Abstract: The iron molybdenum cofactor(FeMoco) of nitrogenase MoFe-protein from Azotobacter vinelandii OP was extracted by N-methylformamide(NMF). The effects of FeMoco(in NMF) on electronic spectrum and fluorescence intensity of fluorescein dimercury acetate(FDMA)(in 1 mol/L NaOH) were investigated by fluorophotometric titrations and compared with those of (NH₄)₂MoS₄ and complexes of (NH₄)₂MoS₄ with Na₂S or Na₂S₂ or (NH₄)₂S_x on the relative properties of FDMA. It was found that the electronic spectrum of FDMA displayed hypsochromic shift(17 nm) in the presence of FeMoco just like that in the presence of other inorganic sulfides and that the titration curve for the quench of FDMA with FeMoco is very similar to that for the quench of FDMA with complex of (NH₄)₂MoS₄ with Na₂S₂(mole ratio is 1∶3). The results showed that FeMoco(N) probably contained S—S bonds and its structure was found to be changed compared with Kim-Rees structural model. This change should profit the formation of the polymer.

Key words: Nitrogenase, Iron molybdenum cofactor, Fluorophotometric titration, Structural change