Arsenic trioxide, marketed as Trisenox (TM), successfully cures 60-85% of relapsed acute promyelocytic leukemia sufferers. However, the mechanisms of action remain unclear. In this work, SR-FTIR microspectroscopy of live HL60 cells was used to monitor biomolecular changes that occur during exposure to arsenite (100 mu M) a period of 2 h. Importantly, the design of the sample holder enabled the collection of high quality spectra that were not dominated by the delta(OH) mode of water. Significant spectral differences in the live treated cells were observed within 40 min after exposure to the drug. In particular, there was a decrease in the nu(C=O) band at 1742 cm(-1) in the spectra of arsenite-treated cells. This is consistent with initial damage to the membrane that leads to later loss of membrane integrity. In addition, there was an initial (40-60 min) decrease in the intensities of the nu(s)(PO2-) and nu(as)(PO2-) bands which was attributed to IR "opaqueness" associated with chromatin condensation. In later stage apoptosis (100-120 min) there was an increase in the intensity of these bands which was consistent with DNA fragmentation. The most significant differences (as determined by the second derivative spectra and PCA plots) were observed in the amide I band where the band was centred at 1639 cm(-1) (beta-sheet) in the control cells and 1650 cm(-1) (alpha-helix) in the arsenite-treated cells. The results of this study indicate that the mechanisms of action of arsenite-induced toxicity include alterations to the protein structure and interactions with DNA. Further studies will be performed to verify whether these effects also occur at therapeutic arsenite concentrations (5-10 mu M). (C) 2010 Elsevier B.V. All rights reserved.