Héctor J. Lozano

Sub‐nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X‐ray Absorption spectroscopy and Near Ambient Pressure X‐ray Photoelectron spectroscopy results reveal that supported Cu₅ clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion‐corrected DFT and first principles thermochemistry revealing that most of the adsorbed O₂ molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude “breathing” motions. A chemical phase diagram for Cu oxidation states of the Cu₅‐oxygen system is presented, clearly different from the already known bulk and nano‐structured chemistry of Cu.

Subnanometer-sized metal clusters often feature a molecule-like electronic structure, which makes their physical and chemical properties significantly different from those of nanoparticles and bulk material. Considering potential applications, there is a major concern about their thermal stability and susceptibility towards oxidation. Cu clusters of only 5 atoms (Cu5 clusters) are first synthesized in high concentration using a new-generation wet chemical method. Next, it is shown that, contrary to what is currently assumed, Cu5 clusters display nobility, beyond resistance to irreversible oxidation, at a broad range of temperatures and oxygen pressures. The outstanding nobility arises from an unusual reversible oxidation which is observed by in situ X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy on Cu5 clusters deposited onto highly oriented pyrolitic graphite at different oxygen pressures and up to 773 K. This atypical property is explained by a theoretical approach combining different state-of-the-art first principles theories. It reveals the essential role of collective quantum effects in the physical mechanism responsible for the nobility of Cu5 clusters, encompassing a structural ‘breathing’ through concerted Cu–Cu elongations/contractions upon O2 uptake/release, and collective charge transfer as well. A predictive phase diagram of their reversible oxidation states is also delivered, agreeing with the experimental observations. The collective quantum effects responsible of the observed nobility are expected to be general in subnanometer-sized metal clusters, pushing this new generation of materials to an upper level.

The interaction of RNA and DNA with two aluminium-containing complexes has been studied at 25 °C using ¹H-NMR, UV and CD spectroscopies, viscosity measurements and DFT simulations. The complex identified at pH = 7, Al₁₃Cac_x, can interact with polynucleotides by external binding. Even though the affinity of the aluminium complex for RNA and DNA is similar, the conformational changes induced upon interaction are greater with RNA. In contrast, at pH = 5 the complex present in solution is Al₂Cac₂, where the aluminium atoms have different configurations, such as octahedral (OH) and distorted square planar (DSP). Al₂Cac₂ is prone to interact covalently with both P (RNA or DNA) to yield Al₂Cac₂/P complexes. These complexes undergo slow decomposition in two consecutive steps to give Al₂Cac₁/P after losing the cacodylate moiety belonging to the Al(OH) and Al₂Cac₀/P when the second cacodylate residue of the Al(DSP) is lost. These steps are (roughly) twice as fast with DNA than with RNA and for both systems the release of the first cacodylate molecule is about 10 times faster than the second. The cacodylate present in the buffer acts not only as a complexing agent of Al but also as an active vehicle that leads the metal to its biological target; once the target is reached, the two cacodylate molecules are released, leaving Al covalently linked to the N7 of guanine and adenine. These features reveal the great complexity of aluminium interaction with biological systems, an important outcome due to the implications of this metal in human health.

Kinetic curve of the binding of aluminum to RNA and metal-induced strand aggregation.

In this work, we report experimental and computational evidence for the intercalation into the DNA base-pairs of the free quinones quinizarin (Q) and naphthazarin (N) and the interstrand covalent binding of their p-cymene di-ruthenium(II) complexes (Cl₂Ru₂X, with X = N, Q bridging ligands). The intercalation extent for the N complex was larger than that for Q, which is in good agreement with the higher relative contour length and melting temperature for the same C_X/C_DNA ratio and with the computational mean stacking distances between the ligand and the nearest base-pair (3.34 Å and 3.19 Å) for N and Q, respectively. However, the apparent binding constant of Q/DNA, two orders higher than that of N/DNA, indicates that the thermal stability of the X/DNA complex is more related to the degree of intercalation than to the magnitude of the binding constant. Cl₂Ru₂X complexes undergo aquation, forming the aqua-derivatives [(H₂O)₂Ru₂X]²⁺. These can further bind covalently to DNA via interstrand crosslinking, through both Ru centres and two N7 sites of consecutive guanines, to give (DNA_1,2)Ru₂X complexes, by a mechanism similar to that of cisplatin. To the best of our knowledge, this type of interaction with dinuclear Ru(II) complexes has not been reported hitherto. The experimental and computational results reveal that the number of rings of the aromatic moiety and the covalent binding to DNA play a key role in the behaviour of the quinones and their Ru(II) derivatives. The cytotoxicity of the ligands and the corresponding Ru(II) complexes was evaluated in MCF-7, A2780, A2780cis tumour cells and in the healthy cell line MRC-5. The cytotoxic activity was notable for N and negligible for Q. The IC₅₀ values and the resistance (RF) and selectivity (SF) factors show that the Cl₂Ru₂N complex is the most promising among the four studied anticancer drugs.

Intercalation of [CuL]⁺into RNA causes an increase of melting temperature of 28 °C in contrast to the 0.6 °C for DNA.

Aluminium ions are believed to play a role in a number of neurological and skeletal disorders in the human body. The study of the biological processes and molecular mechanisms that underlie these pathological disorders is rendered a difficult task due to the wide variety of complex species that result from the hydrolysis of Al³⁺ ions. In addition, this ion displays a pronounced tendency to precipitate as a hydroxide, so certain complexing agents should be envisaged to stabilize Al(III) solutions in near physiological conditions. In this work, we show that the common buffer cacodylic acid (dimethylarsinic acid, HCac) interacts with Al(III) to give stable complexes, even at pH 7. After preliminary analyses of the speciation of the metal ion and also of the ligand, a systematic study of the formation of different Al/Cac complexes at different pH values has been conducted. UV-Vis titrations, mass spectrometry NMR measurements and DTF calculations were performed to enlighten the details of the speciation and stoichiometry of Al/Cac complexes. The results altogether show that Al/Cac dimer complexes prevail, but monomer and trimer forms are also present. Interestingly, it was found that cacodylate promotes the formation of such relatively simple complexes, even under conditions where the polymeric form, Al₁₃O₄(OH)₂₄ ⁷⁺, should predominate. The results obtained can help to shed some light into the reactivity of aluminium ions in biological environments.

The synthesis and full characterization of the new aqua-complex [(η⁶-p-cymene)Ru(OH₂)(κ²-N,N-2-pydaT)](BF₄)₂, [2](BF₄)₂, and the nucleobase derivative [(η⁶-p-cymene)Ru(9-MeG)(κ²-N,N-2-pydaT)](BF₄)₂, [4](PF₆)₂, where 2-pydaT = 2,4-diamino-6-(2-pyridyl)-1,3,5-triazine and 9-MeG = 9-methylguanine, are reported here. The crystal structures of both [4](PF₆)₂ and the chloro complex [(η⁶-p-cymene)RuCl(κ²-N,N-2-pydaT)](PF₆), [1](PF₆), have been elucidated by X-ray diffraction. The former provided relevant information regarding the interaction of the metallic fragment [(η⁶-p-cymene)Ru(κ²-N,N-2-pydaT)]²⁺ and a simple model of DNA. NMR and kinetic absorbance studies have proven that the aqua-complex [2](BF₄)₂ binds to the N7 site of guanine in nucleobases, nucleotides, or DNA. A stable bifunctional interaction (covalent and partially intercalated) between the [(η⁶-p-cymene)Ru(κ²-N,N-2-pydaT)]²⁺ fragment and CT-DNA has been corroborated by kinetic, circular dichroism, viscometry, and thermal denaturation experiments. The reaction mechanism entails the very fast formation of the Ru–O-(PO₃) linkage prior to the fast intercalation of the 2-pydaT fragment. Then, a Ru–N7-(G) covalent bond is formed at the expense of the Ru–O-(PO₃) bond, yielding a bifunctional complex. The dissociation rate of the intercalated fragment is slow, and this confers additional interest to [2](BF₄)₂ in view of the likely correlation between slow dissociation and biological activity, on the assumption that DNA is the only biotarget. Furthermore, [2](BF₄)₂ displays notable pH-dependent cytotoxic activity in human ovarian carcinoma cells (A2780, IC₅₀ = 11.0 μM at pH = 7.4; IC₅₀ = 6.58 μM at pH = 6.5). What is more, complex [2](BF₄)₂ is not cross-resistant with cisplatin, exhibiting a resistance factor, RF(A2780cis), of 0.28, and it shows moderate selectivity toward the cancer cell lines, in particular, A2780cis (IC₅₀ = 3.0 5 ± 0.08 μM), relative to human lung fibroblast cells (MRC-5; IC₅₀ = 24 μM), the model for healthy cells.

The interaction of thionine with triple, double, and single RNA helices has been fully characterized by thermodynamic and kinetic methods. The nature of the interaction of thionine with the synthetic polynucleotides poly(rU), poly(rA)·poly(rU), and poly(rA)·2poly(rU) has been studied at pH = 7.0 and 25 °C by UV absorbance, fluorescence, circular dichroism spectroscopy, viscometry, differential scanning calorimetry, and T-jump kinetic measurements. The results show that at I = 0.1 M thionine binds to a single poly(rU) strand, destabilizes the poly(rA)·2poly(rU) triplex by external binding, and intercalates into poly(rA)·poly(rU) with similar affinity to the thionine/DNA intercalated complex ( Paul , P. ; Kumar , G. S. J. Fluoresc. 2012 , 22 , 71−80). On the other hand, the differential scanning calorimetry measurements performed with thionine display a point in which the heat capacity remains unaltered, revealing the equilibrium of isothermal denaturation: thionine/poly(rA)·2poly(rU) + thionine ⇌ thionine/poly(rA)·poly(rU) + thionine/poly(rU), an outcome supported by the other techniques used. The denaturation equilibrium constant, K_D (25 °C) = 522 M^–1, was evaluated from the affinity with the single, duplex, and triplex RNA.