Maltose
Maltose, also known as malt sugar, is a disaccharide composed of two D-glucose molecules linked by an α-1,4-glycosidic bond between the anomeric carbon of one glucose unit and the hydroxyl group on carbon 4 of the other.[1] This structure, specifically 4-O-α-D-glucopyranosyl-D-glucopyranose, renders it a reducing sugar due to the presence of a free hemiacetal group on the second glucose unit, allowing it to participate in reactions like mutarotation and reduction of Benedict's solution.[2] With the molecular formula C₁₂H₂₂O₁₁·H₂O (monohydrate) and a molar mass of 360.32 g/mol, maltose appears as white, odorless crystals that are highly soluble in water (approximately 1080 g/L at 20°C) but less sweet than sucrose, at about 30-50% of its sweetness.[3][4] Naturally occurring in germinating cereal seeds such as barley, where it arises from the enzymatic breakdown of starch reserves to provide energy for sprouting, maltose also forms during the partial hydrolysis of polysaccharides like starch and glycogen in plants and animals.[4][2] Commercially, it is produced on a large scale through the acid or enzymatic hydrolysis of starch using amylases (such as alpha-amylase and beta-amylase) or diastase, yielding high-maltose corn syrups that contain 40-80% maltose for use in food processing.[2] In brewing, maltose is liberated when germinating barley (malt) acts on starch, contributing to fermentation by yeast.[4] Biologically, maltose serves as an intermediate metabolite in humans, Saccharomyces cerevisiae (yeast), Escherichia coli, and other organisms, where it is hydrolyzed by the enzyme maltase (α-glucosidase) in the small intestine into two glucose molecules for absorption and energy production via glycolysis.[1] Congenital sucrase-isomaltase deficiency, which impairs maltase activity, can cause maltose intolerance and gastrointestinal symptoms upon ingestion.[2][5] Beyond nutrition, maltose finds applications as a sweetening agent in confectionery, beverages, and pharmaceuticals, and in specialized uses like the maltose-neopentyl glycol (MNG) amphiphiles for stabilizing membrane proteins in structural biology research.[2] Its physical properties, including a melting point of 102-103°C (monohydrate) and density of 1.54 g/cm³, make it suitable for these industrial roles.[3]History
Discovery
Maltose's discovery emerged in the context of 19th-century efforts to develop alternative sweeteners from starch, driven by cane sugar shortages in Europe during the Napoleonic era, when Napoleon's continental blockade restricted imports from England and its colonies. This spurred investigations into enzymatic starch hydrolysis as a means to produce fermentable sugars for brewing and food applications.[6] In 1847, French chemist Augustin-Pierre Dubrunfaut first isolated the disaccharide through the action of malt extract (containing diastase) on starch, describing it as a distinct product he termed "glucose de malt" to highlight its relation to glucose but with different properties, such as lower reducing power. Dubrunfaut's observation marked the initial recognition of maltose as a unique entity in saccharification processes, though his characterization was limited by the analytical techniques of the time.[7][8] Dubrunfaut's findings gained widespread acceptance only after confirmation by Irish chemist and brewer Cornelius O'Sullivan in 1872. In his foundational paper "On the Transformation-Products of Starch," O'Sullivan analyzed the saccharification of starch by malt diastase, isolating and crystallizing the disaccharide, which he named maltose to reflect its origin from malt. He demonstrated its composition as a dimer of glucose units, its reducing properties, and specific optical rotation ([α]_D ≈ +130°), distinguishing it definitively from glucose and dextrin. O'Sullivan's rigorous experiments, including hydrolysis studies, established maltose's role in malting and brewing, influencing carbohydrate research thereafter.[9]Scientific development
The scientific study of maltose emerged in the 19th century, closely intertwined with advancements in the sugar and starch industry, spurred by a shortage of cane sugar in continental Europe due to Napoleon's continental blockade that prohibited imports from Britain and its allies.[10] In 1812, Russian chemist Gottlob Kirchhoff demonstrated that starch could be hydrolyzed into a sweet syrup using diluted sulfuric acid, providing an early method for converting starch to fermentable sugars and highlighting the potential of starch as a sugar source.[10] This breakthrough paved the way for enzymatic approaches to starch breakdown. In 1833, French chemist Anselme Payen isolated diastase (now known as α-amylase), the first enzyme identified, which catalyzes the partial hydrolysis of starch into maltose and dextrins, marking a key milestone in understanding biological starch degradation.[10] Maltose itself was first isolated in 1847 by French chemist Augustin-Pierre Dubrunfaut through the action of malt extract (containing amylase) on starch, where he obtained a crystalline product distinct from glucose and termed it "glucose de malt" based on its origin from malt. However, Dubrunfaut's findings were not immediately accepted due to limited characterization. Confirmation came in 1872 from Irish chemist and brewer Cornelius O'Sullivan, who systematically studied the transformation products of starch under enzymatic action, isolated maltose as a well-defined, reducing disaccharide with properties intermediate between glucose and starch, and named it maltose. O'Sullivan's work, including osmotic pressure measurements and optical rotation studies, established maltose as a distinct entity and linked it directly to brewing processes.[7][8][9] In the 20th century, research shifted toward the biochemical and structural aspects of maltose. Mid-century studies by Albert Dahlqvist introduced the first quantitative assay for maltase activity in intestinal mucosa, enabling precise measurement of enzymes that hydrolyze maltose to glucose and advancing knowledge of carbohydrate digestion in mammals.[10] Giorgio Semenza's group further elucidated the molecular basis of maltose hydrolysis by identifying maltase-glucoamylase and sucrase-isomaltase as multi-domain complexes in rabbit small intestine, providing insights into their catalytic mechanisms and evolutionary conservation.[10] The crystal structure of maltose monohydrate was determined in 1970 using X-ray diffraction, revealing its monoclinic lattice and confirming the α-1,4-glycosidic linkage between two D-glucose units in a detailed atomic model.[11] These developments solidified maltose's role as a fundamental unit in starch metabolism and industrial applications.Chemical structure and nomenclature
Molecular structure
Maltose is a disaccharide composed of two D-glucopyranose units linked together by an α-(1→4) glycosidic bond between the anomeric carbon of the first glucose and the hydroxyl group on carbon 4 of the second glucose.[1] The molecular formula of maltose is \ce{C12H22O11}, reflecting the combination of two hexose units minus one water molecule from the dehydration synthesis that forms the glycosidic linkage.[1] In its predominant cyclic form, both glucose residues exist as six-membered pyranose rings, with the non-reducing glucose unit fixed in the α configuration at its anomeric carbon due to the glycosidic bond.[12] The reducing end of the molecule, corresponding to the second glucose unit, features a free anomeric carbon (C1) that can adopt either α or β configurations, resulting in an equilibrium mixture of α-maltose and β-maltose in aqueous solution.[13] This mutarotation occurs via ring opening to an aldehyde intermediate, allowing interconversion between the anomers.[13] The systematic IUPAC name for maltose is (2R,3S,4S,5R,6R)-2-(hydroxymethyl)-6-[(2R,3S,4R,5R)-4,5,6-trihydroxy-2-(hydroxymethyl)oxan-3-yl]oxyoxane-3,4,5-triol, which specifies the stereochemistry at each chiral center in the cyclic structure.[1] Synonyms such as 4-O-α-D-glucopyranosyl-D-glucose highlight the structural relationship to glucose, emphasizing the substitution at the 4-position of the reducing glucose.[1] The molecule's reducing property arises from the free hemiacetal group at the reducing end, enabling reactions typical of aldehydes under basic conditions.[12]Nomenclature and isomers
Maltose, also known as malt sugar or maltobiose, is a disaccharide consisting of two D-glucose units. Its systematic International Union of Pure and Applied Chemistry (IUPAC) name is (2R,3S,4S,5R,6R)-2-(hydroxymethyl)-6-[(2R,3S,4R,5R)-4,5,6-trihydroxy-2-(hydroxymethyl)oxan-3-yl]oxyoxane-3,4,5-triol.[1] In standard carbohydrate nomenclature, it is more commonly expressed as 4-O-α-D-glucopyranosyl-D-glucopyranose or α-D-Glcp-(1→4)-D-Glcp, highlighting the α-glycosidic bond from the anomeric carbon (C1) of the non-reducing glucose to the C4 position of the reducing glucose.[1][2] As a reducing sugar, maltose features a free anomeric carbon on the reducing-end glucose, allowing it to exist in equilibrium between α- and β-anomeric forms in aqueous solution, with the β-anomer typically predominant due to its lower energy state.[14] Maltose represents one structural isomer among several glucobioses—disaccharides formed exclusively from two D-glucose units—differentiated by the position and stereochemistry of the glycosidic linkage. These isomers exhibit varying biological roles and stabilities based on bond configuration; for instance, the α-1,4 linkage in maltose enables enzymatic hydrolysis by amylase, unlike the β-linkages in some counterparts.[15] Common glucobiose isomers include cellobiose (β-1,4 linkage, a cellulose hydrolysis product), isomaltose (α-1,6 linkage, found in amylopectin), gentiobiose (β-1,6 linkage, found in gentian root and some plant glycosides), and trehalose (α-1,1 linkage, a non-reducing stabilizer in organisms).[15][16] The diversity arises from the multiple hydroxyl groups available for linkage on glucose, leading to at least eight positional isomers for glucobioses, though not all occur naturally in significant quantities.[17]| Isomer | Glycosidic Linkage | Notable Occurrence or Property |
|---|---|---|
| Maltose | α-1,4 | Produced during starch digestion; reducing sugar |
| Cellobiose | β-1,4 | Repeat unit in cellulose; hydrolyzed by cellulase |
| Isomaltose | α-1,6 | Branch point in glycogen/amylopectin |
| Gentiobiose | β-1,6 | Found in gentian root and some plant glycosides |
| Trehalose | α-1,1 | Non-reducing; protects against desiccation in insects and fungi |