ReviewFungal laccases as green catalysts for dye synthesis☆
Highlights
► The natural possibility of laccase to produce coloured compounds. ► The connection between research and application. ► The short list of patents related to production of coloured compounds by laccase.
Introduction
Nowadays, there is an increasing demand on the chemical industry to develop eco-friendly processes with the use of biocatalysts that represent an attractive route towards one-step safe synthesis. Biocatalysis not only can improve the selectivity of a reaction but, by doing so, it can also decrease or even remove the need for downstream processing, thereby reducing the material and energy waste associated with product purification. Biocatalysis has many advantages in the context of green chemistry such as mild reaction conditions (physiological pH and temperature) and often fewer steps than conventional chemical procedures. Consequently, classical chemical procedures are being increasingly replaced by cleaner biocatalytic alternatives, especially in the fine chemicals industry. Green catalytic alternatives are particularly needed in oxidation processes, which are still carried out with inorganic (or organic) oxidants such as chromium (VI) compounds, permanganate, manganese dioxide, and periodate. There is clearly a definite need for catalytic alternatives employing clean primary oxidants such as oxygen and environmentally compatible catalysts such as enzymes. The use of enzymes generally circumvents the need for functional group activation and avoids protection and deprotection steps required in traditional procedures. Usually the biocatalytic procedures are shorter, generate less waste and are therefore both environmentally and economically attractive [1].
According to Straathof and co-workers, the industrial biocatalysis process should describe a reaction or a set of simultaneous reactions in which a pre-formed precursor molecule is converted into specific product, rather than a fermentation process with de novo production from a carbon and energy source. Biocatalysis should involve the use of enzymes and/or whole cells, or combinations thereof, either free or immobilized and should lead to production of a fine-chemical or commodity product that is usually recovered after the reaction [2].
In preindustrial times, textiles were dyed primarily with plant and animal dyes. Today synthetic dyes are used almost exclusively. The first synthetic dye, called mauve or mauveine, was discovered by a young English chemist William Henry Perkin in 1856 and it marked the advent of the synthetic dye industry, based on coal tar as the raw material. Mauveine had intense bluish purple colour, and dyed silk in a rich colour that did not wash out nor fade upon exposure to sunlight over one week. This dye was patented and then manufactured; at present it is known as diazine dye “aniline purple” or “Perkin's purple” [3], [4]. In the last 150 years, several million different colour compounds have been synthesized, with about 15,000 colourants produced on an industrial scale over the time. Colourants are synthesized in a reactor, then filtered, dried, and blended with other additives to obtain the final product. A typical synthesis process involves different reactions such as sulfonation, halogenation, amination, diazotization, and coupling, followed by separation processes that may include distillation, precipitation, and, finally, crystallization. Many efforts should be made to substitute degradable and less toxic precursors during synthesis of different colour chemicals. About 40% of globally used colourants contain organically bound chlorine, a known carcinogen [5].
The scale and growth of the dye industry has been linked to that of the textile industry. In 2005, the global market size for dyes, pigments and intermediates was estimated at around $ 23 billion. The global dyestuff production is estimated to be around 34 million tones and the annual global sales of textile dyestuff alone is estimated approximately at around $ 6 billion [6]. The two most important textile fibres are cotton and polyester; consequently, dye manufacturers tend to concentrate their efforts on producing dyes for these two fibres. Among the current field of research on dyes, a marked trend appears focusing on a more exhaustive isolation and study of natural pigments. Studies on eco-friendly oxidation processes represent an important alternative in the synthetic reactions. Enzyme-catalysed oxidations in the presence of air as co-substrate are low cost reactions that use non-toxic reagents in aqueous solutions and are promising in the synthesis of new textile dyes or in new synthesis procedures of known textile dyes.
Colourants are characterized by their ability to absorb or emit light in the visible range from 400 to 700 nm. In terms of the chemical structure, colourants may be either inorganic or organic compounds. Both groups can be further subdivided into natural and synthetic representatives. This latter differentiation is, however, not always meaningful enough, since today, many natural colourants are produced synthetically for example alizarin or indigo [7]. Colourants may also be classified on a colouristic basis, for example in terms of the application method or specific field of application. One group of colourants are dyes that are applied for dyeing various substrates (textiles, leather, paper, hair), and form a solution in which they are completely, or at least partly, soluble. In contrast to the other group of colourants – pigments, which are practically insoluble in the media in which they are applied, dyes must possess a specific affinity to a given substrate [8]. The most appropriate system for classification of dyes is based on their chemical structure. The major classes of dyes are azo (mono-, di-, tri-, etc.), carbonyl (anthraquinone and indigo derivatives), cyanine (di- and triphenylmethane and phtalocyanine) dyes and all of these compounds contain chromophoric groups, which give rise to colour [8].
Laccases from different sources catalyse oxidation of various phenolic and aromatic compounds to products that very often are colourful and may be used as dyes, especially in the textile industry. In literature, there is a lot of information that products of the oxidative action of different laccases in the natural environment are colourful [9], [10], [11], [12], [13]. Because of this, the technological potential of laccases as eco-friendly biocatalysts has long been recognised and numerous studies of their use in various bioconversion processes have been conducted [14]. Especially fungal laccases show very strong oxidative potential, thus facilitating their use as oxidative catalyst in de novo synthesis of dyes for the textile industry [15], [16]. As extracellular enzymes, fungal laccases are easily extractable from different culture media without necessity of application heterogeneous expressions. In the present review, therefore, we aim to describe the natural abilities of laccases to synthesize colour products with respect to the type of the enzymatic reaction, catalyst characterization and possible use of these colour products as dyestuffs. This review summarizes also patent reports relating to the use of laccases for the synthesis of dyes.
Section snippets
The sources of laccases and their functions in the natural environment
Laccases (EC 1.10.3.2) are enzymes belonging to copper-containing oxidoreductases characterized by broad substrate specificity, which determines their biological role in the natural environment. The main sources of laccase are higher plants [17], [18], [19] and most fungi [20]; it is also present in bacteria [21], [22], [23], [24], [25], [26], and even in insects where it participates in sclerotization and pigmentation [27], [28], [29], [30], [31]. Bacterial laccases play a very important role
The mechanism of laccase-mediated reaction
The active centre of a typical fungal laccase contains four atoms of copper (II), representing three different spectroscopic classes, marked as types 1–3 (T1, T2 and T3, respectively). The T1 copper atom shows an absorption band at 610 nm, which is responsible for the blue colour of laccase [49], [50], [51]. The electrons released from substrates are transferred sequentially to the T1 copper and the T2/T3 centre present in the active centre of laccase and then to molecular oxygen, resulting in
Types of laccase-mediated reactions
The reactions catalysed by laccase can be divided into three groups, in all which coloured products are formed [14], [45], [58]:
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direct oxidation of simple phenolic derivatives (the type A reaction; Fig. 2),
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mediated oxidation of phenolic and non-phenolic substrates in the presence of mediators (the type B reaction; Fig. 3),
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coupling of reactive radicals formed by the action of laccase (the type C reaction; Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8).
All mentioned three types of reactions catalysed by
Dyes obtained due to the laccase action
Most natural products synthesised by laccase due to oxidative coupling reactions are colour compounds that play different roles in the physiology of organisms. The ability of laccase to produce reactive quinonic species which can be used subsequently in tandem reactions or dimerisation transformations provides a powerful synthetic route towards coloured natural products and coloured new structures. By mimicking the natural function of laccase, which produces numerous pigments through oxidation
Patents relating to laccase-based synthesis of coloured compounds
During the past few years, biocatalysis has been the focus of intense scientific research and is now a well-established technology within the chemical industry. Associated patents have been published together with scientific papers regarding laccase-mediated synthesis of new structures that can be used as colourants in the textile industry. Most studies on laccase originating from different sources have been patented, including its properties, heterologous production and the molecular cloning
Concluding remarks and prospects
There is a high global research and increasing interest in the development and application of enzymes in the textile industry. This is demonstrated by the high number of published scientific papers and patents in this topic, which continues to increase every year. Because of their versatile biochemical properties, high protein stability, broad of substrate spectrum, laccases are the key enzymes having applications in biotechnological processes, especially in the textile and pulp and paper
Acknowledgments
This work was partially supported by SOPHIED (NMP2-CT-2004-505899) and the National Science Centre (6330/B/P01/2011/40).
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This paper is in memory of Sophie Vanhulle who inspired us in the study of the use of enzymes for dye synthesis. Sophie was the scientific coordinator of the project SOPHIED and this paper is a continuation of the main subject of that large research project (2004–2008). Sophie accidentally deceased on September 12th, 2009.