Extraction and Characterization of Collagen from Elasmobranch Byproducts for Potential Biomaterial Use

Ammodaucus Leucotrichus Seed Extract as a Potential Therapy in Animal Models of Rheumatoid Arthritis Induced by Complete Freund Adjuvant and Chicken Cartilage Collagen

This study assessed the efficacy of an Ammodaucus leucotrichus seed extract to treat rheumatoid arthritis in rat models of this disease. Rheumatoid arthritis was induced in rats using two methods: immunization with 100 µL of Complete Freund Adjuvant (CFA) and immunization with 100 µL of a 3 mg/ml solution of type II collagen (CII) from chicken cartilage. The therapeutic potential of the extract was assessed at different doses (150, 300, and 600 mg/kg/day for 21 days in the CII-induced arthritis model and for 14 days in the CFA-induced arthritis model) and compared with methotrexate (MTX; 0.2 mg/kg for the same periods), a commonly used drug for rheumatoid arthritis treatment in humans. In both models (CII-induced arthritis and CFA-induced arthritis), walking distance, step length, intra-step distance and footprint area were improved following treatment with the A. leucotrichus seed extract (all concentrations) and MTX compared with untreated animals. Both treatments increased the serum concentration of glutathione and reduced that of complement C3, malondialdehyde and myeloperoxidase. Radiographic data and histological analysis indicated that cartilage destruction was reduced already with the lowest dose of the extract (100 mg/kg/dose) in both models. These results show the substantial antiarthritic potential of the A. leucotrichus seed extract, even at the lowest dose, suggesting that it may be a promising alternative therapy for rheumatoid arthritis and joint inflammation. They also emphasize its efficacy at various doses, providing impetus for more research on this extract as a potential therapeutic agent for arthritis.

Marine collagen: Unveiling the blue resource-extraction techniques and multifaceted applications

Marine organisms such as fish and shellfish are composed of compounds with properties and characteristics that have been proven useful in a variety of sectors such as cosmetics, healthcare (wound healing), food industries, and tissue engineering. Collagen extraction from fish waste as a "blue resource" has attracted research attention over the past decade. Around 75 % of fish waste contains a high concentration of collagen. This has driven research in the conversion of these low-cost by-products into valuable products. Collagen extracted by acidic or/and enzymatic methods is gaining a lot of attention today due to its low cost and high yield. Fermentation and enzymatic hydrolysis stand out as one of the most environmentally sustainable and ecologically friendly methods for collagen extraction. Because of its great biocompatibility, excellent bioactivity, and low antigenicity, marine collagen is receiving more attention. Furthermore, collagen-derived peptides may exhibit interesting antioxidant activity, potent antihypertensive activity, and antimicrobial activity against different strains of bacteria. This review focuses on the advancements in extraction and detection methods of marine collagen, both from a technological and legislative standpoint, in addition to exploring its diverse range of application domains.

Current and Expected Trends for the Marine Chitin/Chitosan and Collagen Value Chains

Chitin/chitosan and collagen are two of the most important bioactive compounds, with applications in the pharmaceutical, veterinary, nutraceutical, cosmetic, biomaterials, and other industries. When extracted from non-edible parts of fish and shellfish, by-catches, and invasive species, their use contributes to a more sustainable and circular economy. The present article reviews the scientific knowledge and publication trends along the marine chitin/chitosan and collagen value chains and assesses how researchers, industry players, and end-users can bridge the gap between scientific understanding and industrial applications. Overall, research on chitin/chitosan remains focused on the compound itself rather than its market applications. Still, chitin/chitosan use is expected to increase in food and biomedical applications, while that of collagen is expected to increase in biomedical, cosmetic, pharmaceutical, and nutritional applications. Sustainable practices, such as the reuse of waste materials, contribute to strengthen both value chains; the identified weaknesses include the lack of studies considering market trends, social sustainability, and profitability, as well as insufficient examination of intellectual property rights. Government regulations, market demand, consumer preferences, technological advancements, environmental challenges, and legal frameworks play significant roles in shaping both value chains. Addressing these factors is crucial for seizing opportunities, fostering sustainability, complying with regulations, and maintaining competitiveness in these constantly evolving value chains.

Trends in fisheries waste utilization: a valuable resource of nutrients and valorized products for the food industry

The rise in fisheries production worldwide has caused a remarkable increase in associated anthropogenic waste. This poses significant concerns due to adverse environmental impacts and economic losses. Owing to its renewability, high abundance, and potential as a rich source of many nutrients and bioactive compounds, strategies have been developed to convert fish waste into different value-added products. Conventional and improved methods have been used for the extraction of biomolecules from fish waste. The extracted fish waste-derived value-added products such as enzymes, peptides, fish oil, etc. have been used to fortify different food products. This review aims to provide an overview of the nature and composition of fish waste, strategies for extracting biomolecules from fish waste, and the potential application of fish waste as a source of calcium and other nutrients in food fortification and animal feed has been discussed. In context to fishery waste mitigation, valorization, and circular bioeconomy approach are gaining momentum, aiming to eliminate waste while producing high-quality value-added food and feed products from fishery discards.

Fermented jellyfish (Rhopilema esculentum) collagen enhances antioxidant activity and cartilage protection on surgically induced osteoarthritis in obese rats

Collagen has been considered a key treatment option in preventing damage to the articular cartilage over time and supporting the healing process, following the onset of osteoarthritis (OA). This study aimed to investigate the effect of collagen fermented from jellyfish (FJC) by Bacillus subtilis natto on anterior cruciate ligament transection with medial meniscectomy (ACLT + MMx)-induced knee OA in high-fat diet (HFD)-induced obesity in rats. The male Sprague-Dawley rats were fed an HFD for 6 weeks before ACLT + MMx surgery, after which they were administered a daily oral gavage of saline (control, OA, and OBOA), either with FJC (20 mg/kg, 40 mg/kg, and 100 mg/kg body weight) or glucosamine sulfate as a positive control (GS; 200 mg/kg body weight) for 6 weeks. Treatment with FJC decreased the fat weight, triglyceride, and total cholesterol levels in obese rats. Additionally, FJC downregulated the expression of some proinflammatory cytokines, including tumor necrosis factor-α, cyclooxygenase-2, and nitric oxide; suppressed leptin and adiponectin expression; and attenuated cartilage degradation. It also decreased the activities of matrix metalloproteinase (MMP)-1 and MMP-3. These results demonstrated that FJC showed a protective effect on articular cartilage and also suppressed the degradation of cartilage in an animal OA model, suggesting its potential efficacy as a promising candidate for OA treatment.

In Vivo Skin Hydrating Efficacy of Fish Collagen from Greenland Halibut as a High-Value Active Ingredient for Cosmetic Applications

The industrial processing of fish for food purposes also generates a considerable number of by-products such as viscera, bones, scales, and skin. From a value-added perspective, fish by-products can act also as raw materials, especially because of their collagen content (particularly in fish skin). Interestingly, the potential of marine collagen for cosmetic applications is enormous and, remarkably, the extraction of this protein from fish skins has been established for different species. Using this approach, we investigated the integration of marine collagen (COLRp_I) extracted from the skin of the Greenland halibut as an active ingredient in a cosmetic hydrogel formulation. In this study, extracts of marine collagen at concentrations up to 10 mg/mL showed a non-cytotoxic effect when cultured with fibroblast cells for 3 days. In addition, marine collagen extract, when incorporated into a cosmetic hydrogel formulation, met criterion A of ISO 11930:2019 regarding the efficacy of the preservative system (challenge test). In addition, the cosmetic formulations based on marine collagen at dosages of 0.1, 0.25 and 0.5% were tested in a clinical study on the skin of the forearms of 23 healthy volunteers, showing a sightly hydration effect, suggesting its potential for beauty applications. Moreover, this work illustrates that the circular economy concept applied to the fish processing industry can represent important benefits, at innovation, environmental and economic levels.

3D Biocomposites Comprising Marine Collagen and Silica-Based Materials Inspired on the Composition of Marine Sponge Skeletons Envisaging Bone Tissue Regeneration

Ocean resources are a priceless repository of unique species and bioactive compounds with denouement properties that can be used in the fabrication of advanced biomaterials as new templates for supporting the cell culture envisaging tissue engineering approaches. The collagen of marine origin can be sustainably isolated from the underrated fish processing industry by-products, while silica and related materials can be found in the spicules of marine sponges and diatoms frustules. Aiming to address the potential of biomaterials composed from marine collagen and silica-based materials in the context of bone regeneration, four different 3D porous structure formulations (COL, COL:BG, COL:D.E, and COL:BS) were fabricated by freeze-drying. The skins of Atlantic cod (Gadus morhua) were used as raw materials for the collagen (COL) isolation, which was successfully characterized by SDS-PAGE, FTIR, CD, and amino acid analyses, and identified as a type I collagen, produced with a 1.5% yield and a preserved characteristic triple helix conformation. Bioactive glass 45S5 bioglass® (BG), diatomaceous earth (D.E.) powder, and biosilica (BS) isolated from the Axinella infundibuliformis sponge were chosen as silica-based materials, which were obtained as microparticles and characterized by distinct morphological features. The biomaterials revealed microporous structures, showing a porosity higher than 85%, a mean pore size range of 138-315 μm depending on their composition, with 70% interconnectivity which can be favorable for cell migration and ensure the needed nutrient supply. In vitro, biological assays were conducted by culturing L929 fibroblast-like cells, which confirmed not only the non-toxic nature of the developed biomaterials but also their capability to support cell adhesion and proliferation, particularly the COL:BS biomaterials, as observed by calcein-AM staining upon seven days of culture. Moreover, phalloidin and DAPI staining revealed well-spread cells, populating the entire construct. This study established marine collagen/silica biocomposites as potential scaffolds for tissue engineering, setting the basis for future studies, particularly envisaging the regeneration of non-load-bearing bone tissues.

Characterization of Acid- and Pepsin-Soluble Collagen Extracted from the Skin of Purple-Spotted Bigeye Snapper

Fish processing waste is a prospective source of collagen and a cost-effective environmental pollutant. The skin of the purple-spotted bigeye snapper (Priacanthus tayenus) was extracted utilising various acid soluble collagens (ASC) including acetic acid (AAC), lactic acid (LAC), citric acid (CAC) and pepsin soluble collagens (PSC). In this study, PSC (6.65%) had the highest collagen yield, followed by AAC (5.79%), CAC (4.15%), and LAC (3.19%). The maximum temperatures (Tmax) denaturation of AAC, LAC, CAC, and PSC were 31.4, 31.7, 31.5, and 33.2 °C, respectively. UV-VIS absorption spectra showed all extracted collagens had a range of absorbance at 230 nm, due to the presence of glycine, proline, hydroxyproline, and triple-helical collagen. Additionally, they exhibited amide A, B, amide I, II, and III peaks. SDS−PAGE identified all extracted collagens as type I. The PSC had a significantly higher (p < 0.05) hydroxyproline content than acidic extraction 66.3 ± 1.03 (mg/g sample). Furthermore, all samples were extremely soluble in acetic conditions at pH 5, and all collagen was soluble in NaCl up to 3% (w/v). Therefore, PSC was the best treatment since it did not impact collagen triple helical and acetic acid yielded the most collagen in ASC extraction. Overall, the analysis revealed that fish skin waste might be used as an alternate source of collagen in diverse applications, particularly in food applications.

In Vitro Antioxidant and Antiaging Activities of Collagen and Its Hydrolysate from Mackerel Scad Skin (Decapterus macarellus)

The skin of mackerel scad fish (Decapterus macarellus) is a new source for pepsin-soluble collagen and its hydrolysate, both of which have never been explored. This study aims to characterize and determine the in vitro antioxidant, antiglycation, and antityrosinase activity of pepsin-soluble collagen (PSC) and hydrolyzed collagen (HC) from mackerel scad skin. PSC was extracted using 0.5 M acetic acid containing 0.1% pepsin for 48 h at 4 °C. The obtained PSC was then hydrolyzed with collagenase type II (6250 U/g) to produce HC. The PSC yield obtained was 6.39 ± 0.97%, with a pH of 6.76 ± 0.18, while the HC yield was 96% from PSC. SDS-PAGE and Fourier Transform Infrared (FTIR) analysis showed the typical features of type I collagen. HC demonstrated high solubility (66.75–100%) throughout the entire pH range (1–10). The PSC and HC from mackerel scad skin showed antioxidant activity against 2,2-diphenyl-1-picrylhydrazyl (DPPH), with IC50 values of 148.55 ± 3.14 ppm and 34.966 ± 0.518 ppm, respectively. In the antiglycation test, PSC had an IC50 value of 239.29 ± 15.67 ppm, while HC had an IC50 of 68.43 ± 0.44 ppm. PSC also exhibited antityrosinase activity, with IC50 values of 234.66 ± 0.185 ppm (on the L-DOPA substrate), while HC had an IC50 value of 79.35 ± 0.5 ppm. Taken together, these results suggest that the skin of mackerel scad fish has potential antiaging properties and can be further developed for pharmaceutical and cosmetic purposes.

Bioactive Compounds and Therapeutics from Fish: Revisiting Their Suitability in Functional Foods to Enhance Human Wellbeing

Global public awareness about fish-based diet and its health/nutritional benefits is on the rise. Fish nutritional profile projects promising bioactive and other compounds with innumerable health benefits for human wellbeing. As various reported researches involving fish/marine-derived molecules reveal promising attributes, and as the position of fish-based nutrients as nutraceuticals continue to strengthen, health challenges still confront communities worldwide, from cardiovascular disease, diabetes, and obesity to hypertension. Thus, further understanding of fish-based nutrient impact as functional foods remains crucial given the diverse prevailing compositional/nutraceutical merits. In this review, therefore, we provide important information regarding bioactive compounds and therapeutics obtained from fish, specific to the context of their suitability in functional foods to enhance human health. This contribution is hereby constructed as follows: (a) fish nutraceutical/therapeutic components, (b) constituents of fish-based nutrients and their suitability in functional foods, (c) fish antioxidant/bioactive compounds to help alleviate health conditions, (d) common human ailments alleviated by fish-based nutrients, and (e) role of fish in mental health and immune system. As increased fish consumption should be encouraged, the potential of the quality proteins, omega-3 fatty acids, and other compounds inherent in fish should steadily be harnessed.

Extraction of Biocompatible Collagen From Blue Shark Skins Through the Conventional Extraction Process Intensification Using Natural Deep Eutectic Solvents

The disposal of large amounts of skin waste resulting from the blue shark fishing industry presents several industrial and environmental waste management concerns. In addition, these marine subproducts are interesting sources of collagen, a fibrous protein that shows high social and economic interest in a broad range of biomedical, pharmaceutical, and cosmetic applications. However, blue shark wasted skins are a poorly explored matrix for this purpose, and conventional collagen recovery methodologies involve several pre-treatment steps, long extraction times and low temperatures. This work presents a new green and sustainable collagen extraction approach using a natural deep eutectic solvent composed of citric acid:xylitol:water at a 1:1:10 molar ratio, and the chemical characterization of the extracted collagen by discontinuous electrophoresis, thermogravimetric analysis, Fourier transformed infrared spectroscopy and circular dichroism. The extracted material was a pure type I collagen, and the novel approach presented an extraction yield 2.5 times higher than the conventional one, without pre-treatment of raw material and reducing the procedure time from 96 to 1 h. Furthermore, the in vitro cytotoxicity evaluation, performed with a mouse fibroblasts cell line, has proven the biocompatibility of the extracted material. Overall, the obtained results demonstrate a simple, quick, cheap and environmentally sustainable process to obtain marine collagen with promising properties for biomedical and cosmetic applications.

Process optimization for extraction of avian eggshell membrane derived collagen for tissue engineering applications

The avian eggshell membranes’ composition depicts close resemblance with the extracellular matrix of the cells, and therefore being widely employed as potential biomaterials for tissue engineering applications. However, the optimization of process conditions for collagen extraction, the main constituent of eggshell membranes is still challenging. In the present study, extraction of collagen was performed by an enzymatic method optimized through the one-factor-at-a-time (OFAT) technique for three parameters viz. pepsin concentration, treatment time and pH. The process optimization resulted in the maximum yield of 56% collagen with 350 U/mg pepsin concentration at pH 3 treated for 9 days, not reported yet. The collagen extraction was confirmed by OD at 232 nm; and its viscoelasticity behaviour at pH 5. The physico–chemical characterization of extracted collagen with FESEM, ATR-FTIR, surface roughness analysis and contact angle measurement revealed the morphological and topological alteration during the collagen extraction. The process optimization and characterization of eggshell membrane derived collagen can aid in the significant biomaterials development for tissue regeneration.

A Review on Biphasic Calcium Phosphate Materials Derived from Fish Discards

This review summarizes the results reported on the production of biphasic calcium phosphate (BCP) materials derived from fish wastes (i.e., heads, bones, skins, and viscera), known as fish discards, and offers an in-depth discussion on their promising potential for various applications in many fields, especially the biomedical one. Thus, considerable scientific and technological efforts were recently focused on the capability of these sustainable materials to be transformed into economically attractive and highly valuable by-products. As a consequence of using these wastes, plenty of beneficial social effects, with both economic and environmental impact, will arise. In the biomedical field, there is a strong and continuous interest for the development of innovative solutions for healthcare improvement using alternative materials of biogenic origin. Thus, the orthopedic field has witnessed a significant development due to an increased demand for a large variety of implants, grafts, and/or scaffolds. This is mainly due to the increase of life expectancy and higher frequency of bone-associated injuries and diseases. As a consequence, the domain of bone-tissue engineering has expanded to be able to address a plethora of bone-related traumas and to deliver a viable and efficient substitute to allografts or autografts by combining bioactive materials and cells for bone-tissue ingrowth. Among biomaterials, calcium phosphate (CaP)-based bio-ceramics are widely used in medicine, in particular in orthopedics and dentistry, due to their excellent bioactive, osteoconductive, and osteointegrative characteristics. Recently, BCP materials (synthetic or natural), a class of CaP, which consist of a mixture of two phases, hydroxyapatite (HA) and beta tricalcium phosphate (β-TCP), in different concentrations, gained increased attention due to their superior overall performances as compared to single-phase formulations. Moreover, the exploitation of BCP materials from by-products of fish industry was reported to be a safe, cheap, and simple procedure. In the dedicated literature, there are many reviews on synthetic HA, β-TCP, or BCP materials, but to the best of our knowledge, this is the first collection of results on the effects of processing conditions on the morphological, compositional, structural, mechanical, and biological properties of the fish discard-derived BCPs along with the tailoring of their features for various applications.

Characterization of Marine Organism Extracellular Matrix-Anchored Extracellular Vesicles and Their Biological Effect on the Alleviation of Pro-Inflammatory Cytokines

Representative marine materials such as biopolymers and bioceramics contain bioactive properties and are applied in regenerative medicine and tissue engineering. The marine organism-derived extracellular matrix (ECM), which consists of structural and functional molecules, has been studied as a biomaterial. It has been used to reconstruct tissues and improve biological functions. However, research on marine-derived extracellular vesicles (EVs) among marine functional materials is limited. Recent studies on marine-derived EVs were limited to eco-system studies using bacteria-released EVs. We aimed to expand the range of representative marine organisms such as fish, crustaceans, and echinoderms; establish the extraction process; and study the bioactivity capability of marine EVs. Results confirmed that marine organism ECM-anchored EVs (mEVs) have a similar morphology and cargos to those of EVs in land animals. To investigate physiological effects, lipopolysaccharide (LPS)-infected macrophages were treated with EVs derived from sea cucumber, fish, and shrimp. A comparison of the expression levels of inflammatory cytokine genes revealed that all types of mEVs alleviated pro-inflammatory cytokines, although to different degrees. Among them, the sea cucumber-derived EVs showed the strongest suppression ability. This study showed that research on EVs derived from various types of marine animals can lead to the development of high value-added therapeutics from discarded marine wastes.

Marine-Derived Collagen as Biomaterials for Human Health

Collagen is a kind of biocompatible protein material, which is widely used in medical tissue engineering, drug delivery, cosmetics, food and other fields. Because of its wide source, low extraction cost and good physical and chemical properties, it has attracted the attention of many researchers in recent years. However, the application of collagen derived from terrestrial organisms is limited due to the existence of diseases, religious beliefs and other problems. Therefore, exploring a wider range of sources of collagen has become one of the main topics for researchers. Marine-derived collagen (MDC) stands out because it comes from a variety of sources and avoids issues such as religion. On the one hand, this paper summarized the sources, extraction methods and characteristics of MDC, and on the other hand, it summarized the application of MDC in the above fields. And on the basis of the review, we found that MDC can not only be extracted from marine organisms, but also from the wastes of some marine organisms, such as fish scales. This makes further use of seafood resources and increases the application prospect of MDC.

Diet Supplementation with Fish-Derived Extracts Suppresses Diabetes and Modulates Intestinal Microbiome in a Murine Model of Diet-Induced Obesity

Metabolic syndrome-related diseases affect millions of people worldwide. It is well established that changes in nutritional habits and lifestyle can improve or prevent metabolic-related pathologies such as type-2 diabetes and obesity. Previous reports have shown that nutritional supplements have the capacity to limit glucose intolerance and suppress diabetes development. In this study, we investigated the effect of dietary supplementation with fish-derived extracts on obesity and type 2 diabetes and their impact on gut microbial composition. We showed that nutritional supplements containing Fish Complex (FC), Fish Complex combined with Cod Powder (FC + CP), or Cod Powder combined with Collagen (CP + C) improved glucose intolerance, independent of abdominal fat accumulation, in a mouse model of diet-induced obesity and type 2 diabetes. In addition, collagen-containing supplements distinctly modulate the gut microbiome in high-fat induced obesity in mice. Our results suggest that fish-derived supplements suppress diet-induced type 2 diabetes, which may be partly mediated through changes in the gut microbiome. Thus, fish-derived supplements and particularly the ones containing fish collagen have potential beneficial properties as dietary supplements in managing type 2 diabetes and metabolic syndrome via modulation of the gut microbiome.

Biogenic Calcium Phosphate from Fish Discards and By-Products

Every year, millions of tons of fish waste are generated from fishing activities, and a similar amount is discarded and returned to the sea as unwanted catches. This material can be used as a biological source for many potential new added-value products, such asobtaining hyaluronic acid from fish eyeballs or extracting collagen from fish skin, but there are not many utilities for fish bones yet. This work tackles the transformation of fish discards into calcium phosphates. Discards from scorpionfish (Scorpaena scrofa) and Atlantic horse mackerel (Trachurus trachurus), as well as by-products generated from aquaculture activities (heads and trimmings frames) of salmon (Salmon salar), were used to obtain calcium phosphate. Biphasic carbonated hydroxyapatite (HA) /beta-tricalcium phosphate (TCP) material was obtained. The biphasic HA-TCP material has a promising range of applications in the biomedical field based on its similarity to calcium phosphates found in human bones in terms of crystallite size and carbonate content. The presence of Na, Mg, Sr, and K ions in the HA-TCP material is very beneficial, since they contribute to bone metabolism and cell adhesion.

Fish Waste: From Problem to Valuable Resource

Following the growth of the global population and the subsequent rapid increase in urbanization and industrialization, the fisheries and aquaculture production has seen a massive increase driven mainly by the development of fishing technologies. Accordingly, a remarkable increase in the amount of fish waste has been produced around the world; it has been estimated that about two-thirds of the total amount of fish is discarded as waste, creating huge economic and environmental concerns. For this reason, the disposal and recycling of these wastes has become a key issue to be resolved. With the growing attention of the circular economy, the exploitation of underused or discarded marine material can represent a sustainable strategy for the realization of a circular bioeconomy, with the production of materials with high added value. In this study, we underline the enormous role that fish waste can have in the socio-economic sector. This review presents the different compounds with high commercial value obtained by fish byproducts, including collagen, enzymes, and bioactive peptides, and lists their possible applications in different fields.

Extraction of Type I Collagen from Tilapia Scales Using Acetic Acid and Ultrafine Bubbles

Type I collagen is commonly used in medical materials and cosmetics. While it can be extracted from the skin and bones of mammals, marine collagen has attracted attention recently, since the use of mammalian collagen could result in zoonosis, and products containing mammalian collagen are avoided due to some religious beliefs. Chemical extractions using strong acids and alkalis, thermal extractions, and other nonconventional methods have been used for collagen extraction. However, there are few reports on environmentally friendly methods. Although heat extractions provide higher yields of collagen, they often cause collagen denaturation. Therefore, dilute acetic acid and ultrafine bubbles of oxygen, carbon dioxide, and ozone were used to extract type I collagen from tilapia scales. The extraction performance of the different conditions employed was qualitatively analyzed by SDS-PAGE electrophoresis, and the collagen concentration was quantified using circular dichroism spectroscopy by monitoring the peak intensity at 221 nm, which is specific to the triple helix of type I collagen. Collagen was extracted from tilapia scales with a yield of 1.58% by the aeration of ultrafine bubbles of carbon dioxide gas in a 0.1 M acetic acid solution for 5 h.