Effects of sound energy on proteins and their complexes

Mechanical energy in the form of ultrasound and protein complexes intuitively have been considered as two distinct unrelated topics. However, in the past few years, increasingly more attention has been paid to the ability of ultrasound to induce chemical modifications on protein molecules that further change protein-protein interaction and protein self-assembling behavior. Despite efforts to decipher the exact structure and the behavior-modifying effects of ultrasound on proteins, our current understanding of these aspects remains limited. The limitation arises from the complexity of both phenomena. Ultrasound produces multiple chemical, mechanical, and thermal effects in aqueous media. Proteins are dynamic molecules with diverse complexation mechanisms. This review provides an exhaustive analysis of the progress made in better understanding the role of ultrasound in protein complexation. It describes in detail how ultrasound affects an aqueous environment and the impact of each effect separately and when combined with the protein structure and fold, the protein-protein interaction, and finally the protein self-assembly. It specifically focuses on modifying role of ultrasound in amyloid self-assembly, where the latter is associated with multiple neurodegenerative disorders.

Ultrasonic process affecting interactions between sodium caseinate and whey proteins

Ultrasound has been widely explored for several applications, such as emulsification or structural modification of food materials such as proteins. In this work, the effect of ultrasound on the control of whey proteins (WPI) aggregation was evaluated in the presence of sodium caseinate (NaCas). Solutions of NaCas, WPI and both (1:1) were treated with ultrasound under different power and time conditions and were initially evaluated in terms of particle size distribution, charge density, pH and polyacrylamide gel electrophoresis. Three pairs of conditions were adopted to provide the same energy density - A1 (450 W / 300 s, 6750 MJ/m³), A2 (150 W / 900 s, 6750 MJ/m³), A3 (600 W / 300 s, 900 MJ/m³), A4 (202.5 W / 900 s, 9112.5 MJ/m³), A5 (742.5 W / 300 s, 11137.5 MJ/m³) and A6 (247.5 W / 900 s, 11137.5 MJ/m³). Best conditions of transmitted energy - A1, A3 and A5 - were studied for surface hydrophobicity, circular dichroism and infrared spectroscopy. The decrease of surface hydrophobicity of NaCas:WPI mixtures pointed to a protective effect of NaCas against WPI denaturation, confirmed by the presence of more ordered structures by FTIR analysis that were not observed in the absence of NaCas. Finally, the effect of these structural changes on the gelation capacity of the ultrasound-treated proteins was assessed. Ultrasound was able to reduce the stress at rupture from 1988.59 Pa (control) to 1655.31 Pa (A3) and 1871.24 Pa (A5), and more markedly increase the Young modulus from 113.69 kPa (control) to 243.30 kPa (A3) and 392.44 kPa (A5). This study identified that higher power values with shorter times were able to provide greater protein changes that affected gelation properties, showing that the modulation of ultrasound conditions can produce ingredients with different techno-functional properties.

Ultrasound-induced protein restructuring and ordered aggregation to form amyloid crystals

Amyloid crystals, a form of ordered protein aggregates documented relatively recently, have not been studied as extensively as amyloid fibres. This study investigates the formation of amyloid crystals with low frequency ultrasound (20 kHz) using β-lactoglobulin, as a model protein for amyloid synthesis. Acoustic cavitation generates localised zones of intense shear, with extreme heat and pressure that could potentially drive the formation of amyloid structures at ambient bulk fluid temperatures (20 ± 1 °C). Thioflavin T fluorescence and electron microscopy showed that low-frequency ultrasound at 20 W/cm3 input power induced β-stacking to produce amyloid crystals in the mesoscopic size range, with a mean length of approximately 22 µm. FTIR spectroscopy indicated a shift towards increased intermolecular antiparallel β-sheet content. An increase in sonication time (0-60 min) and input power (4-24 W/cm3) increased the mean crystal length, but this increase was not linearly proportional to sonication time and input power due to the delayed onset of crystal growth. We propose that acoustic cavitation causes protein unfolding and aggregation and imparts energy to aggregates to cross the torsion barrier, to achieve their lowest energy state as amyloid crystals. The study contributes to a further understanding of protein chemistry relating to the energy landscape of folding and aggregation. Ultrasound presents opportunities for practical applications of amyloid structures, presenting a more adaptable and scalable approach for synthesis.

Ultrasonication as a novel processing alternative to pasteurization for camel milk: Effects on microbial load, protein profile, and bioactive properties

Ultrasonic technology presents a promising novel tool in the food industry for the processing of milk and dairy products. In this study, we investigated the effects of ultrasonication (US) as an alternative to thermal pasteurization for stabilization of the bioactive properties of camel milk. Camel and bovine milk samples were subjected to US at 6 different power levels (US1-US6), and 1 set of each type of milk was concurrently subjected to flash heat pasteurization (FHP) for comparative analysis (100 mL; n = 4). The microbiological and bioactive parameters of the samples were analyzed during 7 d of storage at 4°C. In both milk types subjected to US ≥ 140 W (US3), the bacterial load was reduced by almost 4 log cycles and complete reduction of microbial load was achieved with US = 170 W and US = 210 W (US5 and US6 treatments, respectively). No significant changes in protein patterns were observed with either FHP or US treatment. In addition, bioactive properties (cholesteryl esterase and pancreatic lipase inhibition) were either enhanced or retained at US3 or higher. 2,2'-Azino-bis-3-ethylbenzthiazoline-6-sulfonic acid and ferric reducing antioxidant power activities in camel milk were decreased after FHP treatment but increased or retained upon US, particularly at US3 and US4 (160 W). Overall, under our experimental conditions, US4 was effective in completely reducing the microbial count, while concomitantly retaining different bioactive properties of both camel and bovine milk. These outcomes highlight the potential of US at 160 W as an efficient nonthermal alternative processing method for milk.

Mechanism of low-frequency and high-frequency ultrasound-induced inactivation of soy trypsin inhibitors

In this study, the effect of ultrasonic frequency and power on the inactivation of soy trypsin inhibitors (TIs) was investigated to explore the ultrasound-induced inactivation mechanism. It was observed that 20 kHz and 355 kHz ultrasound have better inactivation efficiency than 1056 kHz. First-order rate constants for the inactivation process were obtained, which increased with increasing ultrasonic power at both 20 kHz and 355 kHz. For 20 kHz ultrasound, the formation of TI aggregates resulting from the physical effects of acoustic cavitation decreased the interactions between the active sites of TIs and trypsin, thus reducing the TI activity. For 355 kHz ultrasound, most of the methionine in the TIs was oxidised within 5 mins, resulting in a faster reduction of TI activity. Subsequent aggregation of TIs resulted in further TI inactivation. SDS-PAGE showed that neither disulphide bonds nor CC coupling were involved in the formation of aggregates.