Photochemistry und Phofobiology, 1975, Vol. 21, pp. 27-30.
Pergamon PreTs.
Printed in Greet Britain
OCCURRENCE OF THE SINGLET-OXYGEN MECHANISM IN PHOTODYNAMIC OXIDATIONS OF GUANOSINE" ISAO SAITO, KENZOINOUEt and TERUOMATSUURA Department of Synthetic Chemistry,Faculty of Engineering, Kyoto University, Kyoto 606, Japan (Received 23 April 1974; accepted 2 July 1974) Abstract-Photosensitized oxidations of guanosine in aqueous methanol were investigated with a variety of sensitizers, and several experimental tests for the participation of singlet oxygen were examined. It has been shown that the dye-sensitized photooxidation of guanosine proceeds by both singlet-oxygen and Type I mechanisms, and that the efficiency of the singlet-oxygen mechanism is strongly dependent on photosensitizer type.
Since several studies of dye-sensitized photodynamic oxidation of nucleic acids and nucleotides have shown the guanine residue to be the most readily disrupted (Bohme and Wacker, 1963; Sussenback and Berends, 1963, 1964, 1965), extensive work aimed at an understanding of the chemistry and mechanism of the photodynamic degradation of guanosine and related purines has been carried out (see review, Matsuura et al., 1972). Recent articles by Knowles and Mautner (1972) and by Nilsson et al. (1972a) have suggested that the photodynamic oxidation of guanosine monophosphate in aqueous medium proceeds, rather than by singlet-oxygen involvement, via a complex form between triplet sensitizer and substrate, which is then followed by oxidation with ground-state triplet oxygen, specified as a Type I process by Gollnick (1968). More recently Kornhauser et al. (1973) have indicated that not only a Type I process but also a singlet-oxygen process may operate in the methylene blue-sensitized photooxidation of guanosine in aqueous medium. Indeed, singlet oxygen generated by radiofrequency discharge or by chemical methods has been shown to be highly reactive toward guanosine and its derivatives (Rosenthal and Pitts, 1971; Clagett and Galen, 1971; Hallett et al., 1970). Since the efficiency of the singlet oxygen process, as compared to other processes, will
ultimately depend upon the relative rate of the reaction of singlet oxygen compared to the rates of other processes, it might be expected that the singlet-oxygen process, if involved in the dyesensitized photooxidation of guanosine, will be highly sensitive to the nature of the sensitizers and to the experimental conditions, such as oxygen concentration, solvent, and pH. In order to gain insight into the mechanisms utilized by different sensitizers, we investigated the photooxidation of guanosine in aqueous methanol in the presence of a variety of sensitizers, where several experimental tests for singlet-oxygen reaction were applied; these include (a) inhibition of the photooxidation by a singlet-oxygen quencher, NaN,, (b) competitive reaction between guanosine and a singlet-oxygen acceptor, linalool, and (c) comparison of photooxidation rates in normal and deuterated solvents (see below). In the present communication we report that the sensitized photooxidation of guanosine in aqueous methanol proceeds by both a singlet-oxygen mechanism and a non-singlet-oxygen (Type I) mechanism, and that the efficiency of the singletoxygen process is strongly dependent on photosensitizer type.
*Photoinduced Reactions LXXXI. Part LXXX T. Matsuura and Y. Ito, (to be published). 'TDepartmentof Industrial Chemistry, Niihama Technical College, Niihama, Ehime 792, Japan.
a singlet-oxygen quencher in 10 mm Pyrex test tubes. A slow stream of pure oxygen was passed through the solution and the test tubes were sealed with jointed stoppers. The samples were irradiated with a 500W
MATERIALS AND METHODS
A typical irradiated system consisted of 5ml of CH,OH-H,O (1 : 1) solutions containing guanosine (8.0 X lO-'M), sensitizer (2.9 x M ) and varying amounts of
27
ISAO SAITO,KENZOINOUE,and TERUO MATSUURA
28
1972), were compared with that of the control experiment ( R o ) . As shown in Fig. I , the ratio ( R , / R o )of the rose bengal-sensitized photooxidation decreases with increasing N1 concentration (< 6 X M ) , whereas at higher concentrations M ) the value R , / R o (0.42) is independent (> 7 x on the N1- concentration. If azide ion does not affect any reactive species other than singlet oxygen, e.g. triplet sensitizer, it seems reasonable t o assume that large portions (approximately 58 per cent) of the rose bengal-sensitized photooxidation is a singlet-oxygen-mediated reaction. In fact, the quenching of triplet dyes by azide ion has been shown to be negligibly inefficient under normal RESULTS AND DISCUSSION photooxidation conditions (< lo-’ M NC and Inhibition b y singlet oxygen -quencher - lO-’M Oz) (Nilsson et al., 1972a). At such high M ) as to give a The rate ( R , ) of dye-sensitized photooxidation of concentration of N< (7.5 X guanosine in the presence of varying amounts of constant R , / R o value, the inhibitory effects ( R , / R o ) NaN3, which is known to quench singlet oxygen of NC on photooxidation with different kinds of with a rate constant 108M-’s-’(Hasty et al., sensitizer were obtained (Table 1). The results in Table 1 indicate that inhibition of the photooxidation by N; becomes more significant in the order acridine orange = rhodamine B > rose bengal = thionine > methylene blue, suggesting that a singlet-oxygen mechanism becomes more important in that order. Conversely, methylene blue is the most effective sensitizer for the Type I photooxidation of guanosine. The effect of guanosine concentration on the R , / R o value of the methylene I . blue-sensitized photooxidation at a definite concentration of NaNl is shown in Table 2. The R , / R o value increases with increasing guanosine concentration, indicating that reaction of the methylene I 0 2 4 6 8 10 12 14 blue triplet with guanosine (Type I process) [NaN,) x l o 4 becomes much more important at higher concentrations of the substrate. Nilsson et al. (1972b) have Figure 1. NaN, inhibition of the methylene blue- (0)and rose observed that the methylene blue triplet is e%bengal- (0)sensitized photooxidation of guanosine (8 X M )in methanol-water (1 : 1) at 25°C as a function of NaN, concentra- ciently quenched by guanosine monophosphate, suggesting that the Type I process is a predominant tion.
tungsten-bromine lamp through window glass in a “merry-go-round” irradiation apparatus immersed in a water bath kept at 25 C. Under a fixed set of conditions. the rate of photooxidation of guanosine was followed by monitoring the decrease of the absorption maximum (254 nm). In the case of competitive photooxidation between guanosine and linalool, a stream of oxygen was passed through the solution during irradiation. The extent of photooxidation of linalool was determined by GLC analysis. Under steady-state illumination in the “merry-goround” apparatus as described above, dye-sensitized photooxidations of guanosine (8.0 x lO-’M) in CH,OH-H,O ( I :3) and CH,OD-D,O ( 1 : 3) were compared using different sensitizers.
-
Table 1. Inhibitory effects of NaN, on the dye-sensitized M ) in methanol-water (1 : 1) photooxidation of guanosine (8 x at 25°C Sensitizer
Time of illumination
(2.9 X 10‘’ M )
(hf
RO
4 5 4 10.5 10.5
12.90 11.85 8.80 4.32 12.56
Methylene blue Thionine Rose bengal Rhodamine B Acridine orange
Guanosine reacted* (X
Io-’M) Rq
6.65 5.12 3.68 0.88 2.24
RotRo 0.52 0.43 0.42 0.20 0.17
*Reacted guanosine in the absence (Ro)and presence (R,)of NaN, (7.9 x M).
29
Occurrence of the singlet-oxygen mechanism
sensitizers for singlet oxygen due to very poor yields of intersystem-crossing (Oster et al., 1959), are inefficient sensitizers for the photodynamic oxidation of guanosine compared to methylene blue and rose bengal (Simmon and Vanakis, 1964). Nevertheless, the singlet-oxygen mechanism becomes predominant in the acridine orange or rhodamine B-sensitized photooxidation simply because of an almost complete lack of the type I reaction, whereas methylene blue is an efficient sensitizer for the photooxidation mainly because of the efficient Type I reaction.
Table 2. Effect of guanosine concentration on the R, / R o value of the rnethylene blue-sensitized photooxidation in methanol-water (1 :1) at 25°C Guanosine concentration
(MI
Guanosine reacted* (M) RO R q
RqIRo
664 X 4.42 X
0.85 X 0.51 X
0.57 X 0.38 X
1.37 X lo-'
0.23 X
0.19X lo-'
0.67 0.74 0.83
*Reacted guanosine after 30 min irradiation in the absence (Ro) and presence ( R , ) of NaN, (3.2X lo-" M ) .
Comparison of rates in normal and deuterated solvent As a further check for singlet-oxygen participation, deuterium solvent effects on the photooxidaCompetitive photooxidation tions were examined. Recently, Merkel and Kearns As a further indication of singlet-oxygen partici- (1972) have developed a new technique for pation, a competitive photooxidation between demonstrating singlet-oxygen intermediacy based guanosine and linalool, a typical singlet-oxygen on a comparison of the oxidation rate in normal and acceptor, was carried out in CHIOH-H~O (1 : 1) deuterated solvents. If the acceptor concentration solution. The relative rate of photooxidation of is kept low enough to prevent significant quenching guanosine ( k G ) to linalool ( k ~is) shown in Table 3. of singlet oxygen, an approximately 10-fold inIf singlet oxygen is the only reactive species for the crease in efficiency of photooxidation is expected photooxidation i.e. if a Type I process is not for a pure singlet-oxygen reaction in going from involved in the photooxidation, the ratio k G / k L CHsOH-H20 to CHpOD-D20. As shown in Table 4, should be independent of the concentrations of the rate of photooxidation of guanosine (8 x both substrates and of the type of sensitizer M) in CHIOD-D~O (1 :3) increased approxi(Higgins et al., 1968). Table 3 shows that this is not mately 6- 10 fold compared to that in the normal the case. As expected, the ratio kc/kL decreases in solvent. This also indicates the involvement of the order, rhodamine B acridine orange < rose singlet oxygen in the system. However, the bengal - thionine < methylene blue, in accordance observed deuterium solvent effect was somewhat with the results obtained for the inhibition reac- less than the expected value for a pure singlettions; this means that the singlet-oxygen process oxygen reaction. Although the deuterium solvent becomes more important in that order, regardless effect on the Type I reaction remains to be of the eficiency of the photooxidation reactions. established, these results suggest that the photooxiIndeed, as evident from Table 1, acridine orange dation proceeds by both singlet-oxygen and Type I and rhodamine B, which are known to be poor mechanisms.
pathway of methylene blue-sensitized photooxidation.
-
Table 3. Competitive photooxidation of guanosine (1.54 X lo-' M )and linalool(8.37 x lo-' M ) in the presence of various sensitizers (3.5 x lo-' M ) in methanol-water ( I : 1) at 25 C
kc I k f
Methylene blue
Thionine
Rose bengal
Acridine orange
Rhodamine B
0.29
0.19
0.16
0.09
0.06
*Relative reactivity of guanosine ( k e ) to linalool ( k L ) . Table 4. Comparison of the rate of guanosine photooxidation in 1:3 CH30D-D,O ( R , ) and in I :3 CH,OH-H,O ( R , ) in the presence of various sensitizers at 25 C
&/RH
Methylene blue
Thionine
Rose bengal
Rhodamine B
Acridine orange
5.9
7.5
8.1
11.9
8.1
30
ISAO SAITO, KENZOINOUE, and TERUOMATSUURA
Acknowledgement-We wish to thank the Ministry of Education for a Grant-in-aid for Scientific Research.
REFERENCES
Bohme, H. and A. Wacker (1963) Biochem. Biophys. Res. Commun. 12, 137. Clagett, D. C., and T. J. Galen (1971) Arch. Biochem. Biophys. 146, 196. Gollnick, K., (1968) Adu. Photochern. 6, 1. Hallet, F. R., B. P. Hallet, and W. Snipes (1970) Biophys. J. 10, 305. Hasty, H., P. B. Merkel, R. Radlich, and D. R. Kearns (1972) Tetrahedron Letters 49. Higgins, R., C. S. Foote, and H. Cheng (1968) In Oxidation of Organic Compounds -HI, (Edited by F. R. Mayo) pp. 102. Am. Chem. SOC.Publ., Washington, D.C. Knowles, A. and G. N. Mautner (1972) Photochem. Photobiol. 15, 199.
Kornhauser, A., N. I. Krinsky, P. K. C. Hung, and D. C. Clagett (1973) Photochem. Photobiol. 18, 63. Matsuura, T., I. Saito, and S. Kato (1972) The Purines-Theory and Experiment, pp. 418. The Israel Academy of Sciences and Humanities, Jerusalem. Merkel, P. B., R. Nilsson, and D. R. Kearns (1972) J. Am. Chem. SOC.94, 1030; 7244. Nilsson, R., P. B. Merkel, and D. R. Kearns (1972a) Photochem. Photobiol. 16, 117. Nilsson, R., P. B. Merkel, and D. R. Kearns (1972b) Photochem. Photobiol. 16, 109. Oster, G., J. S. Bellin, R. W. Kimball, and M. E. Schrader (1959) J. Am. Chem. Soc. 81, 5095. Rosenthal, I. and J. N. Pitts, Jr. (1971) Biophys. J. 11, 963. Sussenbach, J. S. and W. Berends (1963) Biochim. Biophys. Acta 76, 154. Sussenbach, J. S. and W. Berends (1964) Biochem. Biophys. Res. Commun. 15, 263. Sussenbach, J. S. and W. Berends (1965) Biochim. Biophys. Acta 95, 184.
Pergamon PreTs.
Printed in Greet Britain
OCCURRENCE OF THE SINGLET-OXYGEN MECHANISM IN PHOTODYNAMIC OXIDATIONS OF GUANOSINE" ISAO SAITO, KENZOINOUEt and TERUOMATSUURA Department of Synthetic Chemistry,Faculty of Engineering, Kyoto University, Kyoto 606, Japan (Received 23 April 1974; accepted 2 July 1974) Abstract-Photosensitized oxidations of guanosine in aqueous methanol were investigated with a variety of sensitizers, and several experimental tests for the participation of singlet oxygen were examined. It has been shown that the dye-sensitized photooxidation of guanosine proceeds by both singlet-oxygen and Type I mechanisms, and that the efficiency of the singlet-oxygen mechanism is strongly dependent on photosensitizer type.
Since several studies of dye-sensitized photodynamic oxidation of nucleic acids and nucleotides have shown the guanine residue to be the most readily disrupted (Bohme and Wacker, 1963; Sussenback and Berends, 1963, 1964, 1965), extensive work aimed at an understanding of the chemistry and mechanism of the photodynamic degradation of guanosine and related purines has been carried out (see review, Matsuura et al., 1972). Recent articles by Knowles and Mautner (1972) and by Nilsson et al. (1972a) have suggested that the photodynamic oxidation of guanosine monophosphate in aqueous medium proceeds, rather than by singlet-oxygen involvement, via a complex form between triplet sensitizer and substrate, which is then followed by oxidation with ground-state triplet oxygen, specified as a Type I process by Gollnick (1968). More recently Kornhauser et al. (1973) have indicated that not only a Type I process but also a singlet-oxygen process may operate in the methylene blue-sensitized photooxidation of guanosine in aqueous medium. Indeed, singlet oxygen generated by radiofrequency discharge or by chemical methods has been shown to be highly reactive toward guanosine and its derivatives (Rosenthal and Pitts, 1971; Clagett and Galen, 1971; Hallett et al., 1970). Since the efficiency of the singlet oxygen process, as compared to other processes, will
ultimately depend upon the relative rate of the reaction of singlet oxygen compared to the rates of other processes, it might be expected that the singlet-oxygen process, if involved in the dyesensitized photooxidation of guanosine, will be highly sensitive to the nature of the sensitizers and to the experimental conditions, such as oxygen concentration, solvent, and pH. In order to gain insight into the mechanisms utilized by different sensitizers, we investigated the photooxidation of guanosine in aqueous methanol in the presence of a variety of sensitizers, where several experimental tests for singlet-oxygen reaction were applied; these include (a) inhibition of the photooxidation by a singlet-oxygen quencher, NaN,, (b) competitive reaction between guanosine and a singlet-oxygen acceptor, linalool, and (c) comparison of photooxidation rates in normal and deuterated solvents (see below). In the present communication we report that the sensitized photooxidation of guanosine in aqueous methanol proceeds by both a singlet-oxygen mechanism and a non-singlet-oxygen (Type I) mechanism, and that the efficiency of the singletoxygen process is strongly dependent on photosensitizer type.
*Photoinduced Reactions LXXXI. Part LXXX T. Matsuura and Y. Ito, (to be published). 'TDepartmentof Industrial Chemistry, Niihama Technical College, Niihama, Ehime 792, Japan.
a singlet-oxygen quencher in 10 mm Pyrex test tubes. A slow stream of pure oxygen was passed through the solution and the test tubes were sealed with jointed stoppers. The samples were irradiated with a 500W
MATERIALS AND METHODS
A typical irradiated system consisted of 5ml of CH,OH-H,O (1 : 1) solutions containing guanosine (8.0 X lO-'M), sensitizer (2.9 x M ) and varying amounts of
27
ISAO SAITO,KENZOINOUE,and TERUO MATSUURA
28
1972), were compared with that of the control experiment ( R o ) . As shown in Fig. I , the ratio ( R , / R o )of the rose bengal-sensitized photooxidation decreases with increasing N1 concentration (< 6 X M ) , whereas at higher concentrations M ) the value R , / R o (0.42) is independent (> 7 x on the N1- concentration. If azide ion does not affect any reactive species other than singlet oxygen, e.g. triplet sensitizer, it seems reasonable t o assume that large portions (approximately 58 per cent) of the rose bengal-sensitized photooxidation is a singlet-oxygen-mediated reaction. In fact, the quenching of triplet dyes by azide ion has been shown to be negligibly inefficient under normal RESULTS AND DISCUSSION photooxidation conditions (< lo-’ M NC and Inhibition b y singlet oxygen -quencher - lO-’M Oz) (Nilsson et al., 1972a). At such high M ) as to give a The rate ( R , ) of dye-sensitized photooxidation of concentration of N< (7.5 X guanosine in the presence of varying amounts of constant R , / R o value, the inhibitory effects ( R , / R o ) NaN3, which is known to quench singlet oxygen of NC on photooxidation with different kinds of with a rate constant 108M-’s-’(Hasty et al., sensitizer were obtained (Table 1). The results in Table 1 indicate that inhibition of the photooxidation by N; becomes more significant in the order acridine orange = rhodamine B > rose bengal = thionine > methylene blue, suggesting that a singlet-oxygen mechanism becomes more important in that order. Conversely, methylene blue is the most effective sensitizer for the Type I photooxidation of guanosine. The effect of guanosine concentration on the R , / R o value of the methylene I . blue-sensitized photooxidation at a definite concentration of NaNl is shown in Table 2. The R , / R o value increases with increasing guanosine concentration, indicating that reaction of the methylene I 0 2 4 6 8 10 12 14 blue triplet with guanosine (Type I process) [NaN,) x l o 4 becomes much more important at higher concentrations of the substrate. Nilsson et al. (1972b) have Figure 1. NaN, inhibition of the methylene blue- (0)and rose observed that the methylene blue triplet is e%bengal- (0)sensitized photooxidation of guanosine (8 X M )in methanol-water (1 : 1) at 25°C as a function of NaN, concentra- ciently quenched by guanosine monophosphate, suggesting that the Type I process is a predominant tion.
tungsten-bromine lamp through window glass in a “merry-go-round” irradiation apparatus immersed in a water bath kept at 25 C. Under a fixed set of conditions. the rate of photooxidation of guanosine was followed by monitoring the decrease of the absorption maximum (254 nm). In the case of competitive photooxidation between guanosine and linalool, a stream of oxygen was passed through the solution during irradiation. The extent of photooxidation of linalool was determined by GLC analysis. Under steady-state illumination in the “merry-goround” apparatus as described above, dye-sensitized photooxidations of guanosine (8.0 x lO-’M) in CH,OH-H,O ( I :3) and CH,OD-D,O ( 1 : 3) were compared using different sensitizers.
-
Table 1. Inhibitory effects of NaN, on the dye-sensitized M ) in methanol-water (1 : 1) photooxidation of guanosine (8 x at 25°C Sensitizer
Time of illumination
(2.9 X 10‘’ M )
(hf
RO
4 5 4 10.5 10.5
12.90 11.85 8.80 4.32 12.56
Methylene blue Thionine Rose bengal Rhodamine B Acridine orange
Guanosine reacted* (X
Io-’M) Rq
6.65 5.12 3.68 0.88 2.24
RotRo 0.52 0.43 0.42 0.20 0.17
*Reacted guanosine in the absence (Ro)and presence (R,)of NaN, (7.9 x M).
29
Occurrence of the singlet-oxygen mechanism
sensitizers for singlet oxygen due to very poor yields of intersystem-crossing (Oster et al., 1959), are inefficient sensitizers for the photodynamic oxidation of guanosine compared to methylene blue and rose bengal (Simmon and Vanakis, 1964). Nevertheless, the singlet-oxygen mechanism becomes predominant in the acridine orange or rhodamine B-sensitized photooxidation simply because of an almost complete lack of the type I reaction, whereas methylene blue is an efficient sensitizer for the photooxidation mainly because of the efficient Type I reaction.
Table 2. Effect of guanosine concentration on the R, / R o value of the rnethylene blue-sensitized photooxidation in methanol-water (1 :1) at 25°C Guanosine concentration
(MI
Guanosine reacted* (M) RO R q
RqIRo
664 X 4.42 X
0.85 X 0.51 X
0.57 X 0.38 X
1.37 X lo-'
0.23 X
0.19X lo-'
0.67 0.74 0.83
*Reacted guanosine after 30 min irradiation in the absence (Ro) and presence ( R , ) of NaN, (3.2X lo-" M ) .
Comparison of rates in normal and deuterated solvent As a further check for singlet-oxygen participation, deuterium solvent effects on the photooxidaCompetitive photooxidation tions were examined. Recently, Merkel and Kearns As a further indication of singlet-oxygen partici- (1972) have developed a new technique for pation, a competitive photooxidation between demonstrating singlet-oxygen intermediacy based guanosine and linalool, a typical singlet-oxygen on a comparison of the oxidation rate in normal and acceptor, was carried out in CHIOH-H~O (1 : 1) deuterated solvents. If the acceptor concentration solution. The relative rate of photooxidation of is kept low enough to prevent significant quenching guanosine ( k G ) to linalool ( k ~is) shown in Table 3. of singlet oxygen, an approximately 10-fold inIf singlet oxygen is the only reactive species for the crease in efficiency of photooxidation is expected photooxidation i.e. if a Type I process is not for a pure singlet-oxygen reaction in going from involved in the photooxidation, the ratio k G / k L CHsOH-H20 to CHpOD-D20. As shown in Table 4, should be independent of the concentrations of the rate of photooxidation of guanosine (8 x both substrates and of the type of sensitizer M) in CHIOD-D~O (1 :3) increased approxi(Higgins et al., 1968). Table 3 shows that this is not mately 6- 10 fold compared to that in the normal the case. As expected, the ratio kc/kL decreases in solvent. This also indicates the involvement of the order, rhodamine B acridine orange < rose singlet oxygen in the system. However, the bengal - thionine < methylene blue, in accordance observed deuterium solvent effect was somewhat with the results obtained for the inhibition reac- less than the expected value for a pure singlettions; this means that the singlet-oxygen process oxygen reaction. Although the deuterium solvent becomes more important in that order, regardless effect on the Type I reaction remains to be of the eficiency of the photooxidation reactions. established, these results suggest that the photooxiIndeed, as evident from Table 1, acridine orange dation proceeds by both singlet-oxygen and Type I and rhodamine B, which are known to be poor mechanisms.
pathway of methylene blue-sensitized photooxidation.
-
Table 3. Competitive photooxidation of guanosine (1.54 X lo-' M )and linalool(8.37 x lo-' M ) in the presence of various sensitizers (3.5 x lo-' M ) in methanol-water ( I : 1) at 25 C
kc I k f
Methylene blue
Thionine
Rose bengal
Acridine orange
Rhodamine B
0.29
0.19
0.16
0.09
0.06
*Relative reactivity of guanosine ( k e ) to linalool ( k L ) . Table 4. Comparison of the rate of guanosine photooxidation in 1:3 CH30D-D,O ( R , ) and in I :3 CH,OH-H,O ( R , ) in the presence of various sensitizers at 25 C
&/RH
Methylene blue
Thionine
Rose bengal
Rhodamine B
Acridine orange
5.9
7.5
8.1
11.9
8.1
30
ISAO SAITO, KENZOINOUE, and TERUOMATSUURA
Acknowledgement-We wish to thank the Ministry of Education for a Grant-in-aid for Scientific Research.
REFERENCES
Bohme, H. and A. Wacker (1963) Biochem. Biophys. Res. Commun. 12, 137. Clagett, D. C., and T. J. Galen (1971) Arch. Biochem. Biophys. 146, 196. Gollnick, K., (1968) Adu. Photochern. 6, 1. Hallet, F. R., B. P. Hallet, and W. Snipes (1970) Biophys. J. 10, 305. Hasty, H., P. B. Merkel, R. Radlich, and D. R. Kearns (1972) Tetrahedron Letters 49. Higgins, R., C. S. Foote, and H. Cheng (1968) In Oxidation of Organic Compounds -HI, (Edited by F. R. Mayo) pp. 102. Am. Chem. SOC.Publ., Washington, D.C. Knowles, A. and G. N. Mautner (1972) Photochem. Photobiol. 15, 199.
Kornhauser, A., N. I. Krinsky, P. K. C. Hung, and D. C. Clagett (1973) Photochem. Photobiol. 18, 63. Matsuura, T., I. Saito, and S. Kato (1972) The Purines-Theory and Experiment, pp. 418. The Israel Academy of Sciences and Humanities, Jerusalem. Merkel, P. B., R. Nilsson, and D. R. Kearns (1972) J. Am. Chem. SOC.94, 1030; 7244. Nilsson, R., P. B. Merkel, and D. R. Kearns (1972a) Photochem. Photobiol. 16, 117. Nilsson, R., P. B. Merkel, and D. R. Kearns (1972b) Photochem. Photobiol. 16, 109. Oster, G., J. S. Bellin, R. W. Kimball, and M. E. Schrader (1959) J. Am. Chem. Soc. 81, 5095. Rosenthal, I. and J. N. Pitts, Jr. (1971) Biophys. J. 11, 963. Sussenbach, J. S. and W. Berends (1963) Biochim. Biophys. Acta 76, 154. Sussenbach, J. S. and W. Berends (1964) Biochem. Biophys. Res. Commun. 15, 263. Sussenbach, J. S. and W. Berends (1965) Biochim. Biophys. Acta 95, 184.
