Random Coil Content Research Articles

Effects of ultrasound-assisted sodium bisulfite pretreatment on the preparation of cholesterol-lowering peptide precursors from soybean protein

In order to take full advantage of the gastrointestinal digestive function, the effects of S-type ultrasound-assisted sodium bisulfite (UASB) pretreatment on the preparation of cholesterol-lowering peptide precursors derived from soybean protein were investigated and the structural characterizations of pretreated proteins were explored. UASB pretreatment with the operational mode of mono-frequency ultrasound at 28 kHz, ultrasonic power density of 200 W/L and ultrasonic time of 50 min exhibited the highest cholesterol-lowering activity (56.90%) of soybean protein hydrolysates (SPH) after simulated gastrointestinal digestion, which increased by 87.17% compared to the control. Under these conditions, the peptide content of SPH after simulated gastrointestinal digestion was not significantly different (p > 0.05) compared to the control. Further FTIR analysis showed that UASB pretreatment increased β-turn and β-sheet content and decreased α-helix and random coil content. The changes in the surface hydrophobicity and microstructures of soybean protein indicated that UASB pretreatment loosened soybean protein structure and exposed more hydrophobic groups. SDS-PAGE indicated that the restriction sites changed after UASB pretreatment. In conclusion, UASB pretreatment is an efficient method for the preparation of cholesterol-lowering peptide precursors.

Effects of japonica rice flour on the mesoscopic and microscopic properties of wheat dough protein

SummaryThe aim of this study was to study the effects of twelve varieties of japonica rice flour on the gluten protein composition and structure of wheat dough. Experimental results indicated that the addition of japonica rice flour significantly decreased the α‐helix structure in the gluten protein sample, but increased the random coil content, indicating that the protein secondary structure became more disordered. The ratio of α‐helix/β‐sheet structures in the dough supplemented with different varieties of japonica rice flour was decreased, contributing to improved dough stability. Increasing the proportion of japonica rice flour from 10% to 40% significantly decreased the sulfhydryl content of the mixed flour dough. However, the total sulfhydryl content was only positively correlated with the amount of Longdao 19 japonica rice flour, reaching a maximum of 25.34 mol g−1 at 40% japonica rice flour. The gluten structure of mixed flour dough typically forms a porous three‐dimensional network, with the Longdao 19, 20, 23 and 25, Longjing 31 and Mudanjiang 31 japonica rice flours leading to improved gluten protein networks. In summary, the supplementation of standard wheat flour with japonica rice flour can alter dough properties and gluten structure.

Effects of high hydrostatic pressure treatment on the emulsifying behavior of myosin and its underlying mechanism

Myosin, a protein originating from muscle, is traditionally used for meat products for its ability to generate emulsions. Small changes in the emulsifying and conformational characteristics of proteins greatly influence the gel-emulsifying meat products. The effect of high hydrostatic pressure (HHP) on the emulsifying behavior of myosin prepared at different pressures (0.1, 100, 150, and 200 MPa) was investigated. The particle size, microstructure, interfacial tension, emulsifying properties, and secondary structure of myosin were analyzed. The results showed that as pressure increased from 0.1 to 150 MPa, the size of oil drops was reduced, the viscoelasticity of interfacial protein film improved, and the emulsifying activity and stability of myosin increased. Meanwhile, as the pressure increased from 0.1 to 150 MPa, the α helix content of the protein decreased significantly (P < 0.05), which was beneficial for the intermolecular interaction between proteins. A further increase in pressure (to 200 MPa) was detrimental to emulsifying activity and stability. As the pressure increased to 200 MPa, the emulsion displayed a decreased interfacial elasticity and increased protein random coil contents, and the particle size noticeably increased (P < 0.05). Therefore, it is concluded that HHP treatment at 150 MPa improves the stability of myosin emulsions.

Impact of pH-shift processing combined with ultrasonication on structural and functional properties of proteins isolated from rainbow trout by-products

The effects of alkaline and acid solubilization pHs (11, 11.5, 3.5 and 2.5) combined with ultrasonication (100, and 400 W for 0, 5, 10 and 20 min) on protein recovery from rainbow trout by-product and its structural and functional properties were investigated. The combination of pH-shift processing with ultrasonication increased protein solubilization and total yield compared with the classic pH-shift processing. The combination also significantly reduced particle size, tryptophan fluorescence intensity, and reactive sulfhydryl groups content of the protein isolates. The secondary structure of the proteins was also influenced by ultrasonication with decreasing in α-helix and increasing in β-sheet, β-turn and random coil content. However, SDS-PAGE demonstrated no difference in the molar masses of the proteins. Emulsifying and foaming properties of the proteins improved by applying the combined method. However, the level of improvements was governed by solubilization pH as well as ultrasound exposure power and time. Altogether, pH-shift processing combined with ultrasonication increased protein recovery from the fish by-products while the structural changes induced with the combination improved functional properties of the recovered proteins.

Conformational changes and functional properties of whey protein isolate-polyphenol complexes formed by non-covalent interaction

The present study was conducted to evaluate the non-ovalent modifications of whey protein isolate (WPI) with gallic acid (GA), chlorogenic acid (CA) and epigallocatechin gallate (EGCG). The structural and functional properties of WPI before and after binding with GA, CA and EGCG were investigated. Results showed that free sulfhydryl groups and surface hydrophobicity significantly decreased in WPI after binding with phenolic compounds. Significant structural alterations in complexes were demonstrated, characterized by a red-shifted maximum emission wavelength in intrinsic fluorescence spectroscopy, and a significant decrease in α-helix and β-sheet and a remarkable increase in β-turn and random coil contents in fourier transform infrared (FTIR) spectroscopy. Moreover, the presence of three polyphenols induced enhanced solubility, foaming and emulsifying capacities of WPI. These findings suggest the feasible application of GA, CA and EGCG to improve the functional properties of WPI and the potential uses of WPI-polyphenol complexes in food industries.

Effect of heat treatment on the structure and digestion properties of oat globulin

AbstractBackground and objectivesThe heat treatment required for deactivation of enzymes was carried out in crop species such as oats to retain its desirable shelf life, but it may lead to a reduction in nutritive value and denaturation of globulin. This study compared the effect of two heat treatments, stirring roasting (240°C, 15 min; OFG) and kiln roasting (140°C, 45 min; OMG), on structure and digestion properties of oat globulins.FindingsThe results showed that compared with control oat globulin (OCG), both heat treatments decreased the content of amino acid in the oat globulins. The increase in roasting temperature (from 140°C to 240°C) led to the change in the secondary structure in oat globulins, with a decrease in β‐sheet and β‐turn contents, and an increase in α‐helix and random coil contents. Scanning electron microscopy showed that the structure of OFG was fragmented into small clumps. Compared with kiln roasting (140°C), stirring roasting (240°C) decreased average molecular weight, peak viscosity, and denaturation enthalpy but increased viscosity and in vitro digestibility of oat globulin.ConclusionsCompared with kiln roasting, stirring roasting increased in vitro digestibility of oat globulin and contributed to the taste, but the nutritional loss was greater.Significance and noveltyTwo heat treatment methods for enzymatic inactivation were compared regarding the effect on the structure and in vitro digestibility of oat globulin, indicating that higher temperature increased digestibility of oat globulin.

Comparison of carrying mechanism between three fat-soluble vitamins and alpha-lactalbumin: Effects on structure and physicochemical properties of alpha-lactalbumin

This study aims to explore the interaction mechanisms between alpha-lactalbumin (ALa) and three fat-soluble vitamins (VA, VD3 and VE) at pH 7.0 and their impacts on structure and functionality of ALa. Analysis of fluorescence spectroscopy showed that vitamins quenched intrinsic fluorescence of ALa in a static mode and cholecalciferol (VD3) had a higher binding affinity for ALa than Retinol (VA) and alpha-tocopherol (VE). Synchronous fluorescence and three-dimensional fluorescence spectra demonstrated the conformation changes of ALa. Meanwhile, fourier transform infrared spectroscopy manifested that three vitamins induced a transition from α-helix to β-sheet in ALa, and VD3 notably increased its random coil content. Furthermore, a combination of VE with ALa generated macromolecular aggregates, based on particle size results. The addition of VA and VD3 increased zeta potential and surface hydrophobicity of ALa, while VE triggered that of ALa initially increased and then decreased with vitamins ranging from 0 to 0.35 mmol/L. Additionally, molecular simulation suggested that VA and VE had a same binding site in the cleft of ALa, but VD3 bound in a hydrophobic pocket in protein, and their binding driving forces were hydrophobic interaction and hydrogen bond. This study reveals that ALa might be a potentially suitable carrier for delivering vitamins, providing a theoretical basis of interaction between proteins and small molecules.

The regulatory mechanism of pulsed electric field (PEF) targeting at C-terminal glutamine of shrimp antioxidant peptide QMDDQ based on MD simulation

The possible mechanism was investigated for the antioxidant activity improvement on a novel peptide QMDDQ obtained from shrimp protein by PEF in this study. It was found that QMDDQ that treated under 40 kV/cm exhibited the best antioxidant activity. After PEF treatment, the primary structure of QMDDQ was not influenced, while the β-sheet content was decreased and the random coil content was correspondingly increased in PEF-treated QMDDQ solution. The cross peak between CαH (4.53 ppm) on 4-Asp and CγH (1.78 ppm) on 5-Gln residue disappeared in PEF-treated peptide. After applying PEF treatment, the side chain of C-terminus residue Glutamine at was stretched to expose more active sites to combine with free radicals, thus enhancing the scavenging effect of radical. Hence, the antioxidant activity improvement of QMDDQ might be attributed to the targeted regulation of PEF treatment at C-terminal.

Effect of extruded starches on the structure, farinograph characteristics and baking behavior of wheat dough

Extruded wheat starch (ES) was obtained by a single-screw extruder to determine its effect on the farinograph, structural properties and baking behaviors of wheat dough. XRD analysis showed that increasing extrusion temperature made the crystalline peaks less pronounced due to the partial gelatinization. In terms of FTIR results, the molecular order of extruded starch was lower than that of native starch. The dough development time was decreased from 3.2 min to 2.7 min while the stability time was increased from 14.4 min to 15.5 min, as 70 ES were added. It was accompanied with increasing levels of α-helix and β-turn transferred from the decreased content of random coil and β -sheet. These effects in bread were to increase loaf volume and reduced loaf hardness. These results indicated that extruded starch had a good potential for producing a high-quality bread.

Structure and functional properties of sunflower seed protein as affected by enzymatic hydrolysis combined with macroporous resin adsorption decolorization

ABSTRACT In order to improve the color and functional properties of sunflower seed protein, low-temperature defatted sunflower seed meal was used as the raw material to study the effects of limited enzyme hydrolysis combined with macroporous resin adsorption decolorization technology on the color and functional properties of sunflower seed protein. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism (CD), particle size, intrinsic fluorescence intensity, surface hydrophobicity (H0) and scanning electron microscopy (SEM) were used to analyze the influence on its structure. The results showed that the molecular weight of sunflower seed protein was reduced after limited enzyme hydrolysis combined with macroporous resin decolorization; the content of α-helix and β-turn was significantly reduced, and the content of β-sheets and random coils was significantly increased; the average particle size was reduced; the fluorescence intensity was increased and the red shift phenomenon occurred. H0 also increased, SEM results showed that the surface of sunflower seed protein changed from a smooth large sheet-like structure to an irregular fragmented fast-like structure. Solubility, foamability, emulsification and emulsification stability were all significantly improved (p < .05), while foam stability and oil retention were reduced. The above results indicate that limited enzyme hydrolysis combined with macroporous resin adsorption and decolorization changes the structure of sunflower seed protein, and then improve its functional properties.

Changes in the thermal stability and structure of myofibrillar protein from quick-frozen pork patties with different fat addition under freeze-thaw cycles

Changes in thermal stability and structure of myofibrillar protein from pork patties with different fat addition (0%, 5%, 10%, 15% and 20%) under freeze-thaw (F-T) cycles were discussed. The results showed that the total sulfhydryl, reactive sulfhydryl, free amino group, α-helix and β-sheet contents, fluorescence intensity (FI), and protein thermal stability (Tmax, ∆Htotal) of samples with the same fat content were significantly decreased, while the β-turn and random-coil content and the maximum fluorescence emission wavelength (λmax) were significantly increased with increasing F-T cycles (P < 0.05). These changes in samples with 20% fat at the 5th F-T cycle were obvious and were verified by the decreases in ∆Htotal (26.1%), reactive sulfhydryl (16.1%), and FI (16.8%) compared with the patties without fat. Therefore, repeated F-T cycles could decline the thermal stability of protein, destroy the protein structure of patty, and the changes were positively correlated with fat content of patty.

Effect of sodium bicarbonate on gel properties and protein conformation of phosphorus-free chicken meat batters

So as to investigate the effect of using sodium bicarbonate (SB) to replace sodium tripolyphosphate (ST) in the chicken meat batters with ST or SB (0.30% and 0.50%), the changes of gel properties and protein conformation were studied. The pH, salt-soluble protein solubility, cooking yield, b* value and texture properties were increased significantly (p < 0.05), the L* and a* values were decreased significantly (p < 0.05) when used the SB to replace ST. The β-sheet structure content was increased, accompanied by the random coil content was decreased (p < 0.05) when used the SB to replace ST. Meanwhile, more hydrophobic interactions were formed and more aliphatic residues were exposed to hydrophilic environment. The secondary and tertiary structures had little affect with the ST or SB were increased from 0.30% to 0.50%. Overall, it was obtained that the use of SB could produce the phosphorus-free chicken meat batters.

Interaction of exopolysaccharide produced by Lactobacillus plantarum YW11 with whey proteins and functionalities of the polymer complex.

Exopolysaccharide (EPS)-producing lactic acid bacteria have been widely used in fermented milk, but interaction between the EPS and milk proteins has not been well studied. In this study, interaction between the EPS from Lactobacillus plantarum YW11 (EPS-YW11) and whey proteins (WP), and functional properties of the EPS-YW11/WP were investigated. The results showed that EPS-YW11 tended to encase WP by ζ-potential analysis with a decrease in the surface charge of the protein fraction (from -26.00mV to 15.30mV), and an increase in the melting temperature of the protein fraction (from 76.31°Cto 84.48°C)asshownby differential scanning calorimetry. Circular dichroism spectrometry showed that the EPS could induce structural change of WP, that is, increment in the content of α-helixes and random coils, There was stronger interaction between EPS-YW11 and WP at higher temperatures (60°C,90°C)due to formation of intermolecular H-bonds and OH stretching vibration as indicated by infrared spectral analysis. A significant improvement in the texture (hardness, springiness, gumminess, resilience, cohesiveness, and chewiness) of the EPS-YW11/WP complex was also observed when compared to that of the EPS or WP alone. This was confirmed by microstructural observation of the EPS-YW11/WP complex that formed branched and porous structures, and it became more complex and stable with increased temperature treatment. Due to the strong interaction the EPS-YW11/WP exhibited improved functionality. This study identifies the potential of the EPS-YW11 to serve as a functional agent in the processing of fermented dairy products with enhanced textural stability and bioactivities such as cholesterol-lowering, antioxidant, and antibiofilm.

Modification of structural and functional characteristics of brewer's spent grain protein by ultrasound assisted extraction

Ultrasound assisted extraction (UAE) was applied to enhance the extraction efficacy of protein from brewer's spent grain (BSG). The effects of ultrasonic extraction conditions including power, time and duty cycle on the techno-functional properties of protein were also evaluated. The results demonstrated that in 110 mM sodium hydroxide solution, UAE working at 250 W for 20 min under duty cycle of 60% could significantly improve the protein yield to 86.16% compared with traditional extraction without ultrasound (45.71%). The structural analysis including CD, UV–Vis, SH content results and SDS-PAGE patterns clearly showed that BSG protein was modified under ultrasonication. The extracted BSG protein increased 5.83% of β-sheet content while decreased α-helix, β-turn and random coil contents by 4.76%, 0.33% and 0.74%, respectively. Besides, SDS-PAGE pattern of protein was slightly strengthened. The enhancement in SH content and UV–Vis spectra further indicated that exposure and unfolding of protein structure occurred and thus the functional properties of BSG protein were enhanced. Indeed, fat absorption capacity, emulsifying and foaming properties of BSG protein treated by UAE were all improved except for the water absorption capacity. Thus, UAE could be a promising method to separate protein from this valuable feedstock for further utilization in the food industry.

Structural basis for high-pressure improvement in depolymerization of interfacial protein from RFRS meat batters in relation to their solubility

High-pressure processing (HPP) can modify the construction of interfacial proteins (IPs) to improve the properties of reduced-fat and reduced-salt (RFRS) meat batters. In this study, the relationship between the construction of IPs and their solubility at fat droplet/water interface in RFRS meat batters with HPP treatments was investigated. When 200 MPa for 2 min was applied, the IPs exhibited the highest solubility due to a high concentration of absorbed myosin with the content of random coil 65.62%, but the particle diameter was in reverse. The microscopy revealed the depolymerization of IPs occurred at low pressure, while macromolecular aggregates were produced as the cross-linking of IPs to some degree at pressure ≥ 200 MPa. This phenomenon was supported by the result of SDS-PAGE and the sulfhydryl of IPs. In conclusion, the HPP induced solubility alteration of IPs was achieved by modifying their construction through adjusting the secondary structures and regulating bond interactions.

New Antibacterial Peptides from the Freshwater Mollusk Pomacea poeyana (Pilsbry, 1927).

Antimicrobial peptides (AMPs) are biomolecules with antimicrobial activity against a broad group of pathogens. In the past few decades, AMPs have represented an important alternative for the treatment of infectious diseases. Their isolation from natural sources has been widely investigated. In this sense, mollusks are promising organisms for the identification of AMPs given that their immune system mainly relies on innate response. In this report, we characterized the peptide fraction of the Cuban freshwater snail Pomacea poeyana (Pilsbry, 1927) and identified 37 different peptides by nanoLC-ESI-MS-MS technology. From these peptide sequences, using bioinformatic prediction tools, we discovered two potential antimicrobial peptides named Pom-1 (KCAGSIAWAIGSGLFGGAKLIKIKKYIAELGGLQ) and Pom-2 (KEIERAGQRIRDAIISAAPAVETLAQAQKIIKGG). Database search revealed that Pom-1 is a fragment of Closticin 574 previously isolated from the bacteria Clostridium tyrobutyrium, and Pom-2 is a fragment of cecropin D-like peptide first isolated from Galleria mellonella hemolymph. These sequences were chemically synthesized and evaluated against different human pathogens. Interestingly, structural predictions of both peptides in the presence of micelles showed models that comprise two alpha helices joined by a short loop. The CD spectra analysis of Pom-1 and Pom-2 in water showed for both structures a high random coil content, a certain content of α-helix and a low β-sheet content. Like other described AMPs displaying a disordered structure in water, the peptides may adopt a helical conformation in presence of bacterial membranes. In antimicrobial assays, Pom-1 demonstrated high activity against the Gram-negative bacteria Pseudomonas aeruginosa and moderate activity against Klebsiella pneumoniae and Listeria monocytogenes. Neither of the two peptides showed antifungal action. Pom-1 moderately inhibits Zika Virus infection but slightly enhances HIV-1 infectivion in vitro. The evaluation of cell toxicity on primary human macrophages did not show toxicity on THP-1 cells, although slight overall toxicity was observed in high concentrations of Pom-1. We assume that both peptides may play a key role in innate defense of P. poeyana and represent promising antimicrobial candidates for humans.

The activation mechanism of peroxidase by ultrasound

The activation mechanism of peroxidase by ultrasound was investigated. The catalysis performance of peroxidase with ultrasound treatment was prior to the controls determined by UV–visible spectra and Fourier transform infrared spectra. The transformation of tryptophan residues in peroxidase led to the increase of a-helix and anti-parallel content in the secondary structure, and the content of p-sheet, p-turn and random coil in the secondary structure. In addition, under the atomic force microscope, under ultrasonic treatment, the large molecular clusters of tyrosinase are broken down into small molecular clusters. The current results showed that the activity of peroxidase is activated under ultrasonic treatment, which is mainly caused by ultrasound without conformational change, the catalytic center is exposed, and the affinity with the substrate is stronger.

Effects of different ultrasound powers on the structure and stability of protein from sea cucumber gonad

The effects of ultrasound with different powers (0-500 W) on the structure and stability of protein from sea cucumber gonad were investigated, according to the changes in phenylalanine (Phe), α-helix, β-sheet, tryptophan (Trp), surface hydrophobicity, particle size distribution, microstructure and amino acid (AA). With the increase in ultrasound power, the contents of Phe, β-sheet and Trp, surface hydrophobicity and particle size first increased and then decreased, while α-helix content was opposite. Compared with the non-ultrasound group, the content of α-helix and random coil significantly decreased by 24.71% and 22.33% in the 200 W ultrasound group, while Phe content, β-sheet content, fluorescence intensity (FI), the surface hydrophobicity and hydrophobic AA significantly increased by 12.77%, 75.91%, 7.90%, 15.61% and 7.00%, respectively (P < 0.05). According to microstructure analysis, after 200 W ultrasound treatment, the protein particles became smaller and more uniform, and the protein structure was much looser than that without ultrasound treatment. Overall, the side chain, secondary, and tertiary structure of protein from sea cucumber gonad were unfolded under ultrasound treatment, which exposed more hydrophobic groups and decreased protein aggregation, and the effect of ultrasound with 200 W on the protein structure was prominent.

Insight into the advantages of premixing yeast-wheat gluten and combining ultrasound and transglutaminase pretreatments in producing umami enzymatic protein hydrolysates

This study aimed to utilize effectively industrial byproducts, yeast suspension (Y) and wheat gluten (W), to produce umami protein hydrolysates as seasonings. Y and W were mixed to yield YW, followed by a pretreatment (ultrasound, transglutaminase (TG), or their combination) and then proteolysis with a yeast extract enzyme and trypsin. Premixing Y and W promoted their dispersibility, and suppressed gluten aggregation and hydrolysate’s bitterness. All pretreatments increased protein recovery. Ultrasound alone or ultrasound with TG increased the embedding of yeasts in W, umami and salty tastes, hydrolysis degree and proportion of molecules < 3 kDa of the YW hydrolysate. For the first time, premixing Y and W, and pretreating YW (by ultrasound then TG-catalyzed protein crosslinking), were found to increase the β-sheet and random coil contents and decreased the β-turn content and surface hydrophobicity, leading to a low-cost umami and non-bitter protein hydrolysate with 56% of species < 1 kDa.

Ultrasonic treatment on physicochemical properties of water-soluble protein from Moringa oleifera seed

Effect of ultrasonic power on the structure and functional properties of water-soluble protein extracted from defatted Moringa oleifera seed were explored. The results showed that ultrasonic treatment could reduce β-sheet and β-turn content of water-soluble protein from Moringa oleifera seed (MOWP) and increase the content of random coil and α-helix. Changes in intrinsic fluorescence spectra, surface hydrophobicity (H0) and thermal behaviors indicated that ultrasonic had significant effect on the tertiary structure of MOWP. The results of SEM and SDS-PAGE showed that the MOWP was aggregated but not significantly degraded by ultrasound. The solubility, foaming properties and emulsifying properties of MOWP increased firstly and then decreased with the increase of ultrasonic power. Ultrasonic treatment altered the functional properties of MOWP, which might be attributed to the exposure of hydrophilic group and the change of and secondary and tertiary structure.