Proc. Natl. Acad. Sci. USA Vol. 91, pp. 12362-12364, December 1994 Chemistry Singlet molecular oxygen evolution upon simple acidification of aqueous hypochlorite: Application to studies on the deleterious health effects of chlorinated drinking water (singlet oxygen/hypochlorite decomposition/chlorinated water supply/DNA mutation) AHSAN ULLAH KHANt AND MICHAEL KASHAt tDepartment of Molecular and Cellular Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138; and tInstitute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, FL 32306-3015 Contributed by Michael Kasha, July 25, 1994 ABSTRACT A study ofthe pH profile ofthe decomposition method is the direct detection of the lAg 31 emission at of aqueous hypochlorite has revealed the evolution (onset at pH 1268 nm in the near infrared, and this method (10, 11) has 8) ofsingle (1Ag) molecular oxygen (singlet spin state dioxygen) been used in the present research. We used a Ge-photodiode detected spectroscopically (1268 nm), prior to the appearance detector with a dedicated near-IR spectrometer in the 1- to of chlorine (onset at pH 5.5). The possible mechanism of the 1.7-gm region, with electronic and data-processing periph- singlet state dioxygen evolution is presented, and the origin of erals (12). Fig. 1 shows the typical 1268-nm chemilumines- its chloride ion dependence is discussed, especially in reference cence spectra recorded upon the acidification of aqueous to chloride ion dependence of singlet molecular oxygen evolu- hypochlorite with HCI at room temperature. tion in biological systems. Recent epidemiological analyses of The pH profiles of 102 and C12 evolution upon acidification the correlation of human cancer with chlorinated water sup- of aqueous hypochlorite solution at room temperature are plies focus attention on the singlet oxygen mechanisms of DNA shown in Fig. 2 (reagent-grade sodium hypochlorite solution, lesion formation. acidified by reagent grade hydrochloric acid). The pH of the solution was measured with a Fisher Accumet pH meter before the acidification and after the completion ofthe singlet Singlet Molecular Oxygen Generation oxygen emission experiment. In each case, the initial hy- pochlorite solution was at pH 11, an aliquot of acid was In a general study of singlet molecular oxygen generation, we added, the integrated (900 sec) luminescence intensity at 1268 discovered that simple acidification ofhypochlorite solutions nm was recorded, and the final pH was measured and yielded typical IR emission characteristic ofsinglet molecular recorded. (excited) oxygen. Currently, an epidemiological study (1) has The highest emission intensity recorded at 1268 nm was correlated the statistics of human cancer with use of chlori- observed by adding 200 ,l of concentrated HCI (12.6 M, nated water supply. The demonstration of singlet oxygen Mallinckrodt analytical reagent) to a 5-ml aqueous solution of attack on DNA components by several laboratories (2-4) 0.6 M sodium hypochlorite [analytical reagent from Alpha 1 suggests a possible mechanistic role for the singlet oxygen Biomedicals (Culver City, CA)] in an optical cuvette, which generated in acidic environments in the correlation between was stirred continuously with a Teflon-covered magnetic chlorinated water and cancer. stirrer; final pH was approximately 5. The limit of detect- A study of the pH profile of the decomposition of aqueous ability of the 1268-nm emission in the current procedure was hypochlorite has revealed the evolution (onset at pH 8) of observed for 0.06 M sodium hypochlorite (5 ml) acidified with singlet ('Ag) molecular oxygen (singlet spin state dioxygen) 150 ,ul of 1 M HCI. detected spectroscopically (1268 nm), prior to the appearance At the higher concentrations used for the data in Figs. 1 and ofchlorine (onset at pH 5.5). This result is surprising because 2, an estimate of the quantum yield of singlet oxygen gener- it has been overlooked in the long history of research on the ated was made by comparing the integrated 1268-nm emission chlorine-hypochlorite equilibria, kinetics, and mechanisms. against that observed for the OCI-/H202 chemiluminescence These observations suggest another focus on mechanisms of reaction (Eq. 1). Because of the very large difference in the redox reactions involving H202 and singlet oxygen species, in rate of the two reactions, OCI-/H+ and OCI-/H202, photo both chemical and biological systems. yield comparisons were made for the same 0C1- concentra- tion consumed, but for differing final volumes. In a typical Experimental Results on Acidification of Hypochlorite experiment, 200 ul of 0.6 M sodium hypochlorite solution was allowed to diffuse slowly out of a disposable glass The generation of singlet molecular oxygen by the chemilu- micropipette (Accu-Fill 90, Micropet) into 5 ml of 20 mM minescent reaction of hydrogen peroxide with aqueous hy- H202 solution for approximately 20 sec, and the emission pochlorite is well known and is now a standard method (5-9), intensity was integrated. The integrated emission intensity at 1268 nm over 1200 sec from the stirred solution of5 ml of0.6 HOOH + OCF -- 10* + HOH + Cl-, [1] M NaOCl solution containing 200 ,ul of 12.6 M HCI was adjusted for equal amounts of OC1-. The average results the singlet oxygen being detected spectroscopically. Initially, indicated a value of0.30 + 0.05 forthe ratio of singlet oxygen this was observed (5, 6) as the 634-nm simultaneous transition yield for the OCI-/H+ vs. OC[-/H202 reactions. (1'A5,) (lAg) -_ (3X ) (3k-) for the electron exchange collision The pH profile for C12 evolution was obtained by acidifying of excited molecular pairs. The preferred and more efficient hypochlorite solution with concentrated HCI. To 20 ml of0.6 M sodium hypochlorite/20 ml of CCL4 in a separatory funnel The publication costs of this article were defrayed in part by page charge was added an aliquot of 12.6 M HCI, the funnel was quickly payment. This article must therefore be hereby marked "advertisement" capped and vigorously shaken for 15 min, and the CCL4 was in accordance with 18 U.S.C. §1734 solely to indicate this fact. then allowed to separate for 30 min. The absorption spectrum 12362 Downloaded by guest on October 1, 2021 Chemistry: Khan and Kasha Proc. Natl. Acad. Sci. USA 91 (1994) 12363 1.0- - molecular oxygen was evolved in an excited singlet state (5, 6). An analogous situation exists in the exhaustive research Ag (0,O)-31:9 reported on the decomposition of hypochlorite (16), in which it had not been noticed that here also a chemiluminescent reaction is involved. The initial mode of decomposition is described (17) by the following reaction 20Cl- 02 + 2C1-, [21 z where the oxygen is presumed to be in its ground state (triplet spin state). However, hypochlorous acid is a weak acid, pKa = 0.5- 7.52, so that upon acidification the net reaction for the z initial mode of decomposition (according to our results), as a 0 chemiluminescent reaction, is 2HOCl -* '0* + 2H+ + 2C1-, [31 with the molecular oxygen in a singlet spin state. Lister (18) proposed that HOCl hydrolysis leads to an intermediate, which can then generate H202, H-0-Cl H-0 - + H++ Cl-, [4] 0 H-O--H J H-0 1200 1270 1340 WAVELENGTH, nm offering a path to singlet oxygen generation via reaction 1, with a pH dependence implied. Another pathway leading to FIG. 1. Near-IR chemiluminescence spectrum of singlet dioxy- evolution from acid has been described gen observed upon acidification of 0.6 M aqueous sodium hypochlo- oxygen hypochlorous rite solution with 12.6 M HCI at room temperature. (19): of the bottom CCL4 layer was measured with a UVIKON 820 2HOCl -> 2H+ + 2Cl + 02, [5] double-beam spectrophotometer. The absorption at 3300 A but the state of the liberated molecular oxygen was not was taken as a measure (13) of dissolved Cl2 in solution. The identified; present knowledge would dictate that in this case final pH ofthe upper aqueous phase was measured with a pH the 02 must be singlet molecular oxygen. It is, of course, not meter. Each of the Cl2 evolution points shown in Fig. 2 was surprising that singlet oxygen was not adduced in the earlier obtained by plotting the final pH ofthe aqueous phase against research (14-17) on hypochlorite reactions and decomposi- the absorption of the CCI4 layer at 3300 A. tion, as the observability and detection methodology were developed subsequently. The hypochlorite decomposition Mechanism of Singlet Oxygen Evolution onset at pH 5.5, In the exhaustive study of the mechanism for reaction (1) by HOCl + H+ + Cl -* HOH + Cl2, [6] kinetic methods by Connick (14) and by isotopic labeling by Cahill and Taube (15), it had not been noticed that the results in lowered pH relative to the stoichiometric value because of the escape of C12. 1.0- Biological Applications of Singlet Oxygen Research z < ~~~~~~0 The catalytic effects in the hypochlorite decomposition are '0J2 Crp 'o - (Ing) noteworthy and suggestive of important extension. The Cl- catalysis was noted early (16) and attributed to the kinetic salt E effect, treated by the Br0nsted theory. The catalysis by Cl- was noted also by Rosen and Klebanoff (20) in their studies z of evolution of singlet oxygen in the myeloperoxidase reac- o pK07.5 tions. They used a chemical scavenger method for the detection of singlet oxygen (2,5-diphenylfuran conversion to cis-dibenzoylethylene) and indeed detected singlet oxygen by o 2. 4. 7080 1001. this method in a buffered solution ofHOCI. Their onset curve for singlet oxygen evolution vs. pH parallels that shown in _ H Fig. 2, with uncertainty ofdata at pH < 6 yielding a deviation <~~~~~~~ from our results. A very strong dependence on chloride ion 0~~~~~~~~~~~~~~~0 concentration for the efficiency of the hypochlorite decom- position was demonstrated.