RESUMO
The interfacial structuring of Acacia gum at various pH values on self-assembled monolayer (SAM) surfaces was investigated in order to evaluate the respective importance of surface versus biopolymer hydration in the adsorption process of the gum. To this end, SAMs with four different ending chemical functionalities (-CH3, -OH, -COOH, and -NH2) were used on gold surfaces, and the gum adsorption was monitored using multiparametric surface plasmon resonance (MP-SPR) and quartz crystal microbalance with dissipation. Surface modification with alkanethiol and the subsequent adsorption of Acacia gum were also characterized by contact angle measurements using both sessile drop and captive bubble methods. According to MP-SPR results, this study demonstrated that gum adsorbed on all surfaces and that adsorption is the most favorable at both acid pH and hydrophobic environments, i.e., when both the surface and the biopolymer are weakly hydrated and more prone to interfacial dehydration. These results reinforce our recent proposal of interfacial dehydration-induced structuring of biopolymers. Increasing the pH logically decreased the adsorption capacity, especially on a hydrophilic surface, enhancing the hydration rate of the layer. A hydrophilic surface is unfavorable to Acacia gum adsorption except if the surface presents a negative surface charge. In this case, interfacial charge dehydration was promoted by attractive electrostatic interactions between the surface and biopolymers. In the aggregate, the water percentage and the viscoelastic properties were closely related to the properties of the surface function: the negative charge and hydrophobicity significantly increased the hydration rate and viscoelastic properties with the pH, while the positive charge induced a rigid and more dehydrated layer.
RESUMO
The adsorption process of Acacia gum (A. senegal), a complex heteropolysaccharide, was followed by using a spectroscopic method to unravel the relative contribution of the protein moieties and the carbohydrate blocks on the adsorption process. In situ ATR-FTIR was used to investigate the kinetics and conformational changes associated with the adsorption of A. senegal gum on gold nanoparticle films (Au-NPs) at different pHs. The results of this thorough study highlighted the adsorption of A. senegal gum through its protein moieties, in particular, AGPs of low molecular weight and high protein content, close to the Au-NPs surface. Isotherm experiments, by gradually increasing the concentration, showed that the gum adsorption was heterogeneous and followed the Freundlich model for the amide part, while the polysaccharide part followed the Langmuir model. In addition, the hydration and structural organization of the gum layer depended on the gum concentration. A. senegal gum adsorbed irreversibly on Au-NPs whatever the pHs, but the adsorbed layer presented a different behavior depending on pH. A more aggregated and less hydrated structure was observed at acidic pH, while a very hydrated and continuous layer was detected at higher pH. The secondary structure analysis through amide III band revealed a change in the gum secondary structure at high pH with the increase in ß-turn while random coil decreased.
Assuntos
Acacia , Nanopartículas Metálicas , Ouro , Senegal , Espectroscopia de Infravermelho com Transformada de Fourier , Amidas , AdsorçãoRESUMO
Adsorption of five different hyperbranched arabinogalactan-protein (AGP) fractions from Acacia senegal gum was thoroughly studied at the solid-liquid interface using a quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance (SPR), and atomic force microscopy (AFM). The impact of the protein/sugar ratio, molecular weight, and aggregation state on the adsorption capacity was investigated by studying AGP fractions with different structural and biochemical features. Adsorption on a solid surface would be primarily driven by the protein moiety of the AGPs through hydrophobic forces and electrostatic interactions. Increasing ionic strength allows the decrease in electrostatic repulsions and, therefore, the formation of high-coverage films with aggregates on the surface. However, the maximum adsorption capacity was not reached by fractions with a higher protein content but by a fraction that contains an average protein quantity and presents a high content of high-molecular-weight AGPs. The results of this thorough study highlighted that the AGP surface adsorption process would depend not only on the protein moiety and high-molecular-weight AGP content but also on other parameters such as the structural accessibility of proteins, the molecular weight distribution, and the AGP flexibility, allowing structural rearrangements on the surface and spreading to form a viscoelastic film.
Assuntos
Acacia , Adsorção , Galactanos , Mucoproteínas , Proteínas de Plantas , Técnicas de Microbalança de Cristal de Quartzo , Senegal , Propriedades de SuperfícieRESUMO
Owing to the suspected toxicity and carcinogenicity of tungstate (VI) oxyanions [i.e. mono tungstate and several polytungstate, generally represented by W (VI)], the environmental fate of W (VI) has been widely studied. Sorption is regarded as a major mechanism by which W (VI) species are retained in the solid/water interface. Iron (hydr)oxides have been considered important environmental sinks for W (VI) species. Here we report sorption mechanisms of W (VI) on a common iron oxide mineral-hematite under environmentally relevant solution properties using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic probes. Initial W (VI) loadings varied from 10 to 200 µM at fixed pH values ranged from 4.6 to 8.1. For pH envelop (pHs = 4.6, 5.0, 5.5, 6.0, 6.5, 7.5, and 8.1) experiments, fixed W (VI) concentrations (i.e. 10 & 200 µM) were used to understand the effects of pH. The results indicated that at acidic pH values (pH < 6.0) the sorbed polytungstate surface species are prominent at 200 µM initial W (VI) conc. The pH envelop experiments revealed that sorbed polytungstates can be present even at lower initial W (VI) conc. (i.e. 10 µM) at pH values <5.5. Overall, our in situ ATR-FTIR experiments indicated that W (VI) forms inner-sphere type bonds on hematite surface and the strength of the interaction increases with decreasing pH. In addition, initial W (VI) concentration affected the sorption mechanisms of W (VI) on hematite. Our study will aid the molecular level understanding of W (VI) retention on iron oxide surfaces.
Assuntos
Poluentes Ambientais/química , Compostos Férricos/química , Modelos Químicos , Espectroscopia de Infravermelho com Transformada de Fourier , Compostos de Tungstênio/química , Adsorção , Poluentes Ambientais/análise , Concentração de Íons de Hidrogênio , Minerais , Soluções , Propriedades de Superfície , Compostos de Tungstênio/análiseRESUMO
In this paper, partial least square (PLS) regression is innovatively applied for a semi-quantitative non invasive study of the most precious dye of Antiquity: Tyrian purple. This original approach for the study of organic dyes in the cultural heritage field, is based on the correlation of spectrophotometric (UV-Visible) and chromatographic (Fast-HT-HPLC-PDA) data from an extensive set of textiles prepared with different snail species according to historical recipes. A cross-validated PLS model, based on the quantity of 6,6'-dibromoindigotin, displays an excellent correlation factor (R(2)Y = 0.987) between values determined by chromatography and those predicted from reflectance spectra. This indicates that the spectral features of Tyrian purple on textile fibre is strictly related to the amount of this indigoid component whose content may be non invasively predicted from reflectance spectrum. The studied correlation also highlights that, independently of the dyeing method and nature of the textile fibre used, the relative content of 6,6'-dibromindigotin may be used as a parameter to distinguish samples prepared with Hexaplex trunculus L. snails from those prepared with further mollusc species. To validate this model, archaeological textile fragments dating from the Roman period were successfully examined. The results achieved open an absolutely new way in Tyrian purple analysis in cultural heritage by non invasive spectroscopic techniques attesting their convergence with HPLC and giving them a semi-quantitative value.