pubs.acs.org/Langmuir © 2009 American Chemical Society
Encapsulation of Ruthenium Nitrosylnitrate and DNA Purines in Nanostructured Sol-Gel Silica Matrices Luı´ s M. F. Lopes,† Ana R. Garcia,†,‡ Alexandra Fidalgo,† and Laura M. Ilharco*,† †
Centro de Quı´mica-Fı´sica Molecular (CQFM) and Institute of Nanoscience and Nanotechnology (IN), Instituto Superior T ecnico, Universidade T ecnica de Lisboa, Avenida Rovisco Pais 1, 1049-001 Lisboa, Portugal, and ‡Departamento de Quı´mica, Bioquı´mica e Farm acia, FCT, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal Received March 25, 2009. Revised Manuscript Received May 8, 2009 The interactions between DNA purines (guanine and adenine) and the ruthenium complex Ru(NO)(NO3)3 were studied within nanostructured silica matrices prepared by a two-step sol-gel process. By infrared analysis in diffuse reflectance mode, it was proved that encapsulation induces a profound modification on the complex, whereas guanine and adenine preserve their structural integrity. The complex undergoes nitrate ligand exchange and co-condenses with the silica oligomers, but the nitrosyl groups remain stable, which is an unusual behavior in Ru nitrosyl complexes. In turn, the doping molecules affect the sol-gel reactions and eventually the silica structure as it forms: the complex yields a microporous structure, and the purine bases are responsible for the creation of macropores due to hydrogen bonding with the silanol groups of the matrix. In a confined environment, the interactions are much stronger for the coencapsulated pair guanine complex. While adenine only establishes hydrogen bonds or van der Waals interactions with the complex, guanine bonds covalently to Ru by one N atom of the imidazole ring, which becomes strongly perturbed, resulting in a deformation of the complex geometry.
1. Introduction The interaction of nucleic acid bases with metal atoms is of particular interest, since the activity of metal-based antitumoral agents appears to be related to their binding with DNA.1-3 Ruthenium complexes are among the most promising antitumor drugs currently being used or under clinical trials.4,5 Different interaction mechanisms between DNA base models and Ru(II) and Ru(III) complexes have been proposed, based on theoretical approaches,6 and experimental evidence has been obtained on the preferred binding targets of ruthenium complexes in homogeneous medium.7-9 A clear understanding of those interactions may be achieved by promoting complex-DNA base encounters in a confined medium, such as a nanostructured matrix. Silica xerogels are good candidates, because they are synthesized by a soft and versatile chemical process (sol-gel), are non toxic, and are biocompatible. In the present work, the model Ru complex Ru(NO)(NO3)3 and the two purine bases (guanine (Gua) and adenine (Ade)) were *Corresponding author. Address: Centro de Quı´ mica-Fı´ sica Molecular and Institute of Nanoscience and Nanotechnology, Complexo I, I.S.T., Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal,.Tel: +351-218419220. Fax: +351-218464455. E-mail:
[email protected]. (1) Keene, F. R.; Smith, J. A.; Collins, J. G. Coord. Chem. Rev., doi:10.1016/ j.ccr.2009.01.004. (2) Zhang, C. X.; Lippard, S. J. Curr. Opin. Chem. Biol. 2003, 7, 481–489. (3) Novakova, O.; Nazarov, A. A.; Hartinger, C. G.; Keppler, B. K.; Brabec, V. Biochem. Pharmacol. 2009, 77, 364–374. (4) Velders, A. H.; Bergamo, A.; Alessio, E.; Zangrando, E.; Haasnoot, J. G.; Casarsa, C.; Cocchietto, M.; Zorzet, S.; Sava, G. J. Med. Chem. 2004, 47, 1110– 1121. (5) Jakupec, M. A.; Reisner, E.; Eichinger, A.; Pongratz, M.; Arion, V. B.; Galanski, M.; Hartinger, C. G.; Keppler, B. K. J. Med. Chem. 2005, 48, 2831–2837. (6) Chen, J.-C.; Chen, L.-M.; Xu, L.-C.; Zheng, K.-C.; Ji, L.-N. J. Phys. Chem. B 2008, 112, 9966–9974. (7) Clarke, M. J. Coord. Chem. Rev. 2003, 236, 209–233. (8) Cebrian-Losantos, B.; Reisner, E.; Kowol, C. R.; Roller, A.; Shova, S.; Arion, V. B.; Keppler, B. K. Inorg. Chem. 2008, 47, 6513–6523. (9) Chen, H.; Parkinson, J. A.; Parsons, S.; Coxall, R. A.; Gould, R. O.; Sadler, P. J. J. Am. Chem. Soc. 2002, 124, 3064–3082.
Langmuir 2009, 25(17), 10243–10250
encapsulated and coencapsulated in a nanoporous sol-gel silica matrix, and their structural modifications induced by the medium and/or by each other were characterized. Infrared spectroscopy appears as a prevailing tool for this purpose. Thus, a reference to the structures and vibrational spectra of the three molecules seems pertinent. Both purine bases (Scheme 1) are heterocyclic molecules containing labile hydrogen atoms, which are responsible for their tautomerism and reactivity.10 The tautomeric distribution has deserved considerable attention, because it is thought to play an important role in stimulating spontaneous mutation in DNA.11 For Gua in low temperature argon matrix, it was proved by infrared analysis that equal proportions of amino-oxo (keto) and amino-hydroxy (enol) tautomers are present.12 In the gas phase, theoretical methods anticipate the higher stability of the ketoN7H tautomer, followed by keto-N9H and two rotamers of the enol-N9H.13 In aqueous solution, and generally in polar solvents, the keto-N9H form predominates, given its higher dipole moment. The keto form is also preferred in crystalline Gua and after incorporation into DNA.14 In the absence of water or any other solvent, solid Gua exists as the keto-N7H tautomer,15 whereas in monohydrate crystals the keto-N9H prevails.16 In both cases, the crystal structure is monoclinic, assured by N-H 3 3 3 N and N-H 3 3 3 O hydrogen bonds between Gua molecules (that may amount to eight), forming sheets that interact by π-π stacking.16,17 Taking into account the Gua tautomeric equilibria, its (10) Bodor, N.; Dewar, M. J. S.; Harget, A. J. J. Am. Chem. Soc. 1970, 92, 2929– 2936. (11) Topal, M. D.; Fresco, J. R. Nature 1976, 263, 285–289. (12) Szczepaniak, K.; Szczesniak, M. J. Mol. Struct. 1987, 156, 29–42. (13) Shukla, M. K.; Leszczynski, J. Chem. Phys. Lett. 2006, 429, 261–265. (14) Sanger, W. Principles of Nucleic Acid Structure; Springer: Berlin, 1984. (15) Guille, K.; Clegg, W. Acta Crystallogr. 2006, C62, o515–o517. (16) Ortmann, F.; Hannewald, K.; Bechstedt, F. J. Phys. Chem. B 2008, 112, 1540–1548. (17) Levy-Lior, A.; Pokroy, B.; Levavi-Sivan, B.; Leiserowitz, L.; Weiner, S.; Addadi, L. Cryst. Growth Des. 2008, 8, 507–511.
Published on Web 06/05/2009
DOI: 10.1021/la901046f
10243
Article Scheme 1. Structures of the Keto-N9H Tautomer of Gua (Left) and of the N9H Tautomer of Ade
vibrational spectra have been predicted theoretically,18 and studied in argon matrix,12 in solution19 and in polycrystalline state,20,21 with satisfactory agreement between theoretical and experimental results when models allowed for the hydrogen bonds.20 The spectrum of keto tautomers is dominated by the bands associated with the carbonyl stretching and the amine deformation modes, whose frequencies and relative intensities depend on the physical state of Gua and on the specific interactions with the environment.20-22 Ade may have eight tautomeric structures. Semiempirical calculations have shown that the imino tautomers, called the “rare” tautomeric forms of Ade, are more energetic than the amino ones,23 but they may be stabilized by metal binding.24 The more important equilibrium is established between the amino N9H (with the lowest energy) and N7H forms. For noninteracting Ade monomers, theoretical calculations predicted and the infrared spectra in argon matrix proved a large predominance of the N9H tautomer.25 In solution, the tautomeric stabilization is solvent dependent; thus, in polar solvents the equilibrium is shifted toward the more polar N7H form, although in aqueous solution the N9H tautomer still predominates.26 In crystalline Ade, a network of intermolecular hydrogen bonds (mostly NH2 3 3 3 N1, NH2 3 3 3 N7, and N9H 3 3 3 N3) is responsible for the three-dimensional organization.14 The vibrational spectrum of Ade has also been simulated by ab initio calculations, and the results were compared to experimental infrared spectra in low temperature matrices.27 In order to overcome assignment ambiguities raised by overlapping of NH2 and NH stretching modes and matrix-induced splitting, isotopic substitution and complexation with methylmercury have been useful.28,29 Resembling other four-coordinate ruthenium complexes,30,31 the geometry around the Ru center in Ru(NO)(NO3)3 is pseudotetrahedral (Scheme 2). Given that the nitrosyl ligand is a σ-donor and a strong π-acceptor, the Ru-NO bond length is very short.32 (18) Setnicka, V.; Novy, J.; Bohm, S.; Sreenivasachary, N.; Urbanova, M.; Volka, K. Langmuir 2008, 24, 7520–7527. (19) Lord, R. C.; Thomas, G. J. Spectrochim. Acta A 1967, 23, 2551–2591. (20) Majoube, M. J. Mol. Struct. 1984, 114, 403–406. (21) Santamaria, R.; Charro, E.; Zacarı´ as, A.; Castro, M. J. Comput. Chem. 1999, 20, 511–530. (22) Ten, G. N.; Bourova, T. G.; Baranov, V. I. J. Struct. Chem. 2005, 46, 998– 1005. (23) Norinder, U. J. Mol. Struct. (THEOCHEM) 1987, 151, 259–269. (24) Wu, Y.; Sa, R.; Li, Q.; Wei, Y.; Wu, K. Chem. Phys. Lett. 2009, 467, 387– 392. (25) Nowak, M. J.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochem. Acta A 1991, 47, 87–103. (26) Cohen, B.; Hare, P. M.; Kohler, B. J. Am. Chem. Soc. 2003, 125, 13594– 13601. (27) Nowak, M. J.; Rostrowska, H.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochem. Acta A 1994, 50, 1081–1094. (28) Nowak, M. J.; Lapinski, L.; Kwiatkowski, J. S.; Leszczynski, J. J. Phys. Chem. 1996, 100, 3527–3534. (29) Savoie, R.; Jutier, J.-J.; Prizant, L.; Beauchamp, A. L. Spectrochim. Acta A 1982, 38, 561–568. (30) Huang, H.; Hughes, R. P.; Rheingold, A. L. Polyhedron 2008, 27, 734–738. (31) Liang, B.; Wang, X.; Andrews, L. J. Phys. Chem. A 2009, 113, 5375–5384. (32) Caramori, G. F.; Frenking, G. Organometallics 2007, 26, 5815–5825.
10244 DOI: 10.1021/la901046f
Lopes et al. Scheme 2. Ball and Stick Model of Ru(NO)(NO3)3, Optimized by the Gaussian Module of Chem 3D Ultra 10.0
The infrared spectra of ruthenium-nitrosyl complexes are characterized by two stretching modes, a very strong (Ru)N-O and a medium-intensity Ru-N(O), whose frequencies depend on whether the coordination is linear or bent.33 The nitrate ligands are monodentade, with the N-O(Ru) bond order close to 1 and the π-electronic density delocalized only on the two (O)N-O bonds, resulting in a bond order of 1.5.34 For the free nitrate ion (NO3-), the N-O bond order is of ∼1.33, and the symmetry is D3h. The symmetric N-O stretching mode is forbidden, but has been reported to appear as a weak band at 1059 cm-1, as a result of ion deformation.35 The corresponding degenerate antisymmetric modes are expected to be a strong band in the range of 1350-1390 cm-1. The infrared spectra of monodentate nitrate complexes have been interpreted in terms of C2v symmetry. Accordingly, all the vibrational modes become infrared active.36 The stabilities of the nitrosyl and nitrate ligands are different, and dependent on the medium. In the present work, it was proven that, for this particular complex, encapsulation in sol-gel silica matrices results in substitution of the nitrate ligand rather than the nitrosyl. The two purine bases interact very differently with the coencapsulated complex: while Ade establishes only hydrogen bonds with the matrix and the complex, Gua binds covalently to the Ru atom, confirming that it is the most reactive DNA site.37 The changes on the matrix induced by encapsulation were also studied.
2. Experimental Section 2.1. Materials. Ruthenium(III) nitrosylnitrate, Ru(NO) (NO3)3 3 xH2O, (Ru 31.96% min) was supplied by Alfa Aesar, and tetraethoxysilane (TEOS, 98%), Ade (99%), and Gua (99.9%) by Sigma Aldrich. The catalysts used in the hydrolysis and condensation steps were hydrochloric acid (HCl) from Riedel-de-Haen and ammonium hydroxide (NH4OH) from Merck, respectively. 2-Propanol (99.8%) was obtained from Fluka. All chemicals were used without further purification. Distilled water was used in the preparation of aqueous solutions. 2.2. Preparation of the Sol-Gel Monoliths. All the monoliths were synthesized by a two-step sol-gel process, aged, and dried, following an adaptation of an experimental protocol previously reported.38 First, the appropriate amount of Ru(NO) (NO3)3 3 xH2O was weighed and dissolved in HCl 0.08 M (6.50 mL; 0.52 mmol) under stirring, until a homogeneous solution was obtained. Then, 2-propanol (6.30 mL; 82.4 mmol), the cosolvent, was added, and the mixture was vigorously stirred. The silica (33) Miessler, G. L.; Tarr, D. A. Inorg. Chem., 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1999. (34) Oberhammer, H. J. Mol. Struct. 2002, 605, 177–185. (35) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; Wiley: New York, 1997; Part A, p 182. (36) Gatehouse, B. M.; Livingstone, S. E.; Nyholm, R. S. J. Inorg. Nucl. Chem. 1958, 8, 75–78. (37) Li, X.; Cai, Z.; Sevilla, M. D. J. Phys. Chem. A 2002, 106, 1596–1603. (38) Fidalgo, A.; Rosa, M. E.; Ilharco, L. M. Chem. Mater. 2003, 15, 2186–2192.
Langmuir 2009, 25(17), 10243–10250
Lopes et al.
Article Table 1. Relevant Molar Ratios and Identification of the Samples samplea
molar ratio
control SiO2
H2O:TEOS 40 HCl:TEOS 0.058 1.00 NH4OH:HCl Gua:TEOS RuNO(NO3)3:TEOS Ade:TEOS a Gua = guanine; Ade = adenine.
Gua/ SiO2
Ade/ SiO2
RuNO(NO3)3/ SiO2
Gua/RuNO(NO3)3/ SiO2
Ade/RuNO(NO3)3/ SiO2
40 0.058 1.00 0.012
40 0.058 1.00
40 0.058 1.00
40 0.058 1.00 0.012 0.012
40 0.058 1.00
0.012
0.013 0.014
0.016
Table 2. Gelation Time (tG), Envelope Density (Ge), Estimated Total Pore Volume (Vp), and Porosity (Pr) of the Control Silica Matrix, and of the Matrices Doped with Gua, Ade, RuNO(NO3)3 and Purine-Complex Pairs control SiO2
Gua/SiO2
Ade/SiO2
RuNO(NO3)3 /SiO2
Gua/RuNO(NO3)3/ SiO2
Ade/RuNO(NO3)3/ SiO2
tG 7 min 17 min 15 min ∼20 h
