Yuriy Sakhno

Coordination chemistry of Ca sites at the surface of nanosized hydroxyapatite: interaction with H2O and CO

The affinity towards water of a selection of well-defined, nanostructured hydroxyapatite (HA) samples was investigated by H2O vapour adsorption microcalorimetry and infrared (IR) spectroscopy. A large hydrophilicity of all investigated materials was confirmed. The surface features of hydrated HA were investigated on the as-synthesized samples pre-treated in mild conditions at T=303 K, whereas dehydrated HA features were characterized on samples activated at T=573 K. The relatively large hydrophilicity of the hydrated surface (−ΔadsH∼100–50 kJ mol−1) was due to the interaction of water with the highly polarized H2O molecules strongly coordinated to the surface Ca2+ cations. At the dehydrated surface, exposing coordinatively unsaturated (cus) Ca2+ cations, H2O was still molecularly adsorbed but more strongly (−ΔadsH∼120–90 kJ mol−1). The use of CO adsorption to quantify the Lewis acidic strength of HA surface sites revealed only a moderate strength of cus Ca2+ cations, as confirmed by both microcalorimetric and IR spectroscopic measurements and ab initio calculations. This result implies that the large HA/H2O interaction energy is due to the interplay between cus Ca2+ sites and nearby hydrophilic PO4 groups, not revealed by the CO probe. The lower density of cus Ca2+ cations at the 573 K activated HA surface with respect to the pristine one did not affect the whole hydrophilicity of the surface, as the polarizing effect of Ca sites is so strong to extend up to the fourth hydrated layer, as confirmed by both high-coverage microcalorimetric and IR spectroscopic data. No specific effects due to the investigated specimen preparation method and/or different morphology were observed.

The Formation and Self-Assembly of Long Prebiotic Oligomers Produced by the Condensation of Unactivated Amino Acids on Oxide Surfaces†

In situ IR and mass spectrometry evidence for the catalytic formation on SiO2 and TiO2 surfaces of glycine oligomers (poly-Gly) up to 16 units long by successive feeding with monomers from the vapor phase is presented. Parallel experiments carried out on hydroxyapatite resulted in the unreactive adsorption of Gly, thus indicating that the oligomerization was specifically catalyzed by the surfaces of SiO2 and TiO2 . Furthermore, the poly-Gly moved on the surface when contacted with H2 O vapor and formed self-assembled aggregates containing both helical and β-sheet-like structural motifs. These results indicate that polypeptides formed by the condensation of amino acids adsorbed on a mineral surface can evolve into structured supramolecular assemblies.

Direct Synthesis of Amides from Carboxylic Acids and Amines by Using Heterogeneous Catalysts: Evidence of Surface Carboxylates as Activated Electrophilic Species

“Rethinking amide-bond synthesis” is one of the most stimulating topics in current chemical research. A number of successful amide-forming reactions have been proposed to meet highly demanding synthetic challenges and/or large-scale production requirements. However, a general problem that affects these methods is the consumption of large amounts of chemicals, which typically include reagents that are required for the activation of carboxylic groups, thereby resulting in increased cost and waste production. In 2007, the American Chemical Society Green Chemistry voted “amide-bond formation avoiding poor atom-economy reagents” as a top priority in the search for better and more-sustainable reagents and methods. Catalyzed reactions are quite attractive for this purpose and many compelling studies of amidation reactions that are promoted by homogeneous and heterogeneous catalysts have been reported in recent years. The amidation mechanism for unactivated carboxylic acids and amines on heterogeneous catalysts, in particular the activation of the carboxylic moiety, has not yet been sufficiently explored. In their assessment of Bernal’s hypothesis by using ab initio methods, Rimola et al. proposed a mechanism that involved undissociated carboxylic groups for the formation of peptide bonds on the surface of an aluminosilicate. Herein, IR spectroscopy and high-resolution mass spectrometry (HRMS) provide evidence for the amidation of carboxylate moieties that are formed by the adsorption of formic and acetic acids onto the surface of TiO2 nanoparticles (Degussa P25) under mild conditions [about 323 K, the temperature of the samples in the IR beam, hereafter “beam temperature” (BT)] . In our view, this finding represents the starting point for the elucidation of the structural and energetic features of one of the potential pathways for amidation reactions on surfaces, thereby promoting the rational design of optimized materials and processes for heterogeneous catalytic approaches. Formic acid was properly dosed from the vapor phase onto dehydrated TiO2 nanoparticles (for the experimental details, see the Supporting Information, Materials and Methods and Figure S1) to ensure the adsorption of only formate species onto the surface. The attainment of this target was confirmed by the selective appearance in the IR spectrum of components at 1560 cm , with shoulders at 1580 and 1540 cm , and at 1361 cm , with a weak partner at 1412 cm , owing to the nasymCOO and nsymCOO stretches of the formate species, respectively; the signal at 1385 cm 1 corresponded to the H C O bending mode (Figure 1, curves a, b; for the full spectra, see the Supporting Information, Figures S1 B and S2). The same pattern was obtained by using HCOOD molecules, thus confirming the selective formation of HCOO species (see the Supporting Information, Figure S1 B’). Higher coverage resulted in the appearance of the nC=O band at about 1682 cm , which corresponded to HCOOH molecules that adsorbed in their non-dissociated form (see the Supporting Information, Figure S1 A). A proposal of the coordi[a] Dr. C. Deiana, Dr. Y. Sakhno, M. Fabbiani, Dr. M. Pazzi, Prof. M. Vincenti, Prof. G. Martra Department of Chemistry University of Torino Via P. Giuria 7, 10125 Torino (Italy) Fax: (+ 39) 0116707855 E-mail : gianmario.martra@unito.it [b] Dr. C. Deiana, Dr. Y. Sakhno, Prof. G. Martra Interdepartmental Centre of Excellence “Nanostructured Interfaces and Surfaces-NIS” University of Torino Via P. Giuria 7, 10125 Torino (Italy) [] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201300164. It contains descriptions of the materials, the IR and HRMS methods, and additional results, that is, IR spectra of: increasing/decreasing amounts of HCOOH on TiO2 ; the adsorption of HCOOH and 1-pentanamine onto TiO2 ; washed samples; reacted sample that had been degassed and hydrated with H2O or D2O; reverse-order admission of HCOOH and 1-pentanamine onto TiO2 ; adsorbed CO; the adsorption of CH3COOH and 1-pentanamine onto TiO2; increasing/decreasing amounts of CH3COOH on TiO2 ; D2O that had been admitted onto CH3COO /TiO2; a summary of the structures of the surface carboxylates; and HRMS spectra. Figure 1. Left : IR spectra within the range 1750–1300 cm 1 of a) TiO2 that had been outgassed at 723 K; b) after contact with HCOOH and subsequent outgassing; and c–e) after the admission of increasing amounts of 1-pentanamine. p, w) Spectra of TiO2 that had been outgassed at 723 K and then treated with 1-pentanamine (40 mbar) and water vapors (20 mbar), respectively, and outgassed at the BT for 1 h. Right: HRMS of the washing solution of the reacted sample. For the original spectrum, see the Supporting Information, Figure S3 A.

Fast TiO2-catalyzed direct amidation of neat carboxylic acids under mild dielectric heating

The development of green protocols for amide bond formation is a major socioeconomic goal for chemical and pharmaceutical industries and an important challenge for academic research. We herein report a protocol for the quantitative conversion of carboxylic acids and amines to form amides at 100 °C in the presence of a TiO2 powder catalyst, under monomodal microwave irradiation. The sustainability of the process appears to be augmented by the ease with which the catalyst is recycled.

Activity patterns of metal oxide catalysts in the synthesis of N-phenylpropionamide from propanoic acid and aniline

The reactivities of various commercial and lab-made oxide samples (e.g., γ-Al2O3, CeO2, ZrO2 and TiO2) in the heterogeneous catalytic synthesis of N-phenylpropionamide (T, 383 K) from aniline and propanoic acid have been investigated. All the materials studied drive the direct synthesis of the amide to an extent depending on both the chemical and structural properties. A 0th-order kinetic dependence on the substrate concentrations suggests that the reaction proceeds via a Langmuir–Hinshelwood (L–H) pathway under kinetic control of the adsorption–desorption steps (the rate determining step, r.d.s.). The comparative analysis of the activity data on the basis of the relative surface specific kinetic constant discloses a superior surface reactivity of TiO2, CeO2 and ZrO2 over the γ-Al2O3 system, and also highlights marked differences in the catalytic functionality of the titania samples. IR spectroscopic studies of the carboxylic acids and amine adsorption and interaction patterns show the formation of the bidentate, bridging, and unidentate carboxylate intermediates accounting for the different amidation functionalities of the studied materials.

On the surface effects of citrates on nano-apatites: evidence of a decreased hydrophilicity

The surface structure and hydrophilicity of synthetic nanocrystalline apatite with strongly bound citrates on their surface are here investigated at the molecular level, by combining advanced IR spectroscopy, microgravimetry and adsorption microcalorimetry. Citrate are found to form unidentate-like and ionic-like complexes with surface Ca2+ ions, with a surface coverage closely resembling that present in bone apatite platelets (i.e., 1 molecule/(n nm)2, with n ranging between 1.4 and 1.6). These surface complexes are part of a hydrated non-apatitic surface layer with a sub-nanometre thickness. Noticeably, it is found that the hydrophilicity of the nanoparticles, measured in terms of adsorption of water molecules in the form of multilayers, decreases in a significant extent in relation to the presence of citrates, most likely because of the exposure toward the exterior of –CH2 groups. Our findings provide new insights on the surface properties of bio-inspired nano-apatites, which can be of great relevance for better understanding the role of citrate in determining important interfacial properties, such as hydrophobicity, of bone apatite platelets. The evaluation and comprehension of surface composition and structure is also of paramount interest to strictly control the functions of synthetic biomaterials, since their surface chemistry strongly affects the hosting tissue response.

One step up the ladder of prebiotic complexity: Formation of non‐random linear polypeptides from binary systems of amino acids on silica

Evidence for the formation of linear oligopeptides with nonrandom sequences from mixtures of amino acids coadsorbed on silica and submitted to a simple thermal activation is presented. The amino acid couples (glutamic acid+leucine) and (aspartic acid+valine) were deposited on a fumed silica and submitted to a single heating step at moderate temperature. The evolution of the systems was characterized by X-ray diffraction, infrared spectroscopy, thermosgravimetric analysis, HPLC, and electrospray ionization mass spectrometry (ESI-MS). Evidence for the formation of amide bonds was found in all systems studied. While the products of single amino acids activation on silica could be considered as evolutionary dead ends, (glutamic acid+leucine) and, at to some extent, (aspartic acid+valine) gave rise to the high yield formation of linear peptides up to the hexamers. Oligopeptides of such length have not been observed before in surface polymerization scenarios (unless the amino acids had been deposited by chemical vapor deposition, which is not realistic in a prebiotic environment). Furthermore, not all possible amino acid sequences were present in the activation products, which is indicative of polymerization selectivity. These results are promising for origins of life studies because they suggest the emergence of nonrandom biopolymers in a simple prebiotic scenario.

Surface Sites of Nanomaterials: Investigation of Local Structures by In Situ IR Spectroscopy

In this chapter, the possibilities to highlight atomic/molecular details of the surface of nanomaterials by IR spectroscopy of their interaction with molecules in controlled conditions are reviewed. For this objective, information on the experimental setups and procedures is also provided; particular attention has been devoted to the use of different probe molecules (CO, CH3CN, and H2O). Such molecules, when adsorbed in controlled conditions, become useful “molecular tools because of the sensitivity of their vibrational features to the physical/chemical properties of surface sites.