Mineral Surfaces as Agents of Environmental Proteolysis: Mechanisms and Controls

We investigated the extent to which contact with mineral surfaces affected the molecular integrity of a model protein, with an emphasis on identifying the mechanisms (hydrolysis, oxidation) and conditions leading to protein alteration. To this end, we studied the ability of four mineral surface archetypes (negatively charged, positively charged, neutral, redox-active) to abiotically fragment a well-characterized protein (GB1) as a function of pH and contact time. GB1 was exposed to the soil minerals montmorillonite, goethite, kaolinite, and birnessite at pH 5 and pH 7 for 1, 8, 24, and 168 h and the supernatant was screened for peptide fragments using Tandem Mass Spectrometry. To distinguish between products of oxidative and hydrolytic cleavage, we combined results from the SEQUEST algorithm, which identifies protein fragments that were cleaved hydrolytically, with the output of a deconvolution algorithm (DECON-Routine) designed to identify oxidation fragments. All four minerals were able to induce protein cleavage. Manganese oxide was effective at both hydrolytic and oxidative cleavage. The fact that phyllosilicates-which are not redox active-induced oxidative cleavage indicates that surfaces acted as catalysts and not as reactants. Our results extend previous observations of proteolytic capabilities in soil minerals to the groups of phyllosilicates and Fe-oxides. We identified structural regions of the protein with particularly high susceptibility to cleavage (loops and β strands) as well as regions that were entirely unaffected (α helix).

Foam-stabilizing properties of the yeast protein PAU5 and evaluation of factors that can influence its concentration in must and wine

The absence of the yeast protein seripauperin 5 (PAU5) from Saccharomyces cerevisiae has been suggested as a biomarker for the occurrence of gushing in sparkling wine as samples lacking PAU5 were found to be more susceptible to gushing. In this study, further characterization of PAU5 regarding its foam-stabilizing properties was performed to elucidate whether PAU5 has foam-stabilizing properties and therefore, to elucidate a direct influence on the gushing potential of sparkling wines. PAU5 was successfully purified from non-gushing sparkling wine using reversed-phase high-performance liquid chromatography (RP-HPLC). Pure protein was added to grape juice as a model system for grape must prior to foam stability testing. The results revealed that the protein PAU5 has foam-stabilizing properties. Furthermore, the influence of heat and sulfur treatment in the presence of Botrytis cinerea was analyzed with regard to the amount of PAU5 produced by S. cerevisiae fermented in grape juice. Fermentation experiments using two different S. cerevisiae strains were performed, and the concentration of PAU5 in the samples was compared by RP-HPLC analysis. Unlike sulfur treatment, heat treatment prevented the protein degradation induced by B. cinerea and resulted in even higher amounts of PAU5 compared to the juice fermented with yeast without a previous botrytization. The two different yeast strains applied secreted PAU5 into the surrounding medium in different amounts. In further experiments, the fining process of the wine with bentonite was examined for its potential to remove PAU5 from the wine. RP-HPLC of wines processed with different fining agents revealed that bentonite treatment affected PAU5 concentrations in the final product. The extent of PAU5 removal depended on the type of bentonite applied and on the time of addition during the production process.

Abiotic Protein Fragmentation by Manganese Oxide: Implications for a Mechanism to Supply Soil Biota with Oligopeptides

The ability of plants and microorganisms to take up organic nitrogen in the form of free amino acids and oligopeptides has received increasing attention over the last two decades, yet the mechanisms for the formation of such compounds in soil environments remain poorly understood. We used Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopies to distinguish the reaction of a model protein with a pedogenic oxide (Birnessite, MnO2) from its response to a phyllosilicate (Kaolinite). Our data demonstrate that birnessite fragments the model protein while kaolinite does not, resulting in soluble peptides that would be available to soil biota and confirming the existence of an abiotic pathway for the formation of organic nitrogen compounds for direct uptake by plants and microorganisms. The absence of reduced Mn(II) in the solution suggests that birnessite acts as a catalyst rather than an oxidant in this reaction. NMR and EPR spectroscopies are shown to be valuable tools to observe these reactions and capture the extent of protein transformation together with the extent of mineral response.

Structural studies of adsorbed protein (betalactoglobulin) on natural clay (montmorillonite)

Adsorption of betalactoglobulin on montmorillonite leads to structural changes of the protein accompanied by a partial exfoliation of clay.

Hydrothermal Synthesis and Characterization of Ni-Al Montmorillonite-Like Phyllosilicates

This work describes the first hydrothermal synthesis in fluoride medium of Ni-Al montmorillonite-like phyllosilicates, in which the only metallic elements in the octahedral sheet are Ni and Al. X-ray diffraction , chemical analysis, thermogravimetric and differential thermal analysis, scanning electron microscopy and transmission electron microscopy confirm that the synthesized samples are montmorillonite-like phyllosilicates having the expected chemical composition. The specific surface areas of the samples are relatively large (>100 m² g⁻¹) compared to naturally occurring montmorillonites. ²⁹Si and ²⁷Al nuclear magnetic resonance (NMR) indicate substitutions of Al for Si in the tetrahedral sheet. ¹⁹F NMR and Ni K-edge extended X-ray absorption fine structure (EXAFS) local probes highlight a clustering of the metal elements and of the vacancies in the octahedral sheet of the samples. These Ni-Al phyllosilicates exhibit a higher local order than in previously synthesized Zn-Al phyllosilicates. Unlike natural montmorillonites, where the distribution of transition metal cations ensures a charge equilibrium allowing a stability of the framework, synthetic montmorillonites entail clustering and instability of the lattice when the content of divalent element in the octahedral sheet exceeds ca. 20%. Synthesis of Ni-Al montmorillonite-like phyllosilicates, was successfully achieved for the first time. These new synthetic materials may find potential applications as catalysts or as materials with magnetic, optical or staining properties.

Solid-state NMR investigation on the interactions between a synthetic montmorillonite and two homopolypeptides

Interactions of two homopolypeptides (polylysine and polyglutamic acid) with a synthetic montmorillonite were studied by 1H MAS, 1H-27Al HETCOR and 1H-13C CP-MAS NMR experiments. 1H-27Al HETCOR with 1H spin-diffusion NMR appears to be a powerful probe for the identification of the polypeptide fragments, which interact with the montmorillonite interlayer surfaces. In particular, selective interactions were observed between the polypeptide side-chains and the montmorillonite octahedral aluminum atoms. 1H-13C CP-MAS NMR experiments were used to assess the dynamics of the two polypeptides through the measurement of the t(1/2) characteristic time of selected carbons. Results indicate that the local mobility of the side chains and their interaction with the montmorillonite layers depend on the nature of the adsorbed polypeptides.

Solid-state NMR studies of collagen-based parchments and gelatin

Historical collagen-based parchments have been studied by solid-state NMR. In addition, new parchment (produced according to traditional methods) and gelatin from bovine skin were also studied. Wideline 1H and MAS 13C measurements were carried out directly on intact parchments. A simple approach is proposed for evaluation of the extent of parchment degradation based on the linewidth changes in the 13C CPMAS spectra relative to new parchment and gelatin. Structural (bound) water content was estimated from wideline 1H NMR lineshape and relaxation time measurements. It was found that the relative water content in parchments correlates linearly with 13C MAS linewidths. Its decrease on parchment degradation indicates that structural water molecules are of primary importance in stabilizing higher order collagen structures. Backbone and side chain dynamics of collagen in parchments were compared to those of gelatin based on the 13C dipolar-dephased experiments. Carbonyl 13C chemical shift anisotropies were measured to deduce the geometry of the collagen backbone motion. Unlike previous studies, we found that the collagen backbone motion is similar to that found in other proteins and occurs primarily via small-angle librations about internal bond directions.