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Stable titanium superoxide radicals in aqueous Ti-peroxy gels and Ti-peroxide solutions
LETTER TO THE EDITOR Stable Titanium Superoxide Radicals in Aqueous Ti-Peroxy Gels and Ti-Peroxide Solutions Titanium superoxide radicals (Ti(IV)O~-) were for the first time observed in situ by electron spin resonance methods in aqueous Ti-peroxy gels made from metallic titanium and hydrogen peroxide and in Ti(III)- and Ti(IV)-salt peroxides. The hyperfine splitting g-values (gl = 2.002, g2 = 2.009, and g3 = 2.022) found were identical in aqueous and in dried Ti-peroxy gels. The radicals in the gels were detectable at both neutral and low pH. The radicals in Ti (III or IV)-peroxide solutions escaped detection at neutral pH but reappeared upon lowering the pH. Excess hydrogen peroxide at preparation of Ti peroxide at pH 7 gave rise to formation of a detectable radical with a differing g3 (g3 = 2.037 ), but the radical escaped detection at pH 2. The detectability and g3-values of the Ti-superoxide radicals formed in Ti(III,IV)-peroxide solutions depend on pH and on the method of preparation. This indicates that Ti-superoxide radicals reside in different forms of polymeric Ti-peroxide complexes and may be formed through charge-transfer reactions. © 1991AcademicPress.Inc. INTRODUCTION In situ observations of stable metal superoxide radicals in aqueous solutions are rare. The presence of such radicals may be of considerable importance in many research areas, such as photochemistry, solar energy conversion (1-4), and the biochemistry of respiration and inflammation (5, 6). Previous investigations of the Ti-H202 system were performed using diluted acidic Ti(III) salts with H202 added and reactions were examined in electron spin resonance (ESR)-spectroscopic fast-flow systems (7-9). The experiments displayed short-lived (tl/2 on the order of ms to s) radicals with g-values near 2.010 and 2.012 that were attributed to reactions with the hydroxyl (OH) and hydroperoxyl (HO2) radicals, respectively. Ragai showed that precipitated and dried titania powders prepared from acidic Ti(III) salts and HzO2 precipitated with NH4OH (base) contain a stable titanium superoxide (Ti(IV) O ~) radical ( 10, 11 ). Furthermore, we have observed the Ti(IV)O~radical also in dried Ti-peroxy gels made from metallic titanium and H202 (12, 13). Tentatively the radical may be formed from Ti(III) + 02, Ti(IV) + O2(HO2), or through complex radical reactions. We report here on what we believe is the first observation of stable titanium superoxide radicals in an unperturbed aqueous Ti-peroxy gel and in aqueous peroxide solutions prepared from Ti(IV). The radicals were observed in aqueous (room temperature) and aqueous frozen (77 K) Ti gels and in Ti-peroxide solutions prepared from titanium salts. The properties of the radical complexes formed depend on pH and on the method of preparation, indicating that they may be found in different forms of aqueous polymeric Tiperoxide complexes. For the first time the Ti-superoxide radical is observed to be formed and to survive at aqueous
physiological conditions. Surprisingly, the g-values found were identical in aqueous and dried Ti-peroxy gels. This unequivocally shows that the Ti-superoxide radical displayed in dried gel powders does not originate from reactions occurring during the structure collapse due to the removal of water, but rather is stable enough not to succumb during the drying procedure. MATERIALS AND METHODS The Ti-peroxy gels studied in the present experiments were prepared according to an earlier described procedure ( 13 ). In short, 10 g metallic titanium powder is incubated in 200 ml 6 M H202. Titanium is then dissolved out of the solution through some complex redox and dissolution mechanisms involving H20, H202, and an increased surface 02 pressure. The increased oxygen tension at the Ti surface is due to the catalytic decomposition of H202 on the surface at room conditions. When most of the excess peroxide has been consumed a transparent yellow-green gel with pH 3.5 is spontaneously formed through a chelation of the bulk solution. The gel contains typically 45 m M Ti and peroxide of the same magnitude (13). The suggested composition of the chelate is [Ti(IV)O~-Ti(IV)O~--Ti(IV)(OH)4],q (12, 14, 15). The gel thus contains about 99% of water by weight. The results reported here were independent of the age of the gel (from less than 24 h to 5 months). Titanium peroxide solutions were made by mixing 1 ml titanous or titanic sulphate (15% W/V) with 100 #1 9.5 M H202. The solutions were diluted with distilled water to approximately a 50 mMTi concentration. The Ti(III)- and Ti(IV)-based solutions behaved similarly as far as the Ti(IV)O~ radical was concerned. When re-
589
Journal of Colloid and Interface Science, Vol. 143,No. 2, May 1991
0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. AIIfightsofreproductionin any formreserved.
590
LETTER TO THE EDITOR
quired, the pH of the preparations was set by addition of HC1 or NaOH. RESULTS AND DISCUSSION Typical observed ESR spectra are shown in Fig. 1. Spectrum (a) was obtained for an unbuffered Ti-peroxy gel (pH 3.5) which was frozen and studied at 77 K. Ac-
{a)
=.
fa=9450MHz
I
g 2 =2' 009
(bl ,10G g3-2.021
/~
~'/*--'-g l
(C} g3i2'037
~ .,~ J
~
2
= 2.002
fc=9435MHz I
~.2.o~o
~
g1=2,002
FIG. 1. ESR spectra of a frozen aqueous, pH 3.5, Tiperoxy gel at 77 K (a); aqueous Ti-peroxy gel at room temperature, pH 3.5 (b); and a frozen Ti-peroxide solution with [H202]:[Ti] = 5 and prepared at pH 7 (c). The radicals in (~) and (b) were observed both at neutral and at low pH; the spectrum in (c) was observed only at neutral pH when excess H202 was used. In an equimolar solution of H202 and Ti salts the radical was observed at low but not at neutral pH (see Table I). Note that g3 is significantly different for the spectra in (a) and (b) compared with the spectruni in (e). The spectra have the appearance reported for the Ti(IV) O ~- radical ( 14, 15 ). The ESR spectra were recorded at 77 K (liquid nitrogen) or at room temperature with a Bruker ER 2000 spectrometer operated at x-band. The correct field strength and microwave frequency were determined by an EIP Model 548 A frequency counter and a Bruker 031 M N M R gaussmeter. Suprasil ESR sample tubes were filled with the aqueous Ti-peroxide solutions/Ti-peroxy gels at room conditions. Journal of Colloid and Interface Science, Vol. 143,No. 2, May 1991
cording to the literature, the spectrum displays the hyperfine splittings of a Ti (IV) O ~ radical ( 14, 15) with obtained g-values of gl = 2.002, g2 = 2.009, and g3 = 2.022, respectively. When the pH was set to 7 before freezing, the radical was still observed with the same set of g-values even after 20 h incubation at the higher pH. Thus, the radical in the gel did not escape detection when the pH was adjusted from 3.5 to 7. This was contrary to Ti superoxide seen in peroxides prepared from Ti salts, in which the radical always changed detectability upon a change in pH. An ESR spectrum similar to that in Fig. la was observed at room temperature, although with a lower signalto-noise ratio (Fig. lb). Freeze-dried gel powders also displayed the same spectrum. A summary of this and other experimental results is presented in Table I. It was observed that frozen Ti-peroxide solutions prepared from Ti salts at acidic conditions contained the same type of radical as found in the Ti-peroxy gels. When the pH was raised in the Ti-salt peroxide solutions prior to freezing, the radical disappeared in contrast to the behavior of the radical in the Ti-peroxy gels prepared from metallic titanium and hydrogen peroxide. By lowering the pH (to below 3 ) the radical appeared again. When Ti-salt peroxides were prepared at pH 7 with an excess of H202 (5:1) a Ti(IV)O~radical with different g-values than those discussed above was observed (Fig. lc; Table I). Unexpectedly, when the pH was lowered to below 3 in such a solution the radical escaped detection and reappeared again after an increase in the pH. Addition of catalase to or boiling of the Tiperoxide solutions did not change the observed behavior of the radicals. This indicates that the observed changes in the spectrum and/or detectability of the radical are not due to free hydrogen peroxide redox reactions during the pH adjustments. We conclude from our experimental observations that: (a) stable Ti(IV)O~ radicals exist in aqueous surroundings; they are detectable both in aqueous and dried Ti gels and in Ti-peroxide solutions; (b) there are obvious differences in the detectability of the superoxide radicals depending on the preparation conditions; the Ti-peroxy gels made from metallic titanium and hydrogen peroxide contain a radical stable in the pH range 2-7 in contrast to the radical in Ti-peroxide solutions, suggesting that the radical complex has another more pH-independent structure in the gel; (c) the same g-values were displayed in aqueous gels and in dried gel powders; i.e., water does not alter the Ti-superoxide coordination; (d) apparently the structure surrounding the Ti(IV)O~ radical in Ti-salt peroxide solutions depends on the preparation conditions as manifested from the detection of radicals with different g3values. Conclusions similar to (b) above can be made from earlier reports on dried gels made from Ti(III)C13 and metallic titanium, respectively. In dried Ti-peroxy gels made from metallic titanium and H202 the radical (gvalues, set 1) was observed also in gels made at neutral pH conditions (16). In titania powders made from acidic
LETTER TO THE EDITOR
591
TABLE I Observations on Titanium Superoxide in Aqueous Ti-Peroxy Gels or Ti-Salt Solutions Analyticcondition Type of preparation Ti-peroxy gel (at room temperature or frozen) Dried gel powder made from metallic Ti and H202 (Ref.(13)) [H202]:[Ti] = 1 solution (frozen) [HzO2]:[Ti] = 5 solution (frozen)
Preparation condition
pH2
pH7
Low pH
Radical
Radical
Set 1
pH 3.5 Low pH pH7 Low pH pH7
-Radical Radical Radical No radical
-No radical No radical No radical Radical
Set 1 Set 1 Set 1 Set 1 Set 2
Dried titania prepared from TIC13, H202, and pH2.5 NH4OH (Ref.(7)) Hyperfine splitting, g-values: Set 1: gl = 2.002, g2 = 2.009, g3 = 2.022 Set 2: gt = 2.002, g2 = 2.010, g3 = 2.037
g-values
Set 1
Note. The results illustrate the presence of three different coordinations of the Ti-superoxide radical in aqueous Tiperoxy systems. In the Ti-peroxy gel the radical is observed both at low and at high pH. In Ti-salt peroxide solutions it is observed only at low pH, except for the last case. A solution prepared at neutral pH with an excess of H202 shows no radical at low pH but does so at pH 7. One of the g-values of this radical (set 2) differs from the g-values observed in the other cases (set 1). Set 1 corresponds to curves (a) and (b) and set 2 corresponds to curve (c) in Fig. I. The radical patterns (for the solutions) did not change if catalase was added to the solutions or if they were heated up and cooled prior to measurements.
Ti (III)C13 and hydrogen peroxide, the radical was observed only for samples prepared at low pH (10). It is known that precipitates of dinuclear Ti(IV)20~(OH)ix2-xl+ complexes with x = 1-6 are formed in Yiperoxide solutions at least in the pH range 2-7 (17). The higher the pH, the larger is x. Our results show that these complexes also incorporate Ti superoxide. The pH dependence of the ESR signal observed for Ti-peroxide solutions (made from Ti salts) is not well understood. A pH-dependent structural change of the complexes surrounding Ti(IV)O£ in Ti-peroxide solutions is one possible explanation for the change in behavior of the radical. Monomeric Ti(IV)O~- is known to dimerize in solutions when pH > 1 (17). In our systems, dimerization or polymerization may result in complexes with (two) Ti(IV)O~- groups with paired, opposite, and ESR-inactive spins as the pH is changed. Alternatively, the appearance of Ti superoxide could be due to a pH-dependent charge transfer to and from a stable peroxide group in the di- or polynuclear peroxotitanium complex. The Ti peroxide and Ti superoxide would then be two oxidation states of the same Ti peroxide possessing a pH-dependent charge and structure. The presence of a Ti(IV)O~- radical both at acidic and at neutral pH in Ti gels obtained from metallic titanium upon interaction with hydrogen peroxide is of particular interest for titanium as a biomaterial. Titanium can, e.g., be implanted in human bone with very good clinical results (18). Hydrogen peroxide formed during an inflammatory response may cause Ti ions to dissolve out of the implant and give rise to Ti peroxide. In vitro
model experiments with metallic Ti and HzO2 show no free deleterious hydroxyl radical (OH) formation (12). Instead Ti(IV)O~-, Ti(IV)O~, Ti(OH)4, and complexes thereof are formed. In contrast metals like Fe and Cu are shown to participate in site-specific free hydroxyl radical formation in similar experiments (6, 19, 20). The biological consequences of the fact that Ti instead forms stable and long-lived oxygen radical complexes are not known. It has been suggested that outside a titanium implant the titanium-hydrogen peroxide chemistry may mimic first a sink for H202, and then during Ti-peroxide degradation act as a source of slowly released H202. Furthermore, through the presence of the Ti(IV)O~- radical, an interaction with extracellular components, e.g., (glyco)proteins, may occur in such a way that the healing process is influenced in a positive way ( 12, 21 ). ACKNOWLEDGMENTS This work is supported by grants from the Swedish National Board for Technical Development (STU), their Engineering Research Council (STUF), and the Institute of Applied Biotechnology, Gothenburg, Sweden. REFERENCES i. Munuera, G., Fernandez, A., and Espinos, J. P., in "Photocatalytic Production of Energy-Rich Compounds" (G. Grassi and D. O. Hall, Eds.), p. 97. Elsevier, London, 1988. Journal of Colloid and Interface Science, VoI. 143,No. 2, May 1991
592
LETTER TO THE EDITOR
2. Tafalla, D., and Salvador, P., in "Photocatalytic Production of Energy-Rich Compounds" (G. Grassi and D. O. Hall, Eds.), p. 148. Elsevier, London, 1988. 3. Gr/itzel, M., in "Heterogenous Photochemical Electron Transfer," p. 87. CRC Press, Boca Raton, FL, 1989. 4. Ohta, T., Int. J. Hydrogen Energy 13, 333 (1988). 5. Symons M. C. R., in "Inorganic and Organic Radicals: Their Biological and Clinical Relevance" (H. A. O. Hill, Ed.), p. 451. The Royal Society, London, 1986. 6. Klebanoff, S. J., in "Inflammation: Basic Principles and Clinical Correlates" (J. I. Gallin, I. M. Goldstein, and R. Snyderman, Eds.), p. 391. Raven Press, New York, 1988. 7. Dixon, W. T., and Norman, R. O. C., Nature 196, 891 (1962). 8. Chiang, Y. S., Craddock, J., Mickewich, D., and Turkevich, J., J. Phys. Chem. 70, 3509 (1966). 9. Takakura, K., and Rhnby, B., J. Phys. Chem. 72, 164 ( 1968 ). 10. Ragai, J., Nature325, 703 (1987). 11. Ragai, J., 9". Chem. Technol. BiotechnoL 40, 143 (1987). 12. Tengvall, P., Lundstr6m, I., Sj6kvist, L., Elwing, H., and Bjursten, L. M., Biomaterials 10, 166 (1989). 13. Tengvall, P., Elwing, H., and Lundstr6m, I., J. Colloid Interface Sci. 130, 405 (1989). 14. Naccache, C., Meriadeau, P., Che, M., and Tench, A. J., J. Trans. Faraday Soc. 67, 506 (1971).
Journalof Colloidand InterfaceScience,Vol.143,No. 2, May 1991
15. Shiotani, M., Moro, G., and Freed, J. H., J. Chem. Phys. 74, 2616 ( 1981 ). 16. Tengvall, P. O., Bertilsson, L., Liedberg, B., Elwing, H., and Lundstr6m, I., J. Colloid Interface Sci. 139, 575 (1990). 17. Mfihlebach, J., MiJller, K., and Schwarzenbach, G., Inorg. Chem. 9, 2381 (1970). 18. Albrektsson, T., Crit. Rev. Biocompat. 1, 53 (1984). 19. Bielski, B. H. J., in "Inorganic and Organic Radicals: Their Biological and Clinical Relevance" (H. A. O. Hill, Ed.), p. 473. The Royal Society, London, 1986. 20. Halliwell, B., Gutteridge, J. M. C., and Blake, D., in "Inorganic and Organic Radicals: Their Biological and Clinical Relevance" (H. A. O. Hill, Ed.), p. 659. The Royal Society, London, 1986. 21. Bjursten, L. M., Tengvall, P., Ericson, L. E., Gretzer, C., and Lundstr6m, I., Agents Actions, in press. P. TENGVALL B. W~LIVAARA J. WESTERLING I. LUNDSTROM Department of Physics and Measurement Technology University of LinkO'ping S-581 83 LinkO'ping Sweden Received October 5, 1990
physiological conditions. Surprisingly, the g-values found were identical in aqueous and dried Ti-peroxy gels. This unequivocally shows that the Ti-superoxide radical displayed in dried gel powders does not originate from reactions occurring during the structure collapse due to the removal of water, but rather is stable enough not to succumb during the drying procedure. MATERIALS AND METHODS The Ti-peroxy gels studied in the present experiments were prepared according to an earlier described procedure ( 13 ). In short, 10 g metallic titanium powder is incubated in 200 ml 6 M H202. Titanium is then dissolved out of the solution through some complex redox and dissolution mechanisms involving H20, H202, and an increased surface 02 pressure. The increased oxygen tension at the Ti surface is due to the catalytic decomposition of H202 on the surface at room conditions. When most of the excess peroxide has been consumed a transparent yellow-green gel with pH 3.5 is spontaneously formed through a chelation of the bulk solution. The gel contains typically 45 m M Ti and peroxide of the same magnitude (13). The suggested composition of the chelate is [Ti(IV)O~-Ti(IV)O~--Ti(IV)(OH)4],q (12, 14, 15). The gel thus contains about 99% of water by weight. The results reported here were independent of the age of the gel (from less than 24 h to 5 months). Titanium peroxide solutions were made by mixing 1 ml titanous or titanic sulphate (15% W/V) with 100 #1 9.5 M H202. The solutions were diluted with distilled water to approximately a 50 mMTi concentration. The Ti(III)- and Ti(IV)-based solutions behaved similarly as far as the Ti(IV)O~ radical was concerned. When re-
589
Journal of Colloid and Interface Science, Vol. 143,No. 2, May 1991
0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. AIIfightsofreproductionin any formreserved.
590
LETTER TO THE EDITOR
quired, the pH of the preparations was set by addition of HC1 or NaOH. RESULTS AND DISCUSSION Typical observed ESR spectra are shown in Fig. 1. Spectrum (a) was obtained for an unbuffered Ti-peroxy gel (pH 3.5) which was frozen and studied at 77 K. Ac-
{a)
=.
fa=9450MHz
I
g 2 =2' 009
(bl ,10G g3-2.021
/~
~'/*--'-g l
(C} g3i2'037
~ .,~ J
~
2
= 2.002
fc=9435MHz I
~.2.o~o
~
g1=2,002
FIG. 1. ESR spectra of a frozen aqueous, pH 3.5, Tiperoxy gel at 77 K (a); aqueous Ti-peroxy gel at room temperature, pH 3.5 (b); and a frozen Ti-peroxide solution with [H202]:[Ti] = 5 and prepared at pH 7 (c). The radicals in (~) and (b) were observed both at neutral and at low pH; the spectrum in (c) was observed only at neutral pH when excess H202 was used. In an equimolar solution of H202 and Ti salts the radical was observed at low but not at neutral pH (see Table I). Note that g3 is significantly different for the spectra in (a) and (b) compared with the spectruni in (e). The spectra have the appearance reported for the Ti(IV) O ~- radical ( 14, 15 ). The ESR spectra were recorded at 77 K (liquid nitrogen) or at room temperature with a Bruker ER 2000 spectrometer operated at x-band. The correct field strength and microwave frequency were determined by an EIP Model 548 A frequency counter and a Bruker 031 M N M R gaussmeter. Suprasil ESR sample tubes were filled with the aqueous Ti-peroxide solutions/Ti-peroxy gels at room conditions. Journal of Colloid and Interface Science, Vol. 143,No. 2, May 1991
cording to the literature, the spectrum displays the hyperfine splittings of a Ti (IV) O ~ radical ( 14, 15) with obtained g-values of gl = 2.002, g2 = 2.009, and g3 = 2.022, respectively. When the pH was set to 7 before freezing, the radical was still observed with the same set of g-values even after 20 h incubation at the higher pH. Thus, the radical in the gel did not escape detection when the pH was adjusted from 3.5 to 7. This was contrary to Ti superoxide seen in peroxides prepared from Ti salts, in which the radical always changed detectability upon a change in pH. An ESR spectrum similar to that in Fig. la was observed at room temperature, although with a lower signalto-noise ratio (Fig. lb). Freeze-dried gel powders also displayed the same spectrum. A summary of this and other experimental results is presented in Table I. It was observed that frozen Ti-peroxide solutions prepared from Ti salts at acidic conditions contained the same type of radical as found in the Ti-peroxy gels. When the pH was raised in the Ti-salt peroxide solutions prior to freezing, the radical disappeared in contrast to the behavior of the radical in the Ti-peroxy gels prepared from metallic titanium and hydrogen peroxide. By lowering the pH (to below 3 ) the radical appeared again. When Ti-salt peroxides were prepared at pH 7 with an excess of H202 (5:1) a Ti(IV)O~radical with different g-values than those discussed above was observed (Fig. lc; Table I). Unexpectedly, when the pH was lowered to below 3 in such a solution the radical escaped detection and reappeared again after an increase in the pH. Addition of catalase to or boiling of the Tiperoxide solutions did not change the observed behavior of the radicals. This indicates that the observed changes in the spectrum and/or detectability of the radical are not due to free hydrogen peroxide redox reactions during the pH adjustments. We conclude from our experimental observations that: (a) stable Ti(IV)O~ radicals exist in aqueous surroundings; they are detectable both in aqueous and dried Ti gels and in Ti-peroxide solutions; (b) there are obvious differences in the detectability of the superoxide radicals depending on the preparation conditions; the Ti-peroxy gels made from metallic titanium and hydrogen peroxide contain a radical stable in the pH range 2-7 in contrast to the radical in Ti-peroxide solutions, suggesting that the radical complex has another more pH-independent structure in the gel; (c) the same g-values were displayed in aqueous gels and in dried gel powders; i.e., water does not alter the Ti-superoxide coordination; (d) apparently the structure surrounding the Ti(IV)O~ radical in Ti-salt peroxide solutions depends on the preparation conditions as manifested from the detection of radicals with different g3values. Conclusions similar to (b) above can be made from earlier reports on dried gels made from Ti(III)C13 and metallic titanium, respectively. In dried Ti-peroxy gels made from metallic titanium and H202 the radical (gvalues, set 1) was observed also in gels made at neutral pH conditions (16). In titania powders made from acidic
LETTER TO THE EDITOR
591
TABLE I Observations on Titanium Superoxide in Aqueous Ti-Peroxy Gels or Ti-Salt Solutions Analyticcondition Type of preparation Ti-peroxy gel (at room temperature or frozen) Dried gel powder made from metallic Ti and H202 (Ref.(13)) [H202]:[Ti] = 1 solution (frozen) [HzO2]:[Ti] = 5 solution (frozen)
Preparation condition
pH2
pH7
Low pH
Radical
Radical
Set 1
pH 3.5 Low pH pH7 Low pH pH7
-Radical Radical Radical No radical
-No radical No radical No radical Radical
Set 1 Set 1 Set 1 Set 1 Set 2
Dried titania prepared from TIC13, H202, and pH2.5 NH4OH (Ref.(7)) Hyperfine splitting, g-values: Set 1: gl = 2.002, g2 = 2.009, g3 = 2.022 Set 2: gt = 2.002, g2 = 2.010, g3 = 2.037
g-values
Set 1
Note. The results illustrate the presence of three different coordinations of the Ti-superoxide radical in aqueous Tiperoxy systems. In the Ti-peroxy gel the radical is observed both at low and at high pH. In Ti-salt peroxide solutions it is observed only at low pH, except for the last case. A solution prepared at neutral pH with an excess of H202 shows no radical at low pH but does so at pH 7. One of the g-values of this radical (set 2) differs from the g-values observed in the other cases (set 1). Set 1 corresponds to curves (a) and (b) and set 2 corresponds to curve (c) in Fig. I. The radical patterns (for the solutions) did not change if catalase was added to the solutions or if they were heated up and cooled prior to measurements.
Ti (III)C13 and hydrogen peroxide, the radical was observed only for samples prepared at low pH (10). It is known that precipitates of dinuclear Ti(IV)20~(OH)ix2-xl+ complexes with x = 1-6 are formed in Yiperoxide solutions at least in the pH range 2-7 (17). The higher the pH, the larger is x. Our results show that these complexes also incorporate Ti superoxide. The pH dependence of the ESR signal observed for Ti-peroxide solutions (made from Ti salts) is not well understood. A pH-dependent structural change of the complexes surrounding Ti(IV)O£ in Ti-peroxide solutions is one possible explanation for the change in behavior of the radical. Monomeric Ti(IV)O~- is known to dimerize in solutions when pH > 1 (17). In our systems, dimerization or polymerization may result in complexes with (two) Ti(IV)O~- groups with paired, opposite, and ESR-inactive spins as the pH is changed. Alternatively, the appearance of Ti superoxide could be due to a pH-dependent charge transfer to and from a stable peroxide group in the di- or polynuclear peroxotitanium complex. The Ti peroxide and Ti superoxide would then be two oxidation states of the same Ti peroxide possessing a pH-dependent charge and structure. The presence of a Ti(IV)O~- radical both at acidic and at neutral pH in Ti gels obtained from metallic titanium upon interaction with hydrogen peroxide is of particular interest for titanium as a biomaterial. Titanium can, e.g., be implanted in human bone with very good clinical results (18). Hydrogen peroxide formed during an inflammatory response may cause Ti ions to dissolve out of the implant and give rise to Ti peroxide. In vitro
model experiments with metallic Ti and HzO2 show no free deleterious hydroxyl radical (OH) formation (12). Instead Ti(IV)O~-, Ti(IV)O~, Ti(OH)4, and complexes thereof are formed. In contrast metals like Fe and Cu are shown to participate in site-specific free hydroxyl radical formation in similar experiments (6, 19, 20). The biological consequences of the fact that Ti instead forms stable and long-lived oxygen radical complexes are not known. It has been suggested that outside a titanium implant the titanium-hydrogen peroxide chemistry may mimic first a sink for H202, and then during Ti-peroxide degradation act as a source of slowly released H202. Furthermore, through the presence of the Ti(IV)O~- radical, an interaction with extracellular components, e.g., (glyco)proteins, may occur in such a way that the healing process is influenced in a positive way ( 12, 21 ). ACKNOWLEDGMENTS This work is supported by grants from the Swedish National Board for Technical Development (STU), their Engineering Research Council (STUF), and the Institute of Applied Biotechnology, Gothenburg, Sweden. REFERENCES i. Munuera, G., Fernandez, A., and Espinos, J. P., in "Photocatalytic Production of Energy-Rich Compounds" (G. Grassi and D. O. Hall, Eds.), p. 97. Elsevier, London, 1988. Journal of Colloid and Interface Science, VoI. 143,No. 2, May 1991
592
LETTER TO THE EDITOR
2. Tafalla, D., and Salvador, P., in "Photocatalytic Production of Energy-Rich Compounds" (G. Grassi and D. O. Hall, Eds.), p. 148. Elsevier, London, 1988. 3. Gr/itzel, M., in "Heterogenous Photochemical Electron Transfer," p. 87. CRC Press, Boca Raton, FL, 1989. 4. Ohta, T., Int. J. Hydrogen Energy 13, 333 (1988). 5. Symons M. C. R., in "Inorganic and Organic Radicals: Their Biological and Clinical Relevance" (H. A. O. Hill, Ed.), p. 451. The Royal Society, London, 1986. 6. Klebanoff, S. J., in "Inflammation: Basic Principles and Clinical Correlates" (J. I. Gallin, I. M. Goldstein, and R. Snyderman, Eds.), p. 391. Raven Press, New York, 1988. 7. Dixon, W. T., and Norman, R. O. C., Nature 196, 891 (1962). 8. Chiang, Y. S., Craddock, J., Mickewich, D., and Turkevich, J., J. Phys. Chem. 70, 3509 (1966). 9. Takakura, K., and Rhnby, B., J. Phys. Chem. 72, 164 ( 1968 ). 10. Ragai, J., Nature325, 703 (1987). 11. Ragai, J., 9". Chem. Technol. BiotechnoL 40, 143 (1987). 12. Tengvall, P., Lundstr6m, I., Sj6kvist, L., Elwing, H., and Bjursten, L. M., Biomaterials 10, 166 (1989). 13. Tengvall, P., Elwing, H., and Lundstr6m, I., J. Colloid Interface Sci. 130, 405 (1989). 14. Naccache, C., Meriadeau, P., Che, M., and Tench, A. J., J. Trans. Faraday Soc. 67, 506 (1971).
Journalof Colloidand InterfaceScience,Vol.143,No. 2, May 1991
15. Shiotani, M., Moro, G., and Freed, J. H., J. Chem. Phys. 74, 2616 ( 1981 ). 16. Tengvall, P. O., Bertilsson, L., Liedberg, B., Elwing, H., and Lundstr6m, I., J. Colloid Interface Sci. 139, 575 (1990). 17. Mfihlebach, J., MiJller, K., and Schwarzenbach, G., Inorg. Chem. 9, 2381 (1970). 18. Albrektsson, T., Crit. Rev. Biocompat. 1, 53 (1984). 19. Bielski, B. H. J., in "Inorganic and Organic Radicals: Their Biological and Clinical Relevance" (H. A. O. Hill, Ed.), p. 473. The Royal Society, London, 1986. 20. Halliwell, B., Gutteridge, J. M. C., and Blake, D., in "Inorganic and Organic Radicals: Their Biological and Clinical Relevance" (H. A. O. Hill, Ed.), p. 659. The Royal Society, London, 1986. 21. Bjursten, L. M., Tengvall, P., Ericson, L. E., Gretzer, C., and Lundstr6m, I., Agents Actions, in press. P. TENGVALL B. W~LIVAARA J. WESTERLING I. LUNDSTROM Department of Physics and Measurement Technology University of LinkO'ping S-581 83 LinkO'ping Sweden Received October 5, 1990