Mechanistic elucidation of monoalkyltin(iv)-catalyzed esterification

Monoalkyltin(iv) complexes are well-known catalysts for esterification reactions and polyester formation, yet the mode of operation of these Lewis acidic complexes is still unknown. Here, we report on mechanistic studies of n-butylstannoic acid in stoichiometric and catalytic reactions, analyzed by NMR, IR and MS techniques. While the chemistry of n-butyltin(iv) carboxylates is dominated by formation of multinuclear tin assemblies, we found that under catalytically relevant conditions only monomeric n-BuSn(OAc)(3) and dimeric (n-BuSnOAc(2)OEt)(2) are present. Density functional theory (DFT) calculations provide support for a mononuclear mechanism, where n-BuSn(OAc)(3) and dimeric (n-BuSnOAc(2)OEt)(2) are regarded as off-cycle species, and suggest that carbon–oxygen bond breaking is the rate-determining step

Experimental Study to Assess Micro-mechanical Behavior of Oil Paintings over Time

The majority of oil paintings in museum collections suffer from various chemo-mechanical degradation phenomena. Oil paint is generally made up of layers of drying oil (binder) and metal-based pigment particles. One of the most important degradation mechanisms in historical oil paintings is metal soap formation. Metal ions released by the pigments interact chemically with the saturated fatty acids present in drying oils to form metal soaps. Metal soaps can develop into big aggregates, which deform paint layers and may cause mechanical damage like flaking and cracking of the paint. Additionally, the mechanical response of oil paint shows a strong dependence on age, influencing mechanical properties such as toughness, stiffness, and ductility. The interaction between metal soap formation and the aging response of oil paintings ultimately affects their degradation response. With the help of cutting-edge micro-mechanics experimental techniques, this study aims to identify the causes of changes in the chemo-mechanical response of oil paintings over time

Experimental Study to Assess Micro-mechanical Behavior of Oil Paintings over Time

The majority of oil paintings in museum collections suffer from various chemo-mechanical degradation phenomena. Oil paint is generally made up of layers of drying oil (binder) and metal-based pigment particles. One of the most important degradation mechanisms in historical oil paintings is metal soap formation. Metal ions released by the pigments interact chemically with the saturated fatty acids present in drying oils to form metal soaps. Metal soaps can develop into big aggregates, which deform paint layers and may cause mechanical damage like flaking and cracking of the paint. Additionally, the mechanical response of oil paint shows a strong dependence on age, influencing mechanical properties such as toughness, stiffness, and ductility. The interaction between metal soap formation and the aging response of oil paintings ultimately affects their degradation response. With the help of cutting-edge micro-mechanics experimental techniques, this study aims to identify the causes of changes in the chemo-mechanical response of oil paintings over time

Quantifying solvent action in oil paint using portable laser speckle imaging

The exposure of oil paintings to organic solvents for varnish removal or to water for the removal of surface dirt can affect the chemical and physical properties of oil paint in an undesired way. Solvents can temporarily plasticise and swell the polymerised oil paint binding medium, enhancing both the thermal mobility and mechanical displacement of pigments embedded in this film. The enhancement of these microscopic motions can affect both the chemical and physical stability of the object as a whole. In order to minimise solvent exposure during cleaning, an analytical method that can quantitatively measure the microscopic motions induced by solvent uptake, is required first. In this study, we use Fourier Transform Laser Speckle Imaging (FT-LSI) and a newly developed portable FT-LSI setup as highly resolved motion detection instruments. We employ FT-LSI to probe pigment motion, with high spatiotemporal resolution, as a proxy for the destabilising effects of cleaning solvents. In this way, we can study solvent diffusion and evaporation rates and the total solvent retention time. In addition, qualitative spatial information on the spreading and homogeneity of the applied solvent is obtained. We study mobility in paint films caused by air humidity, spreading of solvents as a result of several cleaning methods and the protective capabilities of varnish. Our results show that FT-LSI is a powerful technique for the study of solvent penetration during oil paint cleaning and has a high potential for future use in the conservation studio.</p

Time-Dependent ATR-FTIR Spectroscopic Studies on Solvent Diffusion and Film Swelling in Oil Paint Model Systems

In the restoration of paintings, solvent diffusion and swelling of polymeric oil paint binding media are important factors to consider. Common cleaning methods with organic solvents or aqueous solutions could lead to undesirable physicochemical changes in the paint in the long term, though the extent of this effect is not yet clear. We used tailored nonporous model systems for aged oil paint to measure paint swelling and solvent diffusion for a wide range of relevant solvents. Using dynamic mechanical analysis (DMA), the glass transition temperature of our model systems was found to be close to room temperature. Subsequently, with a custom sample cell and time-dependent attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, we were able to accurately track swelling and diffusion processes in the polymer films. To quantify the spectroscopic data, we developed a model that completely describes the solvent migration process, including significant film swelling and non-Fickian solvent diffusion. The relation between solvent properties, the diffusion coefficient, and the swelling capacity proved to be rather complex and could not be explained using a single solvent parameter. However, it was found that strongly swelling solvents generally diffuse faster than weakly swelling solvents and that pigmentation does not significantly influence solvent diffusion. These results contribute to a better understanding of transport phenomena in paintings and support the development of improved paint restoration strategies

The distribution and transport of water in oil paintings: A numerical moisture diffusion model

Oil paintings are complex, multi-layered systems that are prone to chemical degradation. While it is increasingly recognised that water plays an important role in these degradation reactions, little is known about moisture concentrations in oil paint systems and their temporal variation in response to fluctuating ambient air humidity. This knowledge is necessary to further preventive conservation, specifically to establish optimal environmental conditions to safeguard works of art for future generations. We developed a transient one-dimensional moisture transport model based on Fickian diffusion enabling the integration of experimentally recorded relative humidity data. Moisture sorption and transport data for painting materials have been reviewed from literature showing that each component of a painting has rather distinct properties. Including the properties of the individual layers enabled predicting the behaviour of a multi-layered painting system. A sensitivity study indicated that the response of a multilayer is determined by the combination of diffusion coefficients, isotherm shapes, maximum water contents, layer thicknesses, period of RH fluctuation and stacking order of the layers. Finally, the model was employed to investigate a case study of 18th-century painted wall hangings in a historic house to illustrate the insights that can be gained from this approach and the types of conservation-related questions that can be answered.Team Joris Di

CCDC 2049109: Experimental Crystal Structure Determination

Related Article: Lukas A. Wolzak, Joen J. Hermans, Folkert de Vries, Keimpe J. van den Berg, Joost N. H. Reek, Moniek Tromp, Ties J. Korstanje|2021|Cat.Sci.Tech.|11|3326|doi:10.1039/D1CY00184

CCDC 2049109: Experimental Crystal Structure Determination

AMEWIB : tris(benzoato)-(n-butyl)-tin(iv) Space Group: P b c a (61), Cell: a 11.9618(3)Å b 16.8603(4)Å c 23.0411(6)Å, α 90° β 90° γ 90° Related Article: Lukas A. Wolzak, Joen J. Hermans, Folkert de Vries, Keimpe J. van den Berg, Joost N. H. Reek, Moniek Tromp, Ties J. Korstanje|2021|Cat.Sci.Tech.|11|3326|doi:10.1039/D1CY00184

Conserved Pro-Glu (PE) and Pro-Pro-Glu (PPE) Protein Domains Target LipY Lipases of Pathogenic Mycobacteria to the Cell Surface via the ESX-5 Pathway*

The type VII secretion system ESX-5 is a major pathway for export of PE and PPE proteins in pathogenic mycobacteria. These mycobacteria-specific protein families are characterized by conserved N-terminal domains of 100 and 180 amino acids, which contain the proline-glutamic acid (PE) and proline-proline-glutamic acid (PPE) motifs after which they are named. Here we investigated secretion of the triacylglycerol lipase LipY, which in fast-growing mycobacteria contains a signal sequence, but in slow-growing species appears to have replaced the signal peptide with a PE or PPE domain. Selected LipY homologues were expressed in wild-type Mycobacterium marinum and its corresponding ESX-5 mutant, and localization of the proteins was investigated by immunoblotting and electron microscopy. Our study shows that Mycobacterium tuberculosis PE-LipY (LipYtub) and M. marinum PPE-LipY (LipYmar) are both secreted to the bacterial surface in an ESX-5-dependent fashion. After transport, the PE/PPE domains are removed by proteolytic cleavage. In contrast, Mycobacterium gilvum LipY, which has a signal sequence, is not transported to the cell surface. Furthermore, we show that LipYtub and LipYmar require their respective PE and PPE domains for ESX-5-dependent secretion. The role of the PE domain in ESX-5 secretion was confirmed in a whole cell lipase assay, in which wild-type bacteria expressing full-length LipYtub, but not LipYtub lacking its PE domain, were shown to hydrolyze extracellular lipids. In conclusion, both PE and PPE domains contain a signal required for secretion of LipY by the ESX-5 system, and these domains are proteolytically removed upon translocation