Stainless steel is an important transport and processing contact material for bovine milk and dairy products. Release (migration) of metals, ions, complexes or wear debris/particles, and metal-induced protein aggregation in such environments are hence important to consider both from a corrosion and food safety perspective. This study aims on investigating the release of iron (Fe), chromium (Cr), and nickel (Ni) from AISI 316L stainless steel in contact with whey protein solutions relevant for protein drinks, and on how the whey proteins are influenced by stirring with a magnetic stir bar and metal release. Mechanistic insight is gained by parallel investigations of metal release from two reference non-protein containing solutions, a metal-complexing (citrate-containing) simulated milk solution (SMS) and a low complexing phosphate buffered saline solution (PBS). All immersion exposures were conducted at pH 6.8 for 0.5, 4, 24 and 48 hat room temperature at static and stirring conditions. All solutions and samples were investigated using different chemical, spectroscopic, microscopic, and electrochemical methods. Significantly higher amounts of Fe, Cr, and Ni were released into the whey protein solution (80 g/L) as compared to SMS and PBS. Strong enrichment of Cr in the surface oxide and reduction of the surface oxide thickness were associated with a higher amount of Ni release in the metal-complexing solutions (SMS and whey protein) compared with PBS. Stirring conditions resulted in higher amounts of metal release, enrichment of Cr in the surface oxide, and clear signs of wear of the 316L surface in all solutions compared to static conditions. The wear mechanism in the whey protein solution was different as compared to corresponding processes in SMS and PBS, involving an etching-like process and larger-sized wear debris. Electrochemical measurements at static conditions confirmed observed differences between the solutions, with the lowest corrosion resistance observed for coupons exposed in the whey protein solution, followed by SMS and PBS. Released metals in solution from the 316L coupons in contact with the whey protein solution resulted in enhanced rates of protein aggregation and precipitation of protein aggregates from solution. Further studies should be made to investigate other relevant test conditions and assess toxicological risks.

Amyloid-like protein nanofibrils (PNFs) can assemble from a range of different proteins including disease-associated proteins, functional amyloid proteins and several proteins for which the PNFs are neither related to disease nor function. We here examined the core building blocks of PNFs formed by soy proteins. Fibril formation at pH 2 and 90 degrees C is coupled to peptide hydrolysis which allows isolation of the PNF-forming peptides and identification of them by mass spectrometry. We found five peptides that constitute the main building blocks in soy PNFs, three of them from the protein b-conglycinin and two from the protein glycinin. The abilities of these peptides to form PNFs were addressed by amyloid prediction software and by PNF formation of the corresponding synthetic peptides. Analysis of the structural context in the native soy proteins revealed two structural motifs for the PNF-forming peptides: (i) so-called b-arches and (ii) helical segments involved in quaternary structure contacts. However, the results suggest that neither the native structural motifs nor the protein of origin defines the morphology of the PNFs formed from soy protein isolate.

Exposure to cobalt (Co), chromium (Cr), and nickel (Ni) occurs often via skin contact and from different dental and orthopedic implants. The metal ions bind to proteins, which may induce structural changes and aggregation, with different medical consequences. We investigated human serum albumin (HSA) aggregation in the presence of Co-II, Cr-III, and/or Ni-II ions and/or their nanoparticle precipitates by using scattering, spectroscopic, and imaging techniques, at simulated physiological conditions (phosphate buffered saline - PBS, pH 7.3) using metal salts that did not affect the pH, and at HSA:metal molar ratios of up to 1:8. Co ions formed some solid nano particles in PBS at the investigated conditions, as determined by nanoparticle tracking analysis, but the Cr-III anions and Ni-II ions remained fully soluble. It was found that all metal ions induced HSA aggregation, and this effect was significantly enhanced when a mixture of all three metal ions was present instead of any single type of ion. Thus, the metal ions induce aggregation synergistically. HSA aggregates formed linear structures on a mica surface in the presence of Cr-III ions. A clear tendency of aggregation and linearly aligned aggregates was seen in the presence of all three metal ions. Spectroscopic investigations indicated that the majority of the HSA molecules maintained their alpha helical secondary structure and conformation. This study highlights the importance of synergistic effects of metal ions and/or their precipitates on protein aggregation, which are highly relevant for implant materials and common exposures to metals.

Self-assembly of proteins into amyloid-like nanofibrils is not only a key event in several diseases, but such fibrils are also associated with intriguing biological function and constitute promising components for new biobased materials. The bovine whey protein beta-lactoglobulin has emerged as an important model protein for the development of such materials. We here report that peptide hydrolysis is the rate-determining step for fibrillation of beta-lactoglobulin in whey protein isolate. We also explore the observation that beta-lactoglobulin nanofibrils of distinct morphologies are obtained by simply changing the initial protein concentration. We find that the morphological switch is related to different nucleation mechanisms and that the two classes of nanofibrils are associated with variations of the peptide building blocks. Based on the results, we propose that the balance between protein concentration and the hydrolysis rate determines the structure of the formed nanofibrils.

This article describes nanocomposite films with separately grown protein nanofibrils (PNFs) in a nonfibrillar protein matrix from the same protein starting material (whey). Tensile tests on the glycerol-plasticized films indicate an increased elastic modulus and a decreased extensibility with increasing content of PNFs, although the films are still ductile at the maximum PNF content (15 wt %). Infrared spectroscopy confirms that the strongly hydrogen-bonded beta-sheets in the PNFs are retained in the composites. The films appear with a PNF-induced undulated upper surface. It is shown that micrometer-scale spatial variations in the glycerol distribution are not the cause of these undulations. Instead, the undulations seem to be a feature of the PNF material itself. It was also shown that, apart from plasticizing the protein film, the presence of glycerol seemed to favor to some extent exfoliation of stacked beta-sheets in the proteins, as revealed by X-ray diffraction.

Aggregated alpha-synuclein is the main component of Lewy bodies, intraneuronal inclusions found in brains with Parkinson's disease and dementia with Lewy bodies. A body of evidence implicates oxidative stress in the pathogenesis of these diseases. For example, a large excess (30: 1, aldehyde: protein) of the lipid peroxidation end products 4-oxo-2-nonenal (ONE) or 4-hydroxy-2-nonenal (HNE) can induce alpha-synuclein oligomer formation. The objective of the study was to investigate the effect of these reactive aldehydes on alpha-synuclein at a lower molar excess (3: 1) at both physiological (7.4) and acidic (5.4) pH. As observed by size-exclusion chromatography, ONE rapidly induced the formation of alpha-synuclein oligomers at both pH values, but the effect was less pronounced under the acidic condition. In contrast, only a small proportion of alpha-synuclein oligomers were formed with low excess HNE-treatment at physiological pH and no oligomers at all under the acidic condition. With prolonged incubation times (up to 96 h), more alpha-synuclein was oligomerized at physiological pH for both ONE and HNE. As determined by Western blot, ONE-oligomers were more SDS-stable and to a higher-degree cross-linked as compared to the HNE-induced oligomers. However, as shown by their greater sensitivity to proteinase K treatment, ONE-oligomers, exhibited a less compact structure than HNE-oligomers. As indicated by mass spectrometry, ONE modified most Lys residues, whereas HNE primarily modified the His50 residue and fewer Lys residues, albeit to a higher degree than ONE. Taken together, our data show that the aldehydes ONE and HNE can modify alpha-synuclein and induce oligomerization, even at low molar excess, but to a higher degree at physiological pH and seemingly through different pathways.