brain Exhibit a Common Surface Photoionization Threshold

Neuromelanin isolated from the premotor cortex, cerebellum, putamen, globus pallidus and corpus callosum of the human brain is studied by scanning probe and photoelectron emission microscopies and the results are compared with previously published work on neuromelanin from the substantia nigra. Scanning electron microscopy reveals common structure for all neuromelanins. All exhibit spherical entities of diameters between 200 and 400 nm, composed of smaller spherical substructures, ～30 nm in diameter. These features are similar to that observed for many melanin systems including Sepia cuttlefish, bovine eye, and human eye and hair melanosomes. Photoelectron microscopy images were collected for all neuromelanins at specific wavelengths of ultraviolet light between 248 and 413 nm, using the spontaneous emission output from the Duke free electron laser. Analysis of the data establishes a common threshold photoionization potential for neuromelanins of 4.7 ± 0.2 eV, corresponding to an oxidation potential of ?0.3 ± 0.2 V vs the normal hydrogen electrode (NHE). These results are consistent with previously reported potentials for neuromelanin from the substantia nigra of 4.5 ± 0.2 eV (?0.1 ± 0.2 V vs NHE). All neuromelanins exhibit a common low surface oxidation potential, reflecting their eumelanic component and their inability to trigger redox processes with neurotoxic effect.