The structural change of Cu(II)-BSA and Ni(II)-BSA complexes with BSA concentration has been investigated by means of UV spectrophotometry. We have found that the configuration of these complexes transforms from a pentaccordinated square-pyramid into a tetracoordinated square-planar as the BSA concentration increases over 2× 10^-^4～3×10^-^4mol.dm^-^3. This finding confirmed the involvement of the Asp carboxyl oxygen in the coordination of Cu(II) and Ni(II) in the case of BSA for the first time. The optical electronegativities of Cu(II), Ni(II), and relevant ligand orbitals also have been calculated and discussed.

Key words: PLANAR STRUCTURE, SERUM ALBUMIN, METAL, ULTRAVIOLET SPECTROPHOTOMETRY, TETRACOORDINATE