Background: Human serum albumin (HSA) is a soluble blood protein which can bind to small molecules (such as drugs and toxins) and transfer them within the blood circulation.

  Materials and Methods: UV-Vis spectroscopy and FT-IR methods were used to characterize the binding properties of HSA with diazinon(the toxin of organophosphate) and to investigate the changes of protein secondary structure, respectively, in molecular level under physiological condition in two times of first and thirty five days .

  Results: The binding constant (K_Diazinon-HSA = 3.367 ) was have been calculated based UV-Vis spectroscopy data. In FT-IR method, the proportion of decrease in percentage of &alpha-Helix on the first day was 53.97% to 51.88%, other secondary structures increased, such as Turns from 8.49% to10.21%, ß-Sheet from 13.94% to 14.81%, &beta-anti from 8.2% to %8.25 and r-coils from 15.4% to % 17.24. These changes for &alpha-Helixes, Turns, ß-Sheet, &beta- anti and random r-coils after thirty five days were 56.7% to 47.11%, 25.3% to 29.75%, 6.93% to 10.94%, 2% to 2.83% and 9.08% to 10.86%, respectively.

Conclusion: Since the content of protein secondary structure relates closely with its biological activity, therefore, a decrease in &alpha-helix and increase in &beta-sheet structure in the presence of diazinon at high concentration means the decrease of HSA biological activity. Our results suggest that diazinon has a relatively good binding with HSA and it could cause considerable changes in various secondary structures and likely is indicative of a unfolding of protein especially for the samples in thirty five days .