Langmuir 2006, 22, 10109-10117
10109
Infrared Spectroscopic Studies of Siderophore-Related Hydroxamic Acid Ligands Adsorbed on Titanium Dioxide Jing Yang,† Phil J. Bremer,‡ Iain L. Lamont,§ and A. James McQuillan*,† Departments of Chemistry, Food Science, and Biochemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed May 15, 2006. In Final Form: September 2, 2006 The adhesion of bacteria to metal oxide and other mineral surfaces may involve bacterial siderophores, many of which contain hydroxamic acid (Ha) ligands. The adsorption behavior of the siderophore-related ligands acetohydroxamic acid, N-methylformohydroxamic acid, N-methylacetohydroxamic acid, and 1-hydroxy-2-piperidone on titanium dioxide thin films has been investigated using in situ ATR-IR spectroscopy with variation of concentration and pH. All the ligands were found to adsorb strongly on the TiO2 surface as hydroxamate ions and form bidentate surface complexes. Vibrational modes involving CdO stretching and N-O stretching of these ligands were assigned by comparing observed IR spectra with those calculated by the density functional method at the B3LYP/6-31+G(d) level. Calculated spectra of the complex [Ti(Ha)(OH)4]-, used to model the TiO2 surface, were compared with observed spectra of adsorbed hydroxamic acids. These results suggest that hydroxamic acid ligands in siderophores would be expected to bind to metal (oxide) and mineral surfaces during bacterial adhesion processes.
Introduction Many bacteria in iron-deficient situations can produce iron uptake compounds known as siderophores. These siderophores typically contain hydroxamic acid (Ha), catechol, or carboxylic acid groups which can act as bidentate ligands and form highly stable iron(III) chelate complexes. The siderophores sequester iron from the environment and from iron-containing minerals. The roles of siderophores in iron-containing mineral dissolution1-6 and in bacterial iron transport from the extracellular medium to the intracellular medium7-12 are currently active areas of research. The adsorption behavior to minerals of the constituent bidentate ligands within the siderophores is an important factor in determining the role of siderophores in mineral dissolution processes and in other situations where adsorbed siderophores are present. While the adsorption behavior of catechol and carboxylic acid ligands to wet metal oxide surfaces has been frequently studied,13-17 there has been less attention paid to * To whom correspondence should be addressed. Fax: +64 3 4797906. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Food Science. § Department of Biochemistry. (1) Holmen, B. A.; Casey, W. H. Geochim. Cosmochim. Acta 1996, 60, 440316. (2) Kraemer, S. M.; Cheah, S.-F.; Zapf, R.; Xu, J.; Raymond, K. N.; Sposito, G. Geochim. Cosmochim. Acta 1999, 63, 3003-08. (3) Cocozza, C.; Tsao, C. C. G.; Cheah, S.-F.; Kraemer, S. M.; Raymond, K. N.; Miano, T. M.; Sposito, G. Geochim. Cosmochim. Acta 2002, 66, 431-38. (4) Cervini-Silva, J.; Sposito, G. EnViron. Sci. Technol. 2002, 36, 337-42. (5) Cheah, S.-F.; Kraemer, S. M.; Cervini-Silva, J.; Sposito, G. Chem. Geol. 2003, 198, 63-75. (6) Rosenberg, D. R.; Maurice, P. A. Geochim. Cosmochim. Acta 2003, 67, 223-29. (7) Dhungana, S.; Crumbliss, A. L. Geomicrobiol. J. 2005, 22, 87-98. (8) Poole, K.; McKay, G. A. Front. Biosci. 2003, 8, D661-D86. (9) Visca, P.; Leoni, L.; Wilson Megan, J.; Lamont Iain, L. Mol. Microbiol. 2002, 45, 1177-90. (10) Raymond, K. N.; Telford, J. R. NATO ASI Ser., Ser. C 1995, 459, 25-37. (11) Neilands, J. B. J. Biol. Chem. 1995, 270, 26723-6. (12) Guerinot, M. L. Annu. ReV. Microbiol. 1994, 48, 743-72. (13) Redfern, P. C.; Zapol, P.; Curtiss, L. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107, 11419-27. (14) Rodriguez, R.; Blesa, M. A.; Regazzoni, A. E. J. Colloid Interface Sci. 1996, 177, 122-31. (15) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta 1999, 55A, 1395405.
corresponding studies of hydroxamic acid ligands.1,18,19 Hydroxamic acids generally contain the bidentate chelating group -CONOH-, which has a high affinity for Fe3+ ion and other metal ions.18 The synthesis and coordination chemistry of hydroxamic acid ligands have been extensively investigated.20-25 Due to their chelating properties, hydroxamic acids are now widely used in metal ion analysis, mineral flotation, radioactive and heavy metal waste treatments, and therapy of iron-overloadrelated diseases.26-30 Some hydroxamic acids are reported as potential antibacterial, anticancer, and antitumor agents.31-33 Most early studies of the surface chemistry involving hydroxamic acid ligands have focused on the use of alkylhydroxamic acids as flotation collectors, typically using UV-vis spectroscopic measurements to determine amount adsorbed from solution concentration changes.34,35 Pradip18 has reviewed the surface (16) Rosenqvist, J.; Axe, K.; Sjoberg, S.; Persson, P. Colloids Surf., A 2003, 220, 91-104. (17) Vermohlen, K.; Lewandowski, H.; Narres, H. D.; Koglin, E. Colloids Surf., A 2000, 170, 181-89. (18) Pradip. Trans. Indian Inst. Met. 1987, 40, 287-304. (19) Holmen, B. A.; Tejedor-Tejedor, M. I.; Casey, W. H. Langmuir 1997, 13, 2197-206. (20) Crumbliss, A. L. Coord. Chem. ReV. 1990, 105, 155-79. (21) Kurzak, B.; Kozlowski, H.; Farkas, E. Coord. Chem. ReV. 1992, 114, 169-200. (22) Boukhalfa, H.; Brickman, T. J.; Armstrong, S. K.; Crumbliss, A. L. Inorg. Chem. 2000, 39, 5591-602. (23) Brown, D. A.; McKeith, D.; Glass, W. K. Inorg. Chim. Acta 1979, 35, 5-10. (24) Carrano, C. J.; Raymond, K. N. J. Chem. Soc., Chem. Commun. 1978, 501-2. (25) Tufano, T. P.; Raymond, K. N. J. Am. Chem. Soc. 1981, 103, 6617-24. (26) Agrawal, Y. K. ReV. Anal. Chem. 1980, 5, 3-28. (27) Sreenivas, T.; Padmanabhan, N. P. H. Colloids Surf., A 2002, 205, 4759. (28) Barkatt, A.; Olszowka, S. A.; Gmurozyk, M. U.; Brewer, G. A. Removal of radioactive or heavy metal contaminants by means of non-persistent complexing agents. U.S. Patent 93-US11120 9411884, 19931117, 1994. (29) Liu, Z. D.; Hider, R. C. Coord. Chem. ReV. 2002, 232, 151-71. (30) Ghosh, M.; Miller, M. J. Bioorg. Med. Chem. 1996, 4, 43-8. (31) Jain, R.; Chen, D.; White, R. J.; Patel, D. V.; Yuan, Z. Curr. Med. Chem. 2005, 12, 1607-21. (32) Kouraklis, G.; Theocharis, S. Oncol. Rep. 2006, 15, 489-94. (33) Li, Q.; Da Silva, M. F. C. G.; Pombeiro, A. J. L. Chemistry 2004, 10, 1456-62. (34) Eisenlauer, J.; Matijevic, E. J. Colloid Interface Sci. 1980, 75, 199-211.
10.1021/la061365l CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006
10110 Langmuir, Vol. 22, No. 24, 2006
chemistry and applications of alkylhydroxamic acids in mineral flotation. These collectors form chelate complexes with the surface metal ions and give adsorption and mineral flotation responses peaking at pH 9. Their chemisorption at mineral/water interfaces has been confirmed by electrokinetic investigations and ex situ infrared (IR) spectroscopic studies. More recent surface chemical studies have mainly sought to determine the influence of adsorbed hydroxamic acid siderophores on the dissolution of minerals, particularly those containing iron. Holmen and Casey1 found that adsorbed acetohydroxamic acid, as an analogue siderophore, does not strongly influence the goethite dissolution rate. In a subsequent study, using attenuated total reflection infrared (ATR-IR) spectroscopy, Holmen et al.19 reported IR spectra of acetohydroxamic acid adsorbed on goethite in an aqueous suspension. They concluded that acetohydroxamic acid adsorbed to Fe(III) surface ions in a bidentate mode, with the spectrum of the adsorbed species remaining unchanged in the pH range 3-6, over which the solution complex is altered from the bis- to the tris(hydroxamato)iron(III) complex. Sposito and co-workers2-5 have carried out a series of studies of the influence of the adsorption of the trihydroxamate siderophore desferrioxamine B (DFO-B) on mineral dissolution rates. Kraemer et al.2 reported an enhanced rate of Fe release from goethite in the presence of DFO-B, concluding that coordination of the siderophore to an Fe(III) ion at the mineral surface is a precursor to dissolution. From the temperature dependence of goethite dissolution Cocozza et al.3 concluded that only a single hydroxamate group of DFO-B was bound to an Fe(III) center on the goethite surface. Cervini-Silva and Sposito4 found that the rate of dissolution of iron from aluminumsubstituted goethites increased with Al content and with DFO-B concentration and that a rate plateau at 10-4 mol L-1 corresponded to adsorption saturation prior to dissolution. Cheah et al.5 have studied the influence on goethite dissolution kinetics of the adsorption of both oxalate and DFO-B and found preferential oxalate adsorption despite the very large solution association constant of Fe3+ with DFO-B. Rosenberg and Maurice6 showed that the pH adsorption envelope of DFO-B on kaolinite reflected cation-like behavior, with adsorption increasing with negative surface charge. Edwards et al.36 have recently reported an experimental and theoretical vibrational spectroscopic study of acetohydroxamic acid and of DFO-B in aqueous solutions of differing pH. ATRIR and resonance Raman spectra were compared with those calculated from density functional theory at the B3LYP/6-31G* level of theory. The study showed that the cis-keto form of acetohydroxamic acid is dominant and that deprotonation occurs at the oxime group. It was also shown that the oxime and carbonyl groups of DFO-B could be unambiguously identified and that conformational changes between the anion in solution and complexed to Fe3+ are small. A recent in situ ATR-IR spectroscopic study of the adhesion of the bacterium Pseudomonas aeruginosa to titanium dioxide and iron oxide thin films showed that pyoverdin, a hydroxamic acid- and 6,7-dihydroxyquinoline-containing siderophore, forms covalent bonds with metal ions on these surfaces.37 As almost all metal surfaces interact with adhering or adsorbing species via a thin metal oxide film, adsorption behavior at metal surfaces corresponds to that measured at the surface of its metal oxide. (35) Kraemer, S. M.; Xu, J.; Raymond, K. N.; Sposito, G. EnViron. Sci. Technol. 2002, 36, 1287-91. (36) Edwards, D. C.; Nielsen, S. B.; Jarzecki, A. A.; Spiro, T. G.; Myneni, S. C. B. Geochim. Cosmochim. Acta 2005, 69, 3237-48. (37) McWhirter, M. J.; Bremer, P. J.; Lamont, I. L.; McQuillan, A. J. Langmuir 2003, 19, 3575-77.
Yang et al.
This study suggests that bacterial siderophores may play a significant role in the adhesion of bacteria to metal surfaces. However, in this investigation, the interaction between the 6,7dihydroxyquinoline (catechol-like) group in pyoverdin and the surface was clearly evident but that of the two hydroxamic acid ligands was not detected, which has prompted further investigation. An earlier ATR-IR spectroscopic study by McWhirter et al.38 showed that the initial attachment rate of P. aeruginosa to TiO2 films is higher at pH 10 than under neutral conditions. This pH-dependent adhesion may reflect the pH dependence of the siderophore adsorption to the metal oxide. If the role of siderophore adsorption to metals in bacterial adhesion is established, this knowledge may help to create the new strategies for biofilm control that are urgently required in many industrial and medical situations.39-41 The aim of the research described here is to clarify the IR spectral characteristics and adsorption behavior to metal oxides of monohydroxamic acids that are structurally similar to the hydroxamic acid groups in most naturally occurring siderophores. The work has also been carried out to provide a basis for our subsequent reports on the adsorption to metal oxides of siderophores containing multiple hydroxamic acid ligands. In the present work four hydroxamic acids were selected as model molecules: acetohydroxamic acid, N-methylformohydroxamic acid, N-methylacetohydroxamic acid, and 1-hydroxy-2-piperidone. The latter three hydroxamic acids are very similar to those found within the siderophores pyoverdine, rhodotorulic acid, and DFO-B.42 The adsorption of these hydroxamic acid ligands on titanium dioxide thin films was examined with ATR-IR spectroscopy, which can provide in situ information about adsorbed species.43 There have been few vibrational spectroscopic studies of hydroxamic acids adsorbed on wet mineral surfaces. Titanium dioxide was chosen as a model oxide for metal surfaces and oxide minerals as it forms stable particle films, has little tendency to form soluble complexes, exhibits little interference from carbonate ions in adsorption studies compared with other metal oxides, and has been used widely in ATR-IR spectroscopic studies of adsorption.43 The adsorption modes of the siderophore-related hydroxamic acids on titanium dioxide surfaces were investigated over a range of solution pH. Adsorption isotherm measurements were also carried out to determine binding constants for the ligand-surface interaction on the basis of the Langmuir adsorption model. Calculated IR spectra of the cis-formed neutral molecules and anions of the hydroxamic acids were obtained from geometry optimization and harmonic vibrational calculations performed using density functional theory (DFT) with the Spartan or Gaussian 03 programs. Vibrational modes were assigned by comparing observed and calculated IR spectra of the neutral molecules and their anions and using software-generated visualization of the calculated modes. Materials and Methods Materials. Acetohydroxamic acid (Aha) (Sigma, 98%) was used as obtained. N-Methylformohydroxamic acid (Fmha) was prepared (38) McWhirter, M. J.; McQuillan, A. J.; Bremer, P. J. Colloids Surf., B 2002, 26, 365-72. (39) Davey, M. E.; O’Toole G, A. Microbiol. Mol. Biol. ReV. 2000, 64, 84767. (40) Costerton, J. W.; Montanaro, L.; Arciola, C. R. Int. J. Artif. Organs 2005, 28, 1062-68. (41) Jass, J.; Walker, J. T. Ind. Biofouling 2000, 1-12. (42) Drechsel, H.; Winkelmann, G. Transition Met. Microb. Metab. 1997, 1-49. (43) McQuillan, A. J. AdV. Mater. 2001, 13, 1214.
Ha Ligands Adsorbed on Titanium Dioxide Scheme 1
from morpholine and N-methylhydroxylamine.44 N-methylacetohydroxamic acid (Maha) was prepared from N-methylhydroxylamine and acetyl chloride and purifed as reported by Harrison and Richards.45 1-Hydroxy-2-piperidone (Hp) was prepared from cyclopentanone (Merck-Schuchard, 99%) and N-hydroxybenzenesulfonamide (Fluka, 98%) and characterized as reported by Maio and Tardella.46 Scheme 1 illustrates the cis-keto structures of the four model hydroxamic acids with reported N-hydroxyl group dissociation constants at 25 °C.47,48 Titanium(IV) oxide sol (∼0.01 mol L-1) was prepared by hydrolysis of TiCl4 in water as previously reported.49 TiO2 thin films were prepared by placing 100 µL of TiO2 sol on a ZnSe ATRIR prism and drying it in a desiccator under a water pump vacuum for about 25 min. The TiO2 in these films is amorphous, with an isoelectric point of pH ≈ 5 and a high surface area, providing good sensitivity for IR spectroscopic studies.50 In all the experiments with TiO2 thin films a 0.01 mol L-1 aqueous NaOH solution flow was used to remove adsorbed contaminants before IR spectra were recorded. HCl (BDH, AR), NaOH (Merck, AR), and KCl (BDH, AR) were used as received. All solutions were prepared using deionized water (Millipore, Milli-Q, resistivity of 18 MΩ cm). Solution pH measurements were made with a Mettler-Toledo MP220 meter to a precision of (0.1. IR Spectroscopic Measurements. Infrared spectra were recorded using a Harrick FastIR ATR accessory with a 45° single-reflection ZnSe prism in a BioRad/Digilab FTS 60 spectrometer running with Win-IR 4.1 software or a Digilab FTS 4000 spectrometer running with Merlin 3.4 software, each fitted with DTGS detectors. All spectra were calculated from 64 scans at 4 cm-1 resolution and not corrected for the wavelength dependence of absorbance in ATR-IR compared with transmission spectra. Spectra were recorded once equilibrium had been established, typically after solutions were flowed over the film for 30-60 min. Infrared spectra of aqueous solutions at pH 6 and pH 12 were recorded with respect to that of water in contact with the bare ZnSe prism. Adsorption isotherms of these hydroxamic acids on TiO2 were determined by flowing a series of solutions of increasing concentration in a range from 5 × 10-6 to 1 × 10-3 mol L-1 at pH 3.0 and constant ionic strength (0.01 mol L-1 KCl) over the film. The Langmuir isotherm equation was fitted to the data using nonlinear regression analysis with Origin 6.0 software. The Langmuir isotherm equation takes the following form for data collected by internal reflection spectroscopy:51 θ)
A Kc ) Amax (1 + Kc)
where θ is the fractional surface coverage, c is the solution concentration, A is the absorbance of the adsorbed species, Amax is (44) Gate, E. N.; Threadgill, M. D.; Stevens, M. F. G.; Chubb, D.; Vickers, L. M.; Langdon, S. P.; Hickman, J. A.; Gescher, A. J. Med. Chem. 1986, 29, 1046-52. (45) Harrison, P. G.; Richards, J. A. J. Organomet. Chem. 1980, 185, 9-51. (46) Maio, D. G.; Tardella, P. A. Proc. Chem. Soc. 1963, 224. (47) ACD/Labs Calculated using Advanced Chemistry Development Software V8.14 for Solaris, 1994-2006. (48) Fazary, A. E. J. Chem. Eng. Data 2005, 50, 888-95. (49) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1995, 11, 4193-5. (50) Dobson, K. D.; Connor, P. A.; McQuillan, A. J. Langmuir 1997, 13, 2614-16. (51) Degenhardt, J.; McQuillan, A. J. Chem. Phys. Lett. 1999, 311, 179-84.
Langmuir, Vol. 22, No. 24, 2006 10111 the absorbance of the adsorbed species corresponding to θ ) 1, and K is the Langmuir adsorption constant. The pH dependence of adsorption on TiO2 of each hydroxamic acid was recorded by flowing over the film a series of hydroxamic acid solutions of decreasing pH but constant concentration. The pH of these solutions was adjusted using small volumes of 0.1 mol L-1 HCl and 0.1 mol L-1 NaOH solutions. The single-beam spectrum from a 0.01 mol L-1 NaOH solution over TiO2 was used as the background for spectra recorded at each pH. DFT-Calculated Spectra. To analyze the measured vibrational spectra of solution species, geometry optimization and IR spectra of the cis-formed neutral molecules and anions of these monohydroxamic acids were calculated with DFT theory in the Gaussian 94 package using B3LYP functionals and the 6-31+G(d) basis sets. To simulate the adsorption of these hydroxamic acids on TiO2, the model coordination complexes [Ti(Aha)(OH)4]-, [Ti(Fmha)(OH)4]-, [Ti(Maha)(OH)4]-, and [Ti(Hp)(OH)4]- were built up with GaussView 3.0. Geometry optimization and calculation of IR frequencies were carried out with Gaussian 03 at the B3LYP/6-31+G(d) level. The calculated IR spectra were used to interpret the IR spectra of hydroxamic acids adsorbed on TiO2. Similar calculations were performed on a model coordination complex, [Ti(Fmha)3]+, to compare with the frequency results of the model surface complex [Ti(Fmha)(OH)4]-. Vibrational modes were visualized by using GaussView 3.0 to assist mode assignments. A single scale factor of 0.9806 for the 6-31G(d) basis set, obtained from least-squares fitting (rms error of 50 cm-1) to experimental fundamental frequencies of 39 small molecules,52 was applied to all calculated harmonic frequencies. A Lorentzian curve with a half-width of 3 cm-1 was used in calculated spectra to represent each IR absorption with peak height proportional to peak absorbance. The spectra were obtained by transforming the calculated frequencies to Lorenzian curves with GRAMS 5.0 (Galactic Industries). The vibrational mode with the largest IR intensity was set to 100 on the arbitrary absorbance scale, and the spectra were displayed using Merlin 3.4 (Digilab).
Results and Discussion IR Spectra of Hydroxamic Acids in Aqueous Solutions. According to the dissociation constants (Scheme 1), the dominant solution species for each hydroxamic acid under neutral aqueous conditions is the undissociated acid while the hydroxamate anions predominate in strongly alkaline conditions. However, the dominant conformers of these hydroxamic acids and their anions in aqueous solutions were not well characterized because of the existence of their various conformers, among which the cis and trans forms of hydroxamic acids can coexist in the gas phase or in aqueous solutions. Figure 1 shows the IR spectra measured for the four selected hydroxamic acids in aqueous solutions at (a) pH 6 and (b) pH 12. Table 1 also lists the wavenumber of the observed peaks in the neutral and alkaline solutions and band assignments made with the assistance of GaussView visualization of the vibrational modes. All the spectra exhibited four characteristic infrared absorption regions: 1700-1600, 1500-1300, 1200-1000, and 1000-800 cm-1. The major absorption band at 1700-1600 cm-1 for all neutral molecules in Figure 1a is mainly contributed by the CdO stretching mode. A dramatic 14-34 cm-1 downshift of this vibrational mode occurred for all the hydroxamic acids upon the loss of the hydroxyl proton at high pH as shown in Figure 1b. Significant peak shifts with deprotonation (4-10 cm-1) are also present in the regions 1200-1000 and 1000-800 cm-1 such as for Aha (1092 f 1096 cm-1), Fmha (887 f 898 cm-1), and Hp (1096 f 1100 cm-1). The absorption loss evident at ∼1640 cm-1 in some spectra is due to overcompensation for the (52) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502-13.
10112 Langmuir, Vol. 22, No. 24, 2006
Figure 1. IR spectra of 0.1 mol L-1 hydroxamic acid aqueous solutions at (a) pH 6 and (b) pH 12. Spectra not on the same absorbance scale.
bending mode adsorption of solution water using pure water on the ZnSe prism for background spectra. Fitzpatrick and Margeswaran showed,53 with ab initio molecular orbital calculations, that the trans-keto forms of isolated formohydroxamic, acetohydroxamic, and N-methylacetohydroxamic acids have the lowest energy and that the cis-keto forms are less stable. However, on hydration, the cis-keto forms are preferred due to an increase in hydrogen bonding with water.53,54 In aqueous solution, it is generally believed19,21 that the cis-keto form of the acids with adjacent carbonyl and hydroxyl groups, as shown in Scheme 1, is the more stable conformation. However, the 1H NMR spectrum of N-methylacetohydroxamic acid55 indicated that the trans isomer shows higher stability in aqueous (53) Fitzpatrick, N. J.; Mageswaran, R. Polyhedron 1989, 8, 2255-63. (54) Kakkar, R.; Grover, R.; Chadha, P. Org. Biomol. Chem. 2003, 1, 2403. (55) Birus, M.; Gabricevic, M.; Kronja, O.; Klaic, B.; van Eldik, R.; Zahl, A. Inorg. Chem. 1999, 38, 4064-69.
Yang et al.
solution. DFT geometry optimization was also carried out for the isolated hydroxamic acids and the anions. Results are given in the Supporting Information for this paper. The results show that the bond strength of the carbonyl group of all the hydroxamate anions is weakened in comparison with that of the acid while the C-N and N-O bonds gain some double bond character, which would lead to a downshift of CdO stretching and an upshift of N-O stretching frequencies. The results predict that deprotonation of hydroxamic acids, which occurs on chelation of metal ions, leads to the N-O stretching mode band having a frequency upshift. Table 1 shows the DFT-calculated wavenumbers for IR spectral peaks of the isolated hydroxamic acids and their corresponding hydroxamate ions at the B3LYP/6-31+G(d) level. In comparing the calculated frequencies of isolated species with those observed for the aqueous solution species, modes involving the more polar parts of the molecules would be expected to occur at lower wavenumber due to interaction with the polar solvent. This influence is clearly observed for the CdO stretch mode, which has an observed wavenumber significantly lower than calculated. Figure 2 shows examples of the DFT-calculated IR spectra for 1-hydroxy-2-piperidone (Hp) and its anion. The prominent Cd O stretch band at 1663 cm-1 of the neutral ligand shifts down to 1622 cm-1 with deprotonation of the NOH group. Also, the calculated N-O stretching frequency of the Hp anion shifts from 1096 to 1100 cm-1 with deprotonation. On comparison of the observed spectra with the calculated vibrational modes displayed with GaussView, the bands observed at 1096 and 1100 cm-1 in Figure 1 for Hp and its anion, respectively, are associated with the N-O stretching mode. The assignment of the N-O stretching mode is important in this work because the coordination of hydroxamic acid ligands to metal ions invariably involves the N-O bonds. Frequency calculations were also carried out for the cis form of the neutral molecule and the gas-phase anions for the other investigated hydroxamic acids and are listed in Table 2. According to a similar analysis, IR bands at 1092 cm-1 for Aha and at 887 cm-1 for Fmha in Figure 1a can be assigned to the N-O stretching mode while the corresponding anion bands appear at 1096 and 898 cm-1, respectively. However, the N-O stretching mode assignments for Maha and its anion are an exception to this analysis, since both the experimental and calculated results show that the N-O stretching band of the anion is at a lower wavenumber. For Maha, the broad band at 974 cm-1 for the neutral molecule and the 950 cm-1 band for its anion correspond to the N-O stretching mode. It has been reported that in alkaline solutions the deprotonation of the N-hydroxyl group may allow both cis and trans forms of the monoanions of Aha, Fmha, and Maha to coexist in the absence of any hydrogen bonding between the keto and -NOH groups.21 Other frequency downshifts with deprotonation of the hydroxamic acids are also observed for Aha (993 f 988 cm-1), for Fmha (1197 f 1188 cm-1), and for Maha (1207 f 1193 cm-1). Holmen et al.19 have assigned the Aha band at 993 cm-1 to the N-O stretching mode from DFT HF/3-21G calculations. The present DFT calculations using B3LYP functionals and 6-31+G(d) basis sets provide more accurate frequency predictions and, with mode visualization through GaussView, do not support their assignment. Recent IR spectral measurements on aqueous Aha solutions and ab initio MO calculations36 suggest that cisO-deprotonated Aha is the stable form. The dominant forms of the Fmha and Maha anions in aqueous solution have not yet been established. However, hydroxamic acids invariably coordinate to metal ions via the cis-keto form as bidentate ligands, and it
Ha Ligands Adsorbed on Titanium Dioxide
Langmuir, Vol. 22, No. 24, 2006 10113
Table 1. Observed and Calculated Wavenumber (cm-1) for IR Spectral Peaks of Monohydroxamic Acids Aha obsd
calcd
pH 6
pH 12
1642 1544 1450
1613 1548 1447 1434 1381
1378
obsd
cis neutral
cis anion
1700 1537 1481 1463 1399 1382
1647 1516 1481 1455 1371
calcd
assignment
pH 6
pH 12
ν(CdO) δ(OH) + δ(NH) + ν(C-N) δ(NH) δ(CH3)
1317 1092
1318 1096
993
988
δ(OH) + δ(CH2) δ(OH) + δ(NH) + δ(CH3)
cis neutral
cis anion
assignment
1261 1075 1044 992 939
1271 1119 1018 946 931
δ(NH) + δ(CH3) ν(N-O) + δ(CH3) δ(CH3) δ(CH3) δ(CH3)
Fmha obsd pH 6
calcd cis neutral
pH 12
obsd cis anion
calcd
cis cis pH 6 pH 12 neutral anion
assignment
1665 1631 1686 1666 ν(CdO) + δ(CH) 1197 1577 1581 1553/1500 1505 δ(OH) + δ(CH3)/δ(CH) + δ(CH3) 1188 1460/1427 1476/1435 1459/1444 1443 δ(CH3) 1399 1419 1359 1421/1406 δ(CH3) + δ(CH) 1077 1371 1396 δ(OH) + δ(CH3) + δ(CH) 1371 1376 δ(CH3) + δ(CH) + ν(C-N) 887 898 1349 1353 δ(OH) + δ(CH)
1141 1071 944 881
assignment
898
δ(OH) + δ(CH) + ν(C-N) δ(CH3) + δ(CH) + ν(C-N) δ(CH3) δ(CH3) + δ(CH) + ν(C-N) δ(CH) (out of plane) ν(N-O)
cis anion
assignment
1204 1157 1124 1045
Maha obsd pH 6
calcd pH 12
cis neutral
obsd cis anion
assignment
calcd cis neutral
pH 6 pH 12
ν(CdO) + δ(OH) + δ(CH3) 1362 1358 δ(CH3) 1662 ν(CdO) + δ(CH3) 1264 1591 1631 1207 1212 δ(OH) + ν(C-N)a + δ(CH3) 1545 δ(OH) + ν(C-N) + δ(CH3) 1193 1177 ν(N-O) + δ(CH3) 1494/1468/1443 δ(OH) + δ(CH3) 1139 1156 1126 δ(CH3) + δ(OH)/δ(CH3) 1478 1482 1500/1488/1467 δ(CH3) 1135/1040 1121 δ(CH3) 1461 1451 δ(CH3) 974 1012 1019/993 δ(CH3) 1426 1425 δ(CH3) 958 964 δ(CH3) + νas(N-O + CdO) 1398/1365 1397/1367 δ(OH) + δ(CH3) 950 940 δ(CH3) + ν(N-O) 1394 1396 ν(C-N) + δ(CH3) 1604
Hp obsd pH 6
calcd pH 12
neutral
obsd anion
assignment
pH 6
pH 12
calcd neutral
anion
assignment
ν(CdO) + δ(OH) + 1272/1259 1256/1247 δ(CH2) δ(CH2) 1599 1622 ν(CdO) + δ(CH2) 1201 1514 1542 δ(OH) + ν(C-N) + 1182 1182 1192/1181 1185 δ(OH) + δ(CH2)/ δ(CH2) ν(C-N) + δ(CH2) 1473 1505/1490 1498/1484 δ(OH) + δ(CH2)/ 1167 1167 1164 1162 δ(OH) + δ(CH2)/ δ(CH2) δ(CH2) 1467 1465 1481 1476 δ(CH2) 1142 δ(CH2) + ν(C-N) + δ(CH2) 1452 1445 1462 1465 δ(OH) + δ(CH2)/ 1100 δ(CH2) 1419 1404 1412 δ(OH) + δ(CH2)/ 1096 1100 1096 1100 ν(N-O) + δ(CH2) ν(C-N) + δ(CH2) 1371 1357 δ(CH2) 1069 1070 1058 1082 δ(CH2) 1354/1333/1319 1355/1334/1318 1363/1344/1321 1348/1329/1316 δ(CH2) + ν(C-N) 978 978 958 1050/959 δ(CH2) 1272/1259 1273/1256 δ(OH) + δ(CH2) 928 925 918 910 δ(CH2) + νas(N-O + CdO) 1614
a
1663
C-N stretching mode related to N and the methyl group.
was anticipated in this study that calculated spectra for only the cis-keto form would be relevant to adsorption at metal oxide surfaces. IR Spectra of Hydroxamic Acid Species Adsorbed to TiO2. Figure 3 shows the IR spectra of Aha, Fmha, Maha, and Hp adsorbed on TiO2 particle films. From the coordination chemistry of hydroxamic acids,20,21 it is expected that the adsorbed species will be hydroxamate ions bound as bidentate ligands. Considering adsorbed acetohydroxamic acid, there are significant changes in the spectral band wavenumbers and relative band intensities of
adsorbed Aha (Figure 3) compared with those of Aha in solution (Figure 1). Such spectral changes with adsorption for relatively small ligands are consistent with the formation of coordinate covalent bonds and are not found when outer-sphere adsorption occurs. Examples of this contrasting spectral behavior are found in the coordinate adsorption of oxalate ion to TiO2, while only outer-sphere adsorption of perchlorate ion is observed on the same substrate.50 Further evidence for inner-sphere Aha adsorption comes from the very close resemblance between the IR spectrum of Aha adsorbed on TiO2 (Figure 3) and that of Aha
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Table 2. Observed IR Spectral Peaks of Monohydroxamate Ligands (Ha) Adsorbed on TiO2 and Calculated for [TiHa(OH)4]obsd
calcda
1611 1539 1442 1391 1342
1633 1536 1479/1461 1393
1639 1470
1665 1512 1452 1437 1429 1358
1433 1402 1339 1608 1480 1433 1373 1611 1481 1450 1424/1356 1334 1317 a
1623 1516 1481 1476/1465 1457 1424 1379 1626 1510 1493 1483 1479/1460/1365 1363 1345 1330
assignment Aha ν(CdO) + ν(C-N) + δ(CH3) δ(NH) + ν(C-N) + δ(CH3) δ(CH3) + δ(NH) + δ(CH3) δ(NH) + δ(CH3) Fmha ν(CdO) + ν(C-N) + δ(CH3) δ(CH3) δ(CH3) δ(CH3) + ν(C-N) δ(CH3) δ(CH) Maha ν(CdO) + δ(CH3) δ(CH3) + ν(CdO) δ(CH3) ν(C-C) + δ(CH3) δ(CH3) δ(CH3) δ(CH3) Hp ν(CdO) + δ(CH2) δ(CH2) + ν(CdO) δ(CH2) δ(CH2) + ν(C-N) δ(CH2) δ(CH2) + ν(C-C) δ(CH2) + ν(C-N) δ(CH2) + ν(CdO)
obsd
calcda
assignment
1100 1040 1003
1296 1124 1039 994
δ(NH) + ν(C-N) + δ(CH3) ν(N-O) + δ(CH3) δ(CH3) δ(CH3) + νas(N-O + CdO)
1210 1133 1076 924 886
δ(CH) + ν(C-N)b δ(CH3) δ(CH3) ν(N-O) δ(CH) (out of plane)
1229 1174 1127 1035 1029 976
ν(C-N) δ(CH3) + ν(C-C) δ(CH3) δ(CH3) δ(CH3) + ν(N-C) + ν(N-O) δ(CH3) + νas(N-O + CdO)
1268/1254 1224 1178/1167 1122 1095 1056 926
δ(CH2) ν(C-N) + δ(CH2) δ(CH2) ν(N-O) + δ(CH2) δ(CH2) δ(CH2) + ν(C-C) δ(CH2) + νas(N-O + CdO)
1203 1089 927
1174 1037 979
1271 1184/1172 1110 1070 931
Calculated harmonic freqencies with the B3LYP/6-31+G(d) method. b C-N stretching mode related to N and the methyl group.
Figure 2. DFT-calculated IR spectra at the B3LYP/6-31+G(d) level of 1-hydroxy-2-piperidone: (a) neutral species, (b) anion.
in the Fe(Aha)3 complex reported by Edwards et al.36 This confirms the bidentate nature of the Aha adsorption to TiO2, and the same conclusion can similarly be reached about the adsorption to TiO2 of the other hydroxamic acid ligands. To interpret the IR spectra of hydroxamic acids adsorbed on a TiO2 surface, the model complex [Ti(Ha)(OH)4]- shown in Scheme 2 was considered. Geometry optimization and IR spectral
Figure 3. IR spectra of hydroxamic acid species Aha, Fmha, Maha, and Hp adsorbed on TiO2 from 1 × 10-4 mol L-1 aqueous solution. Spectra not on the same absorbance scale.
calculations for the model surface complexes [Ti(Aha)(OH)4]-, [Ti(Fmha)(OH)4]-, [Ti(Maha)(OH)4]-, and [Ti(Hp)(OH)4]- were carried out with Gaussian 03 at the B3LYP/6-31+G(d) level. Comparison of the observed spectra of the hydroxamic acids (Figure 1a), the hydroxamates (Figure 1b), and the corresponding
Ha Ligands Adsorbed on Titanium Dioxide
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Scheme 2. TiO2 Surface/Hydroxamate Adsorption Model Where R and R′ Are Hydrogen Atoms or Methyl Groups
adsorbed species (Figure 3) leads to some general observations. For Aha, as already considered, the adsorbed species spectrum is noticeably different in the 1700 and 1400 cm-1 region from those of its solution species. Using GaussView, it was seen that the peak at 1539 cm-1 is mainly due to the N-H bending mode, which is only present in Aha.19 Compared with the spectra of the other three hydroxamic acids, the two bands for Aha at 1100 and 1003 cm-1 are relatively intense. In the 1700-1400 cm-1 region, the spectra of Fmha, Maha, and Hp are quite similar apart from the lack of a strong peak in the Fmha spectrum at about 1480 cm-1. This suggests that the bands around 1480 cm-1 arise from methyl or methylene group vibrations. The minor absorption loss around 1640 cm-1 in some of the spectra is due to displacement of water molecules from the TiO2 surface when adsorption occurs. For Aha adsorbed on geothite Holmen et al.19 observed an IR spectrum similar to that of Aha adsorbed on TiO2 shown in Figure 3. Using data from the spectrum of an Fe(III)-Aha solution mixture at pH 2 and on the basis of Gaussian 92 calculations at the HF/3-21G level, they assigned the 1100 and 1003 cm-1 bands to the CH3 bend and N-O stretch, respectively. However, a more recent DFT analysis by Edwards et al.36 of the vibrational spectra of Fe(III)-Aha complexes with Gaussian 92 at the B3LYP/6-31+G(d) level assigned the 1100 cm-1 band to the N-O stretch mode. Our frequency calculations for the [Ti(Aha)(OH)4]- model complex also support the assignment of the 1100 cm-1 band of Aha adsorbed on TiO2 to the N-O stretching mode, while the band at 1003 cm-1 is due to a vibrational mode of the chelating ring involving the out-of-phase stretching of N-O and CdO bonds, as shown in Scheme 3. Fmha adsorbed on TiO2 has characteristic IR bands at 1639, 1470, 1433, 1402, 1339, 1203, 1089, and 927 cm-1, which show a significant 30 cm-1 downshift of the CdO stretch mode and a 40 cm-1 upshift of the N-O stretch mode compared with those of the IR spectrum of neutral Fmha in solution. This indicates that surface complex formation on TiO2 leads to strengthening of the N-O bond and weakening of the CdO bond. A new band at 1339 cm-1 appears to arise from formation of the surface complex. To test this hypothesis, we calculated the IR spectra of 1:1 and 1:3 Ti(IV)-Fmha complexes with the models [Ti(Fmha-)(OH)4]- and [Ti(Fmha-)3]+. Figure 4 shows the spectra of Fmha (a) adsorbed on TiO2 and (b) calculated for [Ti(Fmha-)3]+. Results of these calculations support the assignment of the 1339 cm-1 band as being mainly due to a combination of C-H bending and CdO stretching modes (see Table 3 in the Supporting Information). This band is at higher frequency than for the solution species as shown in Figure 1. Due to the chelating effect, the CdO bond is weakened, which results in a downshift not only of the main CdO stretch wavenumber but also of the C-H bending mode. The chelating effect also strengthens the C-N bond between the nitrogen atom and the methyl group and brings an upshift of the C-N stretch mode wavenumber to 1203 cm-1, which is a higher wavenumber
Figure 4. IR spectra (a) calculated for [Ti(IV)(Fmha)(OH)4]-, (b) of Fmha adsorbed on TiO2, and (c) calculated for [Ti(IV)(Fmha)3]+. Scheme 3. Calculated Vibrational Modes and Frequencies of [Ti(Aha)(OH)4]-
than that of the Fmha solution species shown in Figure 1 at 1197 cm-1 and at 1188 cm-1 for the neutral and anionic species, respectively. Maha adsorbed on TiO2 gives IR bands at 1608, 1480, 1433, 1373, 1230, 1174, 1037, and 979 cm-1, similar to those in the IR spectra of the Maha complexes with Cu(II), Ni(II), and Fe(III).56 Thus, a Ti(IV)-Maha surface complex also forms on TiO2 surfaces. The GaussView representation of vibrational modes for [Ti(Maha)(OH)4]- indicates that the 979 cm-1 band can be assigned as the N-O stretch. Hp adsorbed on TiO2 gives IR bands at 1611, 1481, 1450, 1424, 1356, 1334, 1317, 1271, 1184, 1172, 1110, 1070, and 931 (56) Brown, D. A.; McKeith, D.; Glass, W. K. Inorg. Chim. Acta 1979, 35, 57-60.
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Figure 5. Adsorption isotherm of Fmha adsorbed on TiO2: (a) data points, (b) Langmuir fitted curve.
cm-1. The spectrum is similar to the IR spectrum of Maha adsorbed on TiO2 in the 1610-1400 cm-1 region but quite different at lower wavenumber. The N-O stretch of the adsorbed species is assigned at 1110 cm-1. The shift of the CdO stretch upon adsorption was not significant compared to that of Aha and Fmha adsorbed on TiO2. The three peaks at 1355, 1334, and 1317 cm-1 are associated with CH2 deformations on the basis of GaussView visualization. Due to the chelating effect, the intensity of the 1355 cm-1 band is increased compared to that of the solution species. The medium-intensity band at 931 cm-1 is associated with an out-of-phase stretch mode of the N-O and CdO bonds from GaussVew and from analysis of the frequency calculations for the [Ti(Hp)(OH)4]- model complex. This band is at a 5 cm-1 higher wavenumber than that of the solution species. Therefore, analysis of calculated spectra of model Ti(IV) compounds containing the hydroxamate ligands supports the conclusion that bidentate surface complexes are formed on TiO2. Adsorption Isotherms. To further investigate the nature of binding of these hydroxamic acids on TiO2, adsorption isotherms and the pH dependence of the IR spectra of these adsorbed molecules were determined. The relative binding strengths of these ligands to titanium dioxide surfaces can be compared in the binding constants obtained from adsorption isotherms.51,57,58 Here, the simple Langmuir model was used to analyze the adsorption behavior of these ligands on TiO2 surfaces. While adsorption of anionic ligands to TiO2 surfaces is not expected to conform closely to this model, the Langmuir isotherm has been widely used to provide comparative data. The absorbances of characteristic N-O stretch bands of Aha, Fmha, Maha, and Hp adsorbed on TiO2 surfaces at 1102, 930, 979, and 1111 cm-1 respectively, were selected to construct adsorption isotherms. Normally the strongest isolated band is chosen to construct adsorption isotherms. For hydroxamic acids, the CdO stretch band is generally the strongest, but its absorbance is not reliable due to the influence of the water bending mode absorption in the same wavenumber region. Figure 5 shows the adsorption isotherm of Fmha as an example with data points and the Langmuir fitted curve. Langmuir adsorption constants from the nonlinear least-squares fitting of the absorbance vs concentration data points were K(Aha) ) (9 ( 2) × 104 L mol-1, K(Fmha) ) (3 ( 1) × 104 L mol-1, K(Maha) ) (8 ( 2) × 104 (57) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451-55. (58) Roddick-Lanzilotta, A. D.; Connor, P. A.; McQuillan, A. J. Langmuir 1998, 14, 6479-84.
Figure 6. pH dependence of the IR spectra of Hp adsorbed on TiO2 from 1 × 10-4 mol L-1 aqueous solution.
L mol-1, and K(Hp) ) (10 ( 4) × 104 L mol-1. The measured Langmuir adsorption constants have fairly similar values around 105 L mol-1, indicating comparable strengths of surface binding, with that of Fmha being slightly weaker. A Langmuir adsorption constant of 105 L mol-1 corresponds to a fractional surface coverage of about 0.9 for a 10-4 mol L-1 ligand solution concentration. Examples of reported Langmuir adsorption constants from the same TiO2 substrate are 3 × 103 L mol-1 for lysine,58 which adsorbs weakly through outer-sphere interactions, and 1.4 × 106 L mol-1 for the photosensitizer57 [Ru(NCS)2(bipyridyl dicarboxylic acid)2]2+, which adsorbs strongly via two carboxylic acid groups. Thus, the adsorptive binding of the hydroxamic acids to the TiO2 surface is relatively strong, which provides further support for a bidentate interaction. pH Dependence of the IR Spectra of Adsorbed Species on TiO2. The pH dependence of IR spectra and any changes of the species over the pH range can reveal the nature of the adsorbed species. Degenhart et al.51 investigated the pH dependence of the IR spectra of oxalate adsorbed on chromium oxide-hydroxide in aqueous solution and analyzed the data in terms of two types of adsorption modes: an oxalate species coordinated to surface Cr(III) ions and another weakly bound oxalate due to hydrogen bonding which exhibited different IR spectral features. The present study investigated adsorption of the selected hydroxamic acids between pH 3 and pH 11. Figure 6 gives the pH dependence of the IR spectra of Hp adsorbed on TiO2 as an example. Generally, for an outer-sphere interation the amount of adsorption of an anion will decrease dramatically at a pH higher than the isoelectric point of the metal oxide surface if electrostatic interaction predominates. However, the adsorption of the studied hydroxamic acids is evident at pH 10-11, which is above the TiO2 isoelectric point (pH ≈ 5).50 This confirms that these hydroxamic acids are coordinatively bound on TiO2. The ligands also show greater adsorption when pH < pKa. Maximum adsorption was approached for Aha at pH ≈ 6, for Fmha at pH 3, for Maha at pH 7, and for Hp at pH ≈ 6. In the adsorbate spectra over the complete pH range studied, no new bands were observed for all the adsorbed
Ha Ligands Adsorbed on Titanium Dioxide
hydroxamic acids, indicating that in each case there is one unchanged and stable surface complex.
Conclusions The characteristic vibrational modes of four siderophore-related hydroxamic acids and their anions, especially the CdO stretch and N-O stretch modes, were assigned by comparing their IR spectra observed in aqueous solution and DFT-calculated IR spectra. Upon deprotonation, the wavenumber of the N-O stretch modes of all hydroxamate ions upshifted while that of the CdO stretch modes downshifted. This evidence is important for identification of the relevant modes of hydroxamic acid species adsorbed on TiO2. ATR-IR spectra of these four hydroxamic acids adsorbed on TiO2 thin films were also recorded. These are the first reported in situ high-quality measurements of IR spectra of siderophorerelated hydroxamic acids adsorbed on a wet metal oxide surface. A significant upshift of the N-O stretch mode was observed upon adsorption for all the hydroxamic acids and a corresponding downshift of the CdO stretch mode for Aha and Fmha. The coherence between the DFT-calculated IR spectra based on a [Ti(HA)(OH)4]- model and the observed IR spectra of the four hydroxamic acids adsorbed on TiO2 surfaces supports adsorption of all the hydroxamic acids as hydroxamate ions and as bidentate ligands. Measurement of Langmuir adsorption constants and the
Langmuir, Vol. 22, No. 24, 2006 10117
pH dependence of adsorption of these hydroxamic acids on TiO2 confirmed that adsorption occurs as an inner-sphere bidentate interaction. These results all suggest that hydroxamic acids in siderophores could be expected to form covalent bonds in coordinative adsorption to metal (oxide) surfaces in general during bacterial adhesion processes. Characterization of the IR spectrum of adsorbed hydroxamates and the pH dependence of adsorption provide a firmer basis for understanding the more complex behavior of hydroxamate-containing siderophore adsorption on metal oxides and environmental minerals. Acknowledgment. We acknowledge support from the Marsden Fund and the Department of Chemistry of the University of Otago. We thank Associate Professor Henrik G. Kjaegaard and Dr. Daniel Schofield for advice in the geometry optimization and frequency calculations. We also thank Associate Professor Lyall Hanton, Penny Walsh, and John Earles for help with the graphics software. Supporting Information Available: Relative energies and bond lengths of hydroxamic acids and their anions predicted by density functional theory at the B3LYP/6-31+G(d) level. This material is available free of charge via the Internet at http://pubs.acs.org. LA061365L
