catalysts Review Biotechnological Methods of Sulfoxidation: Yesterday, Today, Tomorrow Wanda M ˛aczka,Katarzyna Wi ´nska* and Małgorzata Grabarczyk Department of Chemistry, Wrocław University of Environmental and Life Sciences, Norwida 25, 50-375 Wrocław, Poland; [email protected] (W.M.); [email protected] (M.G.) * Correspondence: [email protected]; Tel.: +48-71-320-5213 Received: 31 October 2018; Accepted: 25 November 2018; Published: 5 December 2018 Abstract: The production of chiral sulphoxides is an important part of the chemical industry since they have been used not only as pharmaceuticals and pesticides, but also as catalysts or functional materials. The main purpose of this review is to present biotechnological methods for the oxidation of sulfides. The work consists of two parts. In the first part, examples of biosyntransformation of prochiral sulfides using whole cells of bacteria and fungi are discussed. They have more historical significance due to the low predictability of positive results in relation to the workload. In the second part, the main enzymes responsible for sulfoxidation have been characterized such as chloroperoxidase, dioxygenases, cytochrome flavin-dependent monooxygenases, and P450 monooxygenases. Particular emphasis has been placed on the huge variety of cytochrome P450 monooxygenases, and flavin-dependent monooxygenases, which allows for pure sulfoxides enantiomers effectively to be obtained. In the summary, further directions of research on the optimization of enzymatic sulfoxidation are indicated. Keywords: sulfoxides; biotransformation; enzymatic oxidation 1. Introduction Sulfur is an element that is about one percent of the dry mass of the human body because many sulfur-organic compounds are involved in metabolism. They are not only amino acids (cysteine, methionine) and their derivatives, but also glutathione and sulfolipids. Important enzyme cofactors also contain sulfur such as biotin, thioredoxin, lipoic acid, and coenzyme A [1]. Strong poisons can be found among the sulfur-containing compounds. For example, the consumption of Amanita mushrooms can cause fatal human poisoning and the toxin responsible for this effect is amanitin, which is an R-sulfoxide [2]. The organo-sulfur compounds are also an important group of secondary plant metabolites. They were isolated, among others, from onions, radishes, and cress. The characteristic smell and healing properties of plants of the genus Allium are attributed to sulfur-containing compounds, including chiral sulfoxides such as a compound characteristic of garlic: alliin (3-(2-propenylsulfinyl)-L-alanine) [3]. Since many compounds containing sulfur in their structure perform important biological functions, an attempt was made to develop methods for the synthesis of sulfur-based compounds, among which, the largest group are sulfoxides. It is noteworthy that the sulfur atom is a stereogenic center if it is linked to two different alkyl or aryl substituents, as well as to an oxygen atom and a lone electron pair. However, the bond between sulfur and oxygen is indirect between the coordination bond and the polarized double bond [4]. It does not form a typical π bond like between carbon and oxygen. It is possible to obtain stable enatiomers of sulfoxides because the lone electron pair is not inverted. The value of energy needed to change the configuration on sulfur is in the range of 35–43 kcal mol−1 Catalysts 2018, 8, 624; doi:10.3390/catal8120624 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 27 Catalysts 2018, 8, 624 2 of 27 of 35–43 kcal mol−1 for a typical sulfoxide [1,5]. The first examples of optically active sulfoxides were described in 1926, and the first general method for the synthesis of enantiomeric sulfoxides was reportedfor a typical in 1962 sulfoxide [6]. [1,5]. The first examples of optically active sulfoxides were described in 1926, andSince the first then, general the chemistry method forof chiral the synthesis sulfoxides of enantiomerichas been very sulfoxides popular am wasong reported researchers in 1962 because [6]. these Sincecompounds then, thehave chemistry found many of chiral applications sulfoxides [7–10]. has been They very are popularused as amongchiral ligands researchers in enantioselectivebecause these compounds catalysis. They have foundhave also many found applications wide applications [7–10]. They as are chiral used asauxiliaries chiral ligands and intermediatesin enantioselective in asymmetric catalysis. synthesis. They have High also asymmetric found wide induction applications was observed as chiral when auxiliaries using chiral and sulfinyl.intermediates In addition, in asymmetric several drugs synthesis. containing High asymmetricchirotopic sulfur induction in sulfoxide was observed are used when in using the pharmaceuticalchiral sulfinyl. industry, In addition, such several as esomeprazole drugs containing or modafinil chirotopic [11]. sulfur in sulfoxide are used in the pharmaceuticalDue to their industry, importance, such a as significant esomeprazole effort or ha modafinils been made [11]. to develop efficient methods for their preparation.Due to their importance,There are three a significant main ways effort to obtain has been optically made active to develop sulfoxides: efficient methods for their preparation. There are three main ways to obtain optically active sulfoxides: • nucleophilic substitution of a chiral precursor; •• asymmetricnucleophilic sulphoxidation substitution of of a chiralprochiral precursor; sulfides; •• kineticasymmetric separation sulphoxidation of racemic ofsulfoxides. prochiral sulfides; • Inkinetic this work, separation we will of focus racemic onsulfoxides. discussing the biotechnological methods of oxidation of sulfides becauseIn thisbiocatalysis work, we is willundoubtedly focus on discussing a method theof gentle biotechnological and environmentally methods of friendly oxidation synthesis of sulfides of sulfoxidesbecause biocatalysis [12]. is undoubtedly a method of gentle and environmentally friendly synthesis of sulfoxides [12]. 2. Biotransformations in Cultures of Microorganisms 2. BiotransformationsThe use of whole cells in Cultures of bacteria, of Microorganismsfungi, and yeast in the asymmetric oxidation of sulfides is an excellentThe alternative use of whole to cellsreactions of bacteria, catalyzed fungi, using and pure yeast enzymes. in the asymmetric Such biotransformations oxidation of sulfides are much is an cheaperexcellent and alternative more convenient to reactions than catalyzed enzymatic using reaction pures. enzymes. In addition, Such the biotransformations costly co-factors, are such much as nicotinamidecheaper and adenine more convenient dinucleotd thane/nicotinamide enzymatic reactions.adenine dinucleo In addition,tide phosphate the costly (NADH/NADPH), co-factors, such as arenicotinamide avoided. The adenine costs dinucleotde/nicotinamide associated with the purifi adeninecation dinucleotide of enzymes phosphate also disappear. (NADH/NADPH), It is also extremelyare avoided. important The costs that associated some withmicroorganism the purifications can of simulate enzymes alsothe disappear.metabolism It isof alsomammalian extremely cytochromeimportant thatP450 some monooxygenases microorganisms (CYPs) can simulate[13–17]. the metabolism of mammalian cytochrome P450 monooxygenases (CYPs) [13–17]. 2.1. Fungus and Yeast 2.1. Fungus and Yeast The reactions of asymmetric sulfoxidation with fungi and yeasts are carried out successfully in the secondThe reactions half of the of last asymmetric century [18] sulfoxidation. In 1982, Holland with fungi and and Carter yeasts published are carried a paper out successfullyin which they in presentedthe second the half results of the of last the centurybiotransformation [18]. In 1982, of Hollandaryl-substituted and Carter phenyl published and benzyl-methyl a paper in which sulfides they usingpresented the Mortierella the results isabellina of the biotransformation strain. These compounds of aryl-substituted have been phenyl oxidized and benzyl-methylto sulfoxides sulfidesduring enzymaticusing the oxidationMortierella with isabellina oxygenstrain. from Thesethe atmosphe compoundsre. In havethe studies, been oxidized the authors to sulfoxides focused mainly during onenzymatic determining oxidation the rate with and oxygen mechanism from the of atmosphere. enzymatic Inoxidation the studies, [19]. the In authors subsequent focused studies, mainly the on authorsdetermining confirmed the rate that and benzylic mechanism hydroxylation of enzymatic and oxidation sulfoxidation [19]. Inreactions subsequent can be studies, catalyzed the authors using theconfirmed same enzyme that benzylic in M. hydroxylation isabellina. This and enzyme sulfoxidation has the reactions characteristics can be catalyzed of cytochrome using the P450 same monooxygenaseenzyme in M. isabellina [20]. This enzyme has the characteristics of cytochrome P450 monooxygenase [20]. FurtherFurther research research by by the the team team led led by by Holland Holland show showeded that that the the phenyl-alkyl phenyl-alkylatedated sulfoxides sulfoxides of of the the (S(S) )configuration configuration are are the the main main biotransformation biotransformation prod productsucts of of the the respective respective sulfides sulfides catalyzed catalyzed using using thethe strain strain HelminthosporiumHelminthosporium sp. NRRL 4671 [[21].21]. SubsequentSubsequent studiesstudies conducted conducted under under the the guidance guidance