The interaction between bovine hemoglobin (BHb) and zinc oxide (ZnO) nanoparticle is investigated by UV-vis absorption, fluorescence, synchronous fluorescence, time-resolved fluorescence, FT-IR and circular dichroism techniques under physiological pH 7.4. The static mode of fluorescence quenching of BHb by ZnO nanoparticle indicates formation of BHb-ZnO complex in the ground state. The process of binding of ZnO nanoparticles on BHb is a spontaneous molecular interaction procedure in which electrostatic interaction plays a major role. The CD spectra reveal that alpha helicity of BHb is reduced by increasing ZnO nanoparticle concentration although the structure of BHb preserves its a-helical structure. (C) 2009 Published by Elsevier B. V.