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Photosensitized water oxidation to dioxygen in artificial biomimetic systems: achievements and prospects
J. Photochem. Photobiol. B: Biol., 13 (1992)
335-339
335
News and Views
Photosensitized water oxidation to dioxygen in artificial biomimetic systems: achievements and prospects 0. V. Gerasimov,
G. L. Elizarova
and V. N. Parrnon+
Institute of Catalysis, Pr. Lavrentieva, 5, Novosibirsk 630090 (Russia)
There are two reasons for the keen interest in the creation of artificial systems for water oxidation to dioxygen. The first reason is connected with the problem of natural photosynthetic oxygen evolution. Photosystem II (PSII) of green plants provides efficient light-driven electron transport from water, which is oxidized to dioxygen, to the pool of plastoquinones. Whereas the sequence of electron relays ensuring rapid and irreversible electron transfer from an oxygen evolving complex (OEC) to the pool of plastoquinones is relatively well established, the detailed structure of OEC and the mechanism of OEC action are rather unclear [l-3]. The second reason is connected with attempts to create artificial photosynthetic units as practical devices for the conversion of solar energy into chemical energy. Among various photochemical reactions that could be used for the purpose, water cleavage to elements attracts the greatest attention [4-71: 2HZ0 2
2Hz + Or
This reaction requires the oxidation of water to dioxygen by one-electron oxidants of moderate power (OS-l.5 V 21s. NHE), which are generated by means of visible light: (D+OxzD++Red) 4D+ +2H,O-
4D+0,+4H+
There are two different approaches to the creation of artificial water-oxidizing systems. First, considerable efforts have been made to synthesize a structural analog of natural OEC, which is a protein containing an oligonuclear manganese core (see [8,9] for reviews). Achievements in the field have mostly been connected with obtaining well-characterized bi-, tri- and tetranuclear manganese complexes with particular structural parameters close to those of the manganese cluster in natural OEC. No observable success has been achieved in attempts to accomplish photo-, electro- or thermal catalytic oxidation of water using these synthetic analogs of OEC. Nevertheless it seems to be very important to continue the work in this field, since this is the only way to obtain unambiguous evidence of our understanding natural processes. A more productive approach was to design efficient water photooxidizing systems without trying to mimic natural processes too closely, i.e. to create functional analogs of PSII. Functional analogs of both OEC (i.e. water oxidation catalysts) and PSI1 (i.e. ‘Author
to whom correspondence
loll-1344/92/$5.00
should be addressed.
0 1992 - Elsevier
Sequoia.
All rights reserved
336
photocatalytic systems that oxidize water at the expense of the irreversible reduction of some electron acceptors) have been suggested within several years after formulating the problem in the mid-seventies. In’ earlier works Ru(bpy):+ (bpy = 2,2’-bipyridine) was used as an analog of chlorophyll [W-12]. It was found that a number of transition metal compounds (mainly hydrated oxides) are capable of catalyzing water oxidation to dioxygen by the oxidized form of the complex mentioned, Ru(bpy)z+, and/or by other one-electron oxidants such as Ce4+. These are compounds of Pt, Ir [13], Ru 111, 141, Fe, Co, Cu, Ni [15, 161 and Mn [15] (see also reviews [17, 181). Further studies have developed in several directions. (1) Study of the mechanisms of water oxidation by known catalysts. The most serious obstacle in these studies is that the nature of the water-oxidizing active sites of heterogeneous hydroxide catalysts is rather unclear. The details of molecular oxygen formation are mostly hypothetical. It seemed that the synthesis of homogeneous catalysts with a well-defined structure could be a solution to the problem. However, there are other difficulties with homogeneous catalysts (see below). The majority of physical methods characterize a bulk material of catalyst, rather than its active sites, but the bulk material, hence only kinetic methods were applied [l&20]. However, since oxygen is evolved at the non-limiting states of the catalytic reaction, a kinetic approach can be applied only to study the oxidantxatalyst interaction rather than the elementary steps of oxygen evolution. Nevertheless, such studies have shown that most probably the reaction centers of most artificial catalysts for water oxidation contain two neighboring hydroxylated transition metal cations capable of reversible multielectron redox transformations. (2) Search for homogeneous catalysts. A number of mono-, bi- and trinuclear complexes of Fe [15], Ru [21-251, and Co [26] were reported to catalyze oxygen evolution. In our opinion more evidence on the homogeneity of the catalysts acting in particular systems should be given, because the redox or ligand-exchange destruction of these ‘homogeneous’ catalysts always results in the production of hydrolyzed species similar to some well-known efficient ‘heterogeneous’ catalysts. Besides, there are other reasons to doubt the catalytic action of the reported individual compounds. All the complexes that are claimed to be homogeneous catalysts contain nitrogen-based ligands, which are known to be better electron donors than oxygen-based ligands, including HZ0 (OH-). It is therefore natural to expect that the oxidation of nitrogen-containing ligands will generally be faster than the oxidation of coordinated water, and will thus promote a redox destruction of the complex. (3) Search for more efficient catalysts. The available catalysts do not ensure the evolution of stoichiometric amounts of oxygen owing to a redox destruction of oxidant and/or catalyst. From the viewpoint of stability and efficiency the best catalysts are heterogeneous RuOz [18] (when Ce4+ is used as the oxidant in strongly acid media) and colloid [20] as well as supported [27] Co(II1) hydroxide (when Ru(bpy),3+ is used as the oxidant in weakly acid, neutral and weakly alkaline media). (4) Attempts to conjugate the hydrogen- and oxygen-evolving photocatalytic systems to ensure the whole cycle of water cleavage. It is now generally acknowledged that this can only be achieved in a system consisting of at least two compartments, separated by a membrane-like structure. The most promising candidates for such membrane elements are considered to be microscopic objects like lipid vesicles [28]. Coupling the processes of light-driven electron transfer across bilayer lipid membrane and water oxidation could be the first step in the construction of the complete system. Although the only known attempt to achieve such a coupling [29] has failed, some progress was made torwards the creation of relatively efficient catalysts immobilized on lipid vesicles
337
[30,31], as well as oxygen photoevolution with participation of a photocatalyst embedded in the lipid bilayer membrane [32]. (5) Attempts to use new photocatalysts instead of Ru(bpy)$+. The ruthenium complex does not seem to be optimal for the purpose. Many porphyrins and their metallo derivatives have photophysical and electrochemical parameters that better suit water photooxidation than Ru(bpy)$+ [33]. However, the use of porphyrins as photocatalysts for water oxidation turned out to be quite a difficult task because of the instability of oxidized forms of the porphyrins. Nevertheless in several cases oxygen evolution was detected [34-361. In conclusion, some predictions can be made about the future developments of studies in the field. The most important issues appear to be: elucidation of the precise mechanism of oxygen evolution reactions which are catalyzed by artificial catalysts; synthesis of homogeneous catalysts of water oxidation and reliable demonstration of the homogeneous nature of their catalytic action (this issue is closely related to the synthesis of structural analogs of OEC); continuation of studies on the synthesis of stable and selective water oxidation catalysts; coupling the processes of photoelectron transfer across bilayer lipid membranes and water oxidation to dioxygen. 1 G. C. Disrnukes, Photo&em. PhotobioZ., 43 (1986) 99-115. 2 G. W. Brudwig, W. F. Beck and J. C. de Paula,Ann. Rev. Biophys. Biophys. Chem., 18 (1989) 25-46. 3 D. F. Ghanotakis and C. F. Yocum, Ann. Rev. Plant Physiol. Plant Mol. Biol., 141 (1990) 25.5-276. 4 J. S. Connoly (Ed.), Photochemical Conversion and Storage of Solar Energy, Academic Press, New York, 1982. 5 M. Grltzel (Ed.), Energy Resources through Photochemistry and Catalysis, Academic Press, New York, 1983. 6 K. I. Zamaraev and V. N. Parmon, Uspekhi Khimii., 49 (1980) 1457-1497. 7 V. N. Parmon and K. I. Zamaraev, in N. Serpone and E. Pelizzetti (Eds.), Photocatalysis: Fundamentals and Applications, Wiley Interscience, New York, 1989, p. 565-601. 8 V. L. Pecoraro, Photochem. Photobiol., 48 (1988) 249-264. 9 K. Wieghardt, Angew. Chem., Int. Ed. Engl., 28 (1989) 1153-1172. 10 K. Kalyanasundaram and M. Grltzel, Angew. Chem., 91 (1979) 759-760. 11 J.-M. Lehn, J.-P. Sauvage and R. Ziessel, Nouv. J. Chim., 3 (1979) 423-427. 12 V. Ya. Shafirovich, N. K. Khannanov and V. V. Strelets, Dokl. Akad. Nauk SSSR, 250 (1980) 1197-1200. 13 J. Kiwi and M. Grltzel, Angew. Chem., 90 (1978) 900-901. 14 J. Kiwi and M. Grltzel, Chimia, 33 (1979) 289-291. 15 G. L. Elizarova, L. G. Matvienko, V. N. Parmon and K. I. Zamaraev, Dokl. Akad. Nauk SSSR, 249 (1979) 863-866. 16 V. Ya. Shafirovich, N. K. Khannanov and V. V. Strelets. Nouv. J. Chim., 4 (1980) 81-84. 17 G. L. Elizarova and V. N. Parmon, in K. I. Zamaraev (Ed.), Photokatahticheskoepreobrazovanie solnechnoi enetgii, Vol. 2, Nauka, Novosibirsk, 1985, pp. 152-186. 18 A. Mills, Chem. Sot. Rev., 18 (1989) 285-316. 19 A. P. Moravskii, N. K. Khannanov, A. V. Khramov, V. Ya. Shafirovich and A. Ye. Shilov, Khim. Fizika, 3 (1984) 1584-1590. 20 G. L. Elizarova, L. G. Matvienko and V. N. Parmon, J. Molec. Catal., 43 (1987) 171-181. 21 S. W. Gersten, G. J. Samuels and T. J. Meyer, J. Am. Chem. Sot., 104 (1982) 402WO30. 22 K. Honda and A. J. Frank, J. Chem. Sot. Chem. Comm., (1984) 1635-1636. 23 R. Ramaraj, A. Kira and M. Kaneko,,Angew. Chem., ht. Ed. Engl., 25 (1986) 1009-ldll. 24 R. Ramaraj, A. Kira and M. Kaneko, J. Chem. Sot. Chem Comm., (1987) 227-228. 25 F. P. Rotzinger, S. Munavalli, P. Comte, J. K. Hurst, M. Grltzel, F.-J. Pem and A. J. Frank, J. Am. Chem. Sot., IO9 (1987) 66196626.
338 26 G. L. Elizarova, L. G. Matvienko, N. V. Lozhkina and V. N. Parmon, React. Kinet. Cutal. Lett., 26 (1984) 67-72. 27 G. L. Elizarova, L. G. Matvienko, N. V. Lozhkina, V. N. Parmon and E. M. Moroz, IN. Sib. Otd. Akod. Nauk SSSR, Ser. Khim. Nauk (1990) 86-93. 28 S. V. Lymar, V. N. Parmon and K. I. Zamaraev, in J. Mattay (ed.), Photoinduced electron transfer ZZZ:Topics in Current Chemistry, Springer, Berlin, 159 (1991) l-65. 29 I. M. Tsvetkov, S. V. Lymar, V. N. Parmon and K. I. Zamaraev, Z&et. Z&t&, 27 (1986)
109-116. 30 N. P. Luneva, Ye. I. Knerelman, V. Ya. Shafirovich and A. E. Shilov, J. Chem. Comm., (1987) 1504-1505. 31 0. V. Gerasimov, I. M. Tsvetkov, S. V. Lymar and V. N. Parmon, React. &et. 36 (1988) 145-149. 32 Ye. I. Knerelman and V. Ya. Shafirovich, finet. Kataf., 28 (1987) 1237-1239. 33 J. R. Darwent, P. Douglas, A. Harriman, G. Porter and M.-C. Richoux, Coord. 44 (1982) 83-126. 34 P. A. Chistensen, A. Harriman, G. Porter and P. Neta, J. Chem. Sot., Faraday
Sot. Chem. Cut& Lett.,
Chem. Rev.,
Trans. 2, 80 (1984) 1451-1464. 35 G. S. Nahor, S. Mosseri, P. Neta and A. Harriman, 1 Phys. Chem., 92 (1988) 4499-4504. 36 0. V. Gerasimov, S. V. Lyrnar and V. N. Parmon, J. Photochem. Photobiol. A: Chem., 56 (1991) 275-285.
Structural
analysis of HPD
David Kessel Department of Pharmacology, Wayne State University School of Medicine, Detroit, MI 48201 (USA)
One of the mysteries concerning the tumor-localizing product HPD (hematoporphyrin derivative) is the ability of HPD components to persist for weeks in tissues after administration, in spite of a high degree of water solubility. In a recent report, Mironov et al. [l] provided some data concerning the relative numbers of porphyrin rings in HPD which occur as monomers, dimers and higher oligomers. Alkaline hydrolysis delineated ester from ether linkages. Based on this report, it is possible to make some calculations concerning the ether US. ester linkages in the HPD preparation employed in this study. Fresh HPD consists mainly of ester linkages, but during subsequent treatment, we have suggested that unhydrolyzed acetate groups form ether linkages [2]. Among the unknown factors is the nature of the mechanism whereby many of the ester linkages in HPD are protected from alkaline hydrolysis in aqueous solution. Pandey et nl. [3] have shown that simple porphyrin diesters cannot survive even mild alkali, and the formation of HPD calls for treatment of HP acetates with 0.1 M NaOH. In the purification step leading to Photofrin II, additional treatment at pH 9 results in a product which contains both ester and ether linkages in the dimer/oligomer fraction [41.
335-339
335
News and Views
Photosensitized water oxidation to dioxygen in artificial biomimetic systems: achievements and prospects 0. V. Gerasimov,
G. L. Elizarova
and V. N. Parrnon+
Institute of Catalysis, Pr. Lavrentieva, 5, Novosibirsk 630090 (Russia)
There are two reasons for the keen interest in the creation of artificial systems for water oxidation to dioxygen. The first reason is connected with the problem of natural photosynthetic oxygen evolution. Photosystem II (PSII) of green plants provides efficient light-driven electron transport from water, which is oxidized to dioxygen, to the pool of plastoquinones. Whereas the sequence of electron relays ensuring rapid and irreversible electron transfer from an oxygen evolving complex (OEC) to the pool of plastoquinones is relatively well established, the detailed structure of OEC and the mechanism of OEC action are rather unclear [l-3]. The second reason is connected with attempts to create artificial photosynthetic units as practical devices for the conversion of solar energy into chemical energy. Among various photochemical reactions that could be used for the purpose, water cleavage to elements attracts the greatest attention [4-71: 2HZ0 2
2Hz + Or
This reaction requires the oxidation of water to dioxygen by one-electron oxidants of moderate power (OS-l.5 V 21s. NHE), which are generated by means of visible light: (D+OxzD++Red) 4D+ +2H,O-
4D+0,+4H+
There are two different approaches to the creation of artificial water-oxidizing systems. First, considerable efforts have been made to synthesize a structural analog of natural OEC, which is a protein containing an oligonuclear manganese core (see [8,9] for reviews). Achievements in the field have mostly been connected with obtaining well-characterized bi-, tri- and tetranuclear manganese complexes with particular structural parameters close to those of the manganese cluster in natural OEC. No observable success has been achieved in attempts to accomplish photo-, electro- or thermal catalytic oxidation of water using these synthetic analogs of OEC. Nevertheless it seems to be very important to continue the work in this field, since this is the only way to obtain unambiguous evidence of our understanding natural processes. A more productive approach was to design efficient water photooxidizing systems without trying to mimic natural processes too closely, i.e. to create functional analogs of PSII. Functional analogs of both OEC (i.e. water oxidation catalysts) and PSI1 (i.e. ‘Author
to whom correspondence
loll-1344/92/$5.00
should be addressed.
0 1992 - Elsevier
Sequoia.
All rights reserved
336
photocatalytic systems that oxidize water at the expense of the irreversible reduction of some electron acceptors) have been suggested within several years after formulating the problem in the mid-seventies. In’ earlier works Ru(bpy):+ (bpy = 2,2’-bipyridine) was used as an analog of chlorophyll [W-12]. It was found that a number of transition metal compounds (mainly hydrated oxides) are capable of catalyzing water oxidation to dioxygen by the oxidized form of the complex mentioned, Ru(bpy)z+, and/or by other one-electron oxidants such as Ce4+. These are compounds of Pt, Ir [13], Ru 111, 141, Fe, Co, Cu, Ni [15, 161 and Mn [15] (see also reviews [17, 181). Further studies have developed in several directions. (1) Study of the mechanisms of water oxidation by known catalysts. The most serious obstacle in these studies is that the nature of the water-oxidizing active sites of heterogeneous hydroxide catalysts is rather unclear. The details of molecular oxygen formation are mostly hypothetical. It seemed that the synthesis of homogeneous catalysts with a well-defined structure could be a solution to the problem. However, there are other difficulties with homogeneous catalysts (see below). The majority of physical methods characterize a bulk material of catalyst, rather than its active sites, but the bulk material, hence only kinetic methods were applied [l&20]. However, since oxygen is evolved at the non-limiting states of the catalytic reaction, a kinetic approach can be applied only to study the oxidantxatalyst interaction rather than the elementary steps of oxygen evolution. Nevertheless, such studies have shown that most probably the reaction centers of most artificial catalysts for water oxidation contain two neighboring hydroxylated transition metal cations capable of reversible multielectron redox transformations. (2) Search for homogeneous catalysts. A number of mono-, bi- and trinuclear complexes of Fe [15], Ru [21-251, and Co [26] were reported to catalyze oxygen evolution. In our opinion more evidence on the homogeneity of the catalysts acting in particular systems should be given, because the redox or ligand-exchange destruction of these ‘homogeneous’ catalysts always results in the production of hydrolyzed species similar to some well-known efficient ‘heterogeneous’ catalysts. Besides, there are other reasons to doubt the catalytic action of the reported individual compounds. All the complexes that are claimed to be homogeneous catalysts contain nitrogen-based ligands, which are known to be better electron donors than oxygen-based ligands, including HZ0 (OH-). It is therefore natural to expect that the oxidation of nitrogen-containing ligands will generally be faster than the oxidation of coordinated water, and will thus promote a redox destruction of the complex. (3) Search for more efficient catalysts. The available catalysts do not ensure the evolution of stoichiometric amounts of oxygen owing to a redox destruction of oxidant and/or catalyst. From the viewpoint of stability and efficiency the best catalysts are heterogeneous RuOz [18] (when Ce4+ is used as the oxidant in strongly acid media) and colloid [20] as well as supported [27] Co(II1) hydroxide (when Ru(bpy),3+ is used as the oxidant in weakly acid, neutral and weakly alkaline media). (4) Attempts to conjugate the hydrogen- and oxygen-evolving photocatalytic systems to ensure the whole cycle of water cleavage. It is now generally acknowledged that this can only be achieved in a system consisting of at least two compartments, separated by a membrane-like structure. The most promising candidates for such membrane elements are considered to be microscopic objects like lipid vesicles [28]. Coupling the processes of light-driven electron transfer across bilayer lipid membrane and water oxidation could be the first step in the construction of the complete system. Although the only known attempt to achieve such a coupling [29] has failed, some progress was made torwards the creation of relatively efficient catalysts immobilized on lipid vesicles
337
[30,31], as well as oxygen photoevolution with participation of a photocatalyst embedded in the lipid bilayer membrane [32]. (5) Attempts to use new photocatalysts instead of Ru(bpy)$+. The ruthenium complex does not seem to be optimal for the purpose. Many porphyrins and their metallo derivatives have photophysical and electrochemical parameters that better suit water photooxidation than Ru(bpy)$+ [33]. However, the use of porphyrins as photocatalysts for water oxidation turned out to be quite a difficult task because of the instability of oxidized forms of the porphyrins. Nevertheless in several cases oxygen evolution was detected [34-361. In conclusion, some predictions can be made about the future developments of studies in the field. The most important issues appear to be: elucidation of the precise mechanism of oxygen evolution reactions which are catalyzed by artificial catalysts; synthesis of homogeneous catalysts of water oxidation and reliable demonstration of the homogeneous nature of their catalytic action (this issue is closely related to the synthesis of structural analogs of OEC); continuation of studies on the synthesis of stable and selective water oxidation catalysts; coupling the processes of photoelectron transfer across bilayer lipid membranes and water oxidation to dioxygen. 1 G. C. Disrnukes, Photo&em. PhotobioZ., 43 (1986) 99-115. 2 G. W. Brudwig, W. F. Beck and J. C. de Paula,Ann. Rev. Biophys. Biophys. Chem., 18 (1989) 25-46. 3 D. F. Ghanotakis and C. F. Yocum, Ann. Rev. Plant Physiol. Plant Mol. Biol., 141 (1990) 25.5-276. 4 J. S. Connoly (Ed.), Photochemical Conversion and Storage of Solar Energy, Academic Press, New York, 1982. 5 M. Grltzel (Ed.), Energy Resources through Photochemistry and Catalysis, Academic Press, New York, 1983. 6 K. I. Zamaraev and V. N. Parmon, Uspekhi Khimii., 49 (1980) 1457-1497. 7 V. N. Parmon and K. I. Zamaraev, in N. Serpone and E. Pelizzetti (Eds.), Photocatalysis: Fundamentals and Applications, Wiley Interscience, New York, 1989, p. 565-601. 8 V. L. Pecoraro, Photochem. Photobiol., 48 (1988) 249-264. 9 K. Wieghardt, Angew. Chem., Int. Ed. Engl., 28 (1989) 1153-1172. 10 K. Kalyanasundaram and M. Grltzel, Angew. Chem., 91 (1979) 759-760. 11 J.-M. Lehn, J.-P. Sauvage and R. Ziessel, Nouv. J. Chim., 3 (1979) 423-427. 12 V. Ya. Shafirovich, N. K. Khannanov and V. V. Strelets, Dokl. Akad. Nauk SSSR, 250 (1980) 1197-1200. 13 J. Kiwi and M. Grltzel, Angew. Chem., 90 (1978) 900-901. 14 J. Kiwi and M. Grltzel, Chimia, 33 (1979) 289-291. 15 G. L. Elizarova, L. G. Matvienko, V. N. Parmon and K. I. Zamaraev, Dokl. Akad. Nauk SSSR, 249 (1979) 863-866. 16 V. Ya. Shafirovich, N. K. Khannanov and V. V. Strelets. Nouv. J. Chim., 4 (1980) 81-84. 17 G. L. Elizarova and V. N. Parmon, in K. I. Zamaraev (Ed.), Photokatahticheskoepreobrazovanie solnechnoi enetgii, Vol. 2, Nauka, Novosibirsk, 1985, pp. 152-186. 18 A. Mills, Chem. Sot. Rev., 18 (1989) 285-316. 19 A. P. Moravskii, N. K. Khannanov, A. V. Khramov, V. Ya. Shafirovich and A. Ye. Shilov, Khim. Fizika, 3 (1984) 1584-1590. 20 G. L. Elizarova, L. G. Matvienko and V. N. Parmon, J. Molec. Catal., 43 (1987) 171-181. 21 S. W. Gersten, G. J. Samuels and T. J. Meyer, J. Am. Chem. Sot., 104 (1982) 402WO30. 22 K. Honda and A. J. Frank, J. Chem. Sot. Chem. Comm., (1984) 1635-1636. 23 R. Ramaraj, A. Kira and M. Kaneko,,Angew. Chem., ht. Ed. Engl., 25 (1986) 1009-ldll. 24 R. Ramaraj, A. Kira and M. Kaneko, J. Chem. Sot. Chem Comm., (1987) 227-228. 25 F. P. Rotzinger, S. Munavalli, P. Comte, J. K. Hurst, M. Grltzel, F.-J. Pem and A. J. Frank, J. Am. Chem. Sot., IO9 (1987) 66196626.
338 26 G. L. Elizarova, L. G. Matvienko, N. V. Lozhkina and V. N. Parmon, React. Kinet. Cutal. Lett., 26 (1984) 67-72. 27 G. L. Elizarova, L. G. Matvienko, N. V. Lozhkina, V. N. Parmon and E. M. Moroz, IN. Sib. Otd. Akod. Nauk SSSR, Ser. Khim. Nauk (1990) 86-93. 28 S. V. Lymar, V. N. Parmon and K. I. Zamaraev, in J. Mattay (ed.), Photoinduced electron transfer ZZZ:Topics in Current Chemistry, Springer, Berlin, 159 (1991) l-65. 29 I. M. Tsvetkov, S. V. Lymar, V. N. Parmon and K. I. Zamaraev, Z&et. Z&t&, 27 (1986)
109-116. 30 N. P. Luneva, Ye. I. Knerelman, V. Ya. Shafirovich and A. E. Shilov, J. Chem. Comm., (1987) 1504-1505. 31 0. V. Gerasimov, I. M. Tsvetkov, S. V. Lymar and V. N. Parmon, React. &et. 36 (1988) 145-149. 32 Ye. I. Knerelman and V. Ya. Shafirovich, finet. Kataf., 28 (1987) 1237-1239. 33 J. R. Darwent, P. Douglas, A. Harriman, G. Porter and M.-C. Richoux, Coord. 44 (1982) 83-126. 34 P. A. Chistensen, A. Harriman, G. Porter and P. Neta, J. Chem. Sot., Faraday
Sot. Chem. Cut& Lett.,
Chem. Rev.,
Trans. 2, 80 (1984) 1451-1464. 35 G. S. Nahor, S. Mosseri, P. Neta and A. Harriman, 1 Phys. Chem., 92 (1988) 4499-4504. 36 0. V. Gerasimov, S. V. Lyrnar and V. N. Parmon, J. Photochem. Photobiol. A: Chem., 56 (1991) 275-285.
Structural
analysis of HPD
David Kessel Department of Pharmacology, Wayne State University School of Medicine, Detroit, MI 48201 (USA)
One of the mysteries concerning the tumor-localizing product HPD (hematoporphyrin derivative) is the ability of HPD components to persist for weeks in tissues after administration, in spite of a high degree of water solubility. In a recent report, Mironov et al. [l] provided some data concerning the relative numbers of porphyrin rings in HPD which occur as monomers, dimers and higher oligomers. Alkaline hydrolysis delineated ester from ether linkages. Based on this report, it is possible to make some calculations concerning the ether US. ester linkages in the HPD preparation employed in this study. Fresh HPD consists mainly of ester linkages, but during subsequent treatment, we have suggested that unhydrolyzed acetate groups form ether linkages [2]. Among the unknown factors is the nature of the mechanism whereby many of the ester linkages in HPD are protected from alkaline hydrolysis in aqueous solution. Pandey et nl. [3] have shown that simple porphyrin diesters cannot survive even mild alkali, and the formation of HPD calls for treatment of HP acetates with 0.1 M NaOH. In the purification step leading to Photofrin II, additional treatment at pH 9 results in a product which contains both ester and ether linkages in the dimer/oligomer fraction [41.