Which compound has the same molecular formula?

2.33.4 Biotransformations as a Source of Chiral Compounds

Isomeric compounds have the same molecular formulas but they present a different sequence of bonding of their atoms or different arrangements of their atoms in space. Compounds having the same sequence of covalent bonds but different disposition of their atoms in space are called stereoisomers. If the pair of stereoisomers presents nonsuperimposable mirror images, they are referred to as enantiomers. When a molecule has more than one chiral center, and the compounds that are stereoisomers to one another are not enantiomers, they are called diastereomers. The four diastereomers of carveol are depicted as example in Figure 1.

The organoleptic and other properties of stereoisomers and diastereomers often differ (Figure 2). The three-dimensional structure of a compound can also greatly affect its therapeutic action. Biological receptors are generally proteins showing a preference for one of the enantiomeric or diastereomeric ligands, and, in some cases, the inactive isomer can block the site that should be occupied by the active isomer. To obtain effective and desired chiral building blocks, agrochemicals and pharmaceuticals, and fragrances and food additives, a great effort should be done to produce the necessary enantiomerically pure compounds. Furthermore, while in some cases one enantiomer of the pair is just inactive, in other cases toxicity has been linked to one of the enantiomers, not necessarily the active isomer [15]. Although a large number of drugs, aromatic compounds, and pharmaceuticals are racemic mixtures, in most cases, only one of the enantiomers carries out the pretended function, and different pharmacokinetic behavior of the enantiomers may be observed. Besides, in some cases, one enantiomer of the pair is just inactive, while in other cases toxicity could be linked to one of the enantiomers. Secondary effects can even be provoked by the unwanted enantiomeric compound; granulocytopenia, vomiting, and myasthenia are related to the d-isomer of levodopa, levamisole, and carnitine, respectively [15].

Successful processes of kinetic resolutions of racemic substances using enzymes and microorganisms are becoming common. The isomerization, racemization, epimerization, and rearrangement of molecules, which may occur under the conditions required for chemical synthesis, are prevented with biocatalysis. The biotransformations can be carried out under mild conditions and are usually highly enantio- and regioselective. Furthermore, most of the fragrances and food additives produced by biocatalysis can be recognized as GRAS (generally recognized as safe) substances. This can be an important asset to increasingly health- and nutrition-aware consumers.

The use of several enzymes employed in biocatalysis, with emphasis also on the enantioselectivity of the processes, has been reviewed by Davis and Boyer [8]. The examples provided include optically pure compounds produced by lipases, esterases, acylases, and nitrilases. Patel has reviewed the synthesis of chiral pharmaceutical intermediates by biocatalysis, including the enzymatic synthesis of the antianxiety drug 6-hydroxybuspirone and the antidiabetic drug 2-(3-hydroxy-1-adamantyl)-2-oxoethanoic acid, and the human immunodeficiency virus (HIV) protease inhibitor Atazanavir [28].

In the case of whole cells, the selection of a biocatalyst for a desired kinetic resolution can be achieved by making the enantioselectivity of an expressed enzyme variant as the condition for the survival of the cells [27]. If the desired enantiomer is produced, a nutrient to facilitate cell growth is released, while if the unwanted enantiomer is obtained, a toxic compound is produced to inhibit the growth of the unable cells. This will result in an overgrowth of the cells expressing the desired enantiomer. In the future, it should be possible to design and engineer biotransformation processes to produce chiral compounds in high yields.

1.1 Molecular Chirality

The chemical compounds that possess identical molecular formulae, although they differ in their chemical structures, are called isomers [1–4]. The physical, chemical, and biological properties of these isomers can be different. For a given chemical formula, there could be different isomeric structures with similar or different functional groups. On the basis of structural arrangement the isomers are further classified as structural isomers or constitutional isomers and stereoisomers. In structural isomers the constituting atoms are identical, but they possess different connectivity or arrangement of groups or atoms. On the other hand the isomers having similar atomic connectivity with different spatial arrangements are known as stereoisomers. An sp3-hybridized carbon with four different substituents gives rise to nonsuperimposable mirror images, which are called enantiomers. The enantiomers are a subclass of stereoisomers and the molecules with nonsuperimposable mirror images are called chiral. These enantiomers can exhibit different physical, chemical, and biological properties. On the other hand the mirror images of the molecules, such as CCl4 and CHCl3, are superimposable and are called achiral. The carbon atom containing four different substituents is known as chiral carbon and also called the stereogenic carbon. The enantiomers are optically active molecules, and hence, they can rotate the plane-polarized light in opposite directions by the identical amount. Enantiomers that rotate the plane-polarized light in the clockwise direction that is to the right are called dextrorotatory (d) and given symbol (+). Those enantiomers that rotate plane-polarized light in the anticlockwise direction, that is, to the left, are called as levorotatory (l) and are given the symbol (−). Except for the difference in the optical rotation property, all other physical properties of enantiomers, such as melting point, boiling point, and solubility, are identical. The absolute configurations of such molecules are represented by “R” and “S.” As far as carbohydrates or amino acids are concerned, the configuration is represented by “d” and “l,” instead of R and S. The equal proportional mixture of enantiomers (1:1 ratio) is called racemic and is optically inactive. In nature the preferential occurrence of l- and d-type carbohydrates and amino acids is not the same. The naturally occurring carbohydrates are always d type and the naturally occurring amino acids are always l type. The molecules with one preferred type of enantiomeric form are called homochiral. There is a method adopted for assigning the configuration of stereogenic unit(s) in a given molecule [5–7].

1 Introduction

Butanol is a C4 alcohol with molecular formula C4H9OH. Butanol has four isomeric structures, including n-butanol, 2-butanol, isobutanol, and tet-butanol, all of which can be produced by thermochemical reactions with inorganic catalysts from fossil fuels. Butanol as a bulk fundamental chemical is widely used as precursor in organic synthesis, and as solvents in food industry and various chemical processes. n-Butanol is considered superior to ethanol as a fuel. n-Butanol has a high energy density of 29.2 MJ/L (compared to 19.5 MJ/L of ethanol and 16 MJ/L of methanol), and can replace gasoline (energy density 32 MJ/L) without modifying current internal combustion engines. Butanol is less hygroscopic and less corrosive than ethanol, therefore can be easily transported in pipelines as compared to ethanol, which must be transported in trucks or barges. The low volatility of butanol also makes it less explosive than ethanol. The world butanol demand is estimated to be worth $247 billion by the 2020s (Green, 2011).

Bio-based n-butanol can be produced by ABE (acetone–butanol–ethanol) fermentation by solventogenic Clostridium. Isobutanol can be produced by fermentation via 2-keto acid-based pathway, which has been introduced into Escherichia coli and Clostridium. Clostridium are Gram-positive, spore-forming, and anaerobic bacteria that can produce a variety of chemicals including butanol, ethanol, hexanol, acetone, acetic acid, butyric acid, hexanoic acid, lactic acid, 1,3-propanediol, 2,3-butanediol, isopropanol, propionic acid, etc. (Papoutsakis, 2008; Tracy et al., 2012). Acetone, n-butanol, and ethanol are produced in a weight ratio of 3:6:1 by solventogenic Clostridium (primarily C. acetobutylicum, Clostridium beijerinckii, Clostridium saccharoperbutylacetonicum, or Clostridium saccharobutylicum) in ABE fermentation (Cho et al., 2015; Papoutsakis, 2008). ABE fermentation can be divided into two phases: acidogenic phase, where cell growth occurs and acids (acetic acid, butyric acid) are the main metabolites; and solventogenic phase, where acids are reassimilated and solvents are produced. Later, fermentation ceases and cells form endospores. Due to their biphasic metabolism, it is difficult to operate ABE fermentation in a continuous process; therefore, batch or fed-batch fermentation is typically used (Papoutsakis, 2008).

The first report of n-butanol production was in 1862 (Pasteur, 1862). Solventogenic Clostridium, C. acetobutylicum, was isolated in 1915 by Chaim Weizmann after the implementation of ABE fermentation in the United Kingdom in 1912 (Durre, 2007, 2008). The aim of ABE fermentation during the World War I was to produce acetone for cordite explosive propellants. Later, butanol gradually became the main target product, and two-thirds of the world butanol were produced via ABE fermentation in the 1950s with maize or molasses as substrates (Durre, 2011). Later, butanol production gradually shifted to petrochemical routes due to decreased crude oil prices and increased substrate prices. Conventional ABE fermentation utilizes corn or sugarcane molasses as substrates, which are expensive and may compete with food supply, and it was estimated that substrate cost for biobutanol accounts for 60%–80% of the overall biobutanol cost (Lee et al., 2016; Pfromm et al., 2010). Clostridium can utilize a large array of substrates, including hexose, such as glucose, fructose, and galactose; pentose, such as xylose; glycerol; disaccharide, such as cellobiose, sucrose, and lactose; polysaccharide, such as cellulose, hemicellulose, and xylan; syngas; amino acid; etc. (Tracy et al., 2012). While current biobutanol is produced from corn or sugarcane, lignocellulosic- and syngas-based butanol have been extensively studied and may provide a more cost-competitive and environment-friendly biobutanol production process (Lee et al., 2016). The substrate diversity of Clostridium makes them ideal hosts for biofuel production.

Biobutanol production by ABE fermentation suffers from low titer (usually < 15 g/L) due to the high cytotoxicity of butanol, which causes high separation cost (Xue et al., 2017). n-Butanol and water form a heterogeneous azeotrope, and is generally separated by two distillation columns and a decanter, which is energy intensive and increases capital cost. A number of researchers worked on improving butanol tolerance by metabolic engineering, or relieving the toxicity of butanol by high-cell-density fermentation or in situ butanol removal.

In recent years, the interest of biobutanol production revived due to world energy and environment crisis; however, biobutanol production by Clostridium still cannot compete with petrochemical-based butanol for price, mainly attributed to its high production cost and operation complexity. An array of researches have been conducted aiming at improving fermentation yield, titer and productivity, constructing strains suitable for industrial applications, utilization of various inexpensive substrates, and optimizing the overall production process. This chapter reviews recent advances in the abovementioned aspects of butanol production by Clostridium, focusing on metabolic engineering for high-production strains, process engineering for high fermentation efficiency, and utilization of cheap feedstocks for economic butanol production. Industrial biobutanol producers and potential for industrialization are also discussed. Finally, possible future directions of butanol production by Clostridium are suggested.

4.3.2.1 Stereoisomers

Isomers are different compounds that have the same molecular formula but the atoms are attached in different ways. There are two classes of isomers (Figure 4.4): (1) constitutional isomers and (2) stereoisomers. Constitutional isomers (or structural isomers) differ in their bonding sequence, and their atoms are connected differently and the number of constitutional isomers increases dramatically with the increase of carbon atoms in each compound. For example, there are 2 constitutional isomers of butane (C4H10: n-butane and isobutane), 3 isomers of pentane (C5H12: n-pentane, isopentane and neopentane), respectively, 5 isomers of hexane, 18 isomers of octane, 75 possible isomers of decane, and 355 possible isomers of eicosane (C20H42), respectively.

Stereoisomers are isomers whose atoms are bonded together in the same sequence but differ from each other in the orientation of the atoms in space. Stereoisomers that are mirror images of each other (i.e., differing in the same manner as right and left hands) are called enantiomers; while all other stereoisomers, which are not mirror images, are diastereomers. The cis–trans geometric isomers (such as cis- and trans-1,2-dimethylcyclopentane) are special types of diastereomers. Enantiomer molecules are not superimposable. Many pairs of biomarkers with the same molecular formula (such as 22R and 22S homohopane homologous series in C31–C35 range) are enantiomers. Differences in special orientation might seem unimportant, but stereoisomers often have remarkably different physical, chemical, and biological properties.

6.2 The Preparation of Xylose

Xylose is a kind of pentose; its molecular formula is C5H10O5 and its structural formula is shown in Fig. 6.2. Hemicellulose is a polysaccharide of d-xylose in nature. The industrial production of xylose has been gradually maturing. Xylose is often used as a food sweetener and is the source of xylitol. The utilization of xylose and hemicellulose has been paid much attention with the development of high-value utilization of biomass. As a widespread natural sugar unit, the utilization of xylose as a carbon resource in the fermentation industry has become a “hot spot.” The technology of the fermentation of xylose to produce xylitol is mature. Acetone–butanol fermentation from xylose is also feasible. The use of xylose to produce ethanol is a global research hot spot. Consequently, the crisis of energy and food can be solved, if xylose is used as a carbon source in fermentation.

Commonly, industrial microorganisms can metabolize glucose for their growth, but not all industrial microorganisms are able to utilize xylose. For example, in bioethanol production, the most widely used ethanol-producing bacterium is yeast (Saccharomyces cerevisiae). Yeast does not have the ability to rapidly metabolize xylose. To solve this problem, many researchers try to transform the existing microbial metabolism process in order to obtain optimal strains by genetic engineering (Hamacher, Becker, & Gardonyi, 2002).

The metabolic pathway of xylose in vivo can be briefly stated as follows: first, the xylose is transported into the cell by a special membrane, then xylose is transformed into xylulose by the action of an enzyme, and then the xylulose is metabolized by the PPP. The bottlenecks in the utilization of xylose in many microorganisms are the transport of xylose and the transformation of xylose to xylulose.

Can 2 compounds have the same chemical formula?

What are compounds with the same molecular formula but different arrangements?