Etiket Arşivleri: Carbohydrates

Carbohydrates ( Prof. Dr. Artemis KARAALİ )

Chapter 3
CARBOHYDRATES
Carbohydrates
Simple Sugars(Monosaccharides)
Smallest unit: Cannot be broken down by mild acid hydrolysis
(CH2O)n where n≥3
When n = 5 then C5H10O5, xylose
When Cn=6, then C6H112O6, glucose, fructose
Polyalcohols with aldehyde or ketone functional group
Nomenclature
Chiral Carbons
A carbon is chiral if it has four different groups attached to it
Chiral compounds have the same composition(isomers), but they are not superimposable. They are optically active, meaning that they rotate the plane of polarized light. If the direction of rotation is to the right(+), we call it “dextro-” or D-, if to the left(-), we call it “levo-”, or L-.
in Fisher projection
Ring Formation
Some Important Monosaccharides
PENTOSES: Monosaccharides with five C atoms
Occur rarely as free monosaccharides; widely as component of polysaccharides(these are known as pentosans).
ALDOPENTOSES:Xylose, arabinose, ribose
KETOPENTOSES:Ribulose, xylulose
XYLOSE: occurs in corn cobs and fruits like cherry, peach, pear and plum, brans .
ARABINOSE: occurs in gums, pectins, mucilages, in fruits like apples, figs, grapefruits.
RIBOSE: occurs as component of nucleic acids, coenzymes, part of riboflavine, and DNA(deoxyribonucleic acid)
HEXOSES: Monosaccharides with six C atoms
More widely distributed in plants and animals
ALDOHEXOSES: 4 chiral centres and a total of 16 possible aldohexose stereoisomers (24), but only three commonly occur in nature: D-glucose, D-galactose, and D-mannose.
KETOHEXOSES:3 chiral centres and therefore eight possible stereoisomers (23). Commonly occurring ones are:
D-fructose and D-sorbose.
D-Glucose(dextrose, blood sugar, grape sugar)
D-Fructose(Levulose-Rotation in polarimeter is to the left)
Mannose
An aldohexose
Has been detected in oranges, olives and also in some polysaccharides produced by microorganisms.
Oligosaccharides: Consist of 2-10 monosaccharide units linked together by covalent bond called “glycosidic bond”
-Disaccharides
-Trisaccharides
Disaccharides
In forming a disaccharide, the anomeric C of one sugar molecule interacts with hydroxyl group of the other sugar molecule, resulting in the loss of a H2O molecule(hydrogen atom from one monosaccharide and a hydroxyl group from the other).
R-OH + HO-R’ à R-O-R’ + H2O
The general formula of disaccharides is C12H22O11.
Three most common disaccharides are maltose, lactose and sucrose
When hydrolyzed using acid or an enzyme, the following monosaccharides are produced from these disaccharides:
Sucrose(Cane or beet sugar)
Commercially obtained from sugar cane or sugar beet,also known as table sugar
Because the glucose units are linked at their anomeric C’s (O points down from ring), the glycosidic linkage is called a(1®2).In this linkage, neither monosaccharide can undergo mutorotation(it is fixed), and so sucrose is not a reducing sugar(Sugars which form open chain structures with free carbonyl group)
Sucrose
Sucrose is easily hydrolyzed by acids and the enzyme sucrase, which is secreted in the small intestine.The glucose and fructose can then be absorbed into the bloodstream (disaccharides are too large to be absorbed)
Sugar hydrolysis yield glucose and fructose (invert sugar)
( sucrose is dextrorotatory: +66.5o ; glucose +52.5o; fructose –92o; so the angle of rotation of invert sugar is: -19.8o. We say it iss “inverted”)
Used to make confectionary
Maltose(Malt sugar)
Obtained from sprouted barley(malt) : Malt sugar
Two glucose pyranose rings linked by an a-1-4 bond
Maltase enzyme splits it
Mild sweetness characteristic flavor
The glucose unit on the left is fixed, but the right one is free; so since its right ring can open and close and undergo mutarotation, maltose is a REDUCING SUGAR
Lactose(milk sugar)
~5% in cow milk (~50% milk solids). Does not occur elsewhere.
Commercially prepared from whey left after cheese-making and is used in infant formulations
Composed of galactose & glucose, with b(1®4) glycosidic linkage from the anomeric OH of galactose. Its full name is b-D-galactopyranosyl-(1® 4)-a-D-glucopyranose
Since galactose part opens and closes, lactose is a REDUCING sugar
Lactase deficiency leads to lactose intolerance. (More resistant than sucrose to acid hydrolysis).Undigested lactose is broken down by lactic acid bacteri in the large intestines to lactic acid and causes flatulence and diarhea.
Other Oligosaccharides
Trisaccharide: raffinose (glucose +galactose + fructose)
Tetrasaccharide: stachyose (2 galactoses, +glucose + fructose)
Pentasaccharide: verbascose (3 galactoses+ glucose + fructose)

Laboratory‎ > ‎Carbohydrates

CARBOHYDRATES

The term carbohydrate was originally used to describe compounds that were literally “hydrates of carbon” because they had the empirical formula Cn(H2O)n. Carbohydrates are the most abundant class of organic compounds found in living organisms. They originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll.

The Chemistry of Food Lecture 1 ( Dr. AN Boa )

Carbohydrates – general The Chemistry of Food • Source of energy and sweetness (see later) Lecture 1 Chemistry in Context • Carbohydrates consist of single units known as monosaccharides or multiple units thereof. 06525/06529/06509 • Simple sugars (mono- and disaccharides) • Oligo- and polysaccharides (sugar polymers) Dr AN Boa • Many stereoisomers possible 1 3 Topics in Food Chemistry Polarimetry analyzing Physical and chemical properties of: monochromator l polarizer α • Carbohydrates α – saccharides (mono-, di-, oligo- and poly-) • Proteins c c – amino acids and polypeptides • Lipids sample cell – oils and fats light source polarizer • Colourings • Flavour chemicals Observer rotates analyzing polarizer to view maximum / minimum • Vitamins light and records angle α • Minerals • Water Length of sample cell l(dm) is known, concentration c (g.cm-3) • Preservatives may be known or unknown

Simple Sugars Monosaccharides • Glucose • Monosaccharides mainly exist as cyclic hemiacetals – also known as grape sugar, corn sugar or dextrose • Attributed names on the basis of the ring size, e.g. furanose (5-ring, c.f. furan) and pyranose (6 ring, c.f. pyran) – dextrorotatory (+) – made commercially by hydrolyzing starch • Fructose – also known as fruit sugar or laevulose – laevorotatory (-) (but in the D-sugar series!) – made by hydrolyzing starch and isomerizing the glucose from the corn syrup hemiacetal 5 • α- and β- anomers of the cyclic forms exist 7 Monosaccharides Cyclisation of linear sugars H • General Formula (CH O) O 2 n O O O • Polyhydroxy carbonyl 5 H 5 compounds, e.g. aldoses 1 1 (aldehyde based) and ketoses (ketone based) Carbonyl carbon freely rotates • Carbon content can vary →O can attack either (typically 3-8) for each type H face of C=O OH of saccharide, e.g. pentoses (C5) and hexoses 5 O OH 5 O H (C6) 1 1 FISCHER PROJECTIONS α-anomer β-anomer

The anomeric effect Mutarotation • When compared to the β-anomer, the α-anomer is often much more stable than • Single molecule in solution can interchange expected so that in many cases [α-anomer] > [β-anomer]. between – straight chain and ring α-pyranose β-pyranose – different ring sizes – αand βanomers straight chain • Mutarotation is α-furanose β-furanose – a dynamic equilibrium – all isomers can potentially exist in solution • There is a stereoelectronic preference for conformations in which the best – energy / stability of different forms vary donor lone pair, or bond, is antiperiplanar to the best acceptor bond. 9 11 Sucrose and Lactose: Monosaccharides disaccharides • Fructose • Sucrose – Also known as cane sugar or beet sugar – Obtained commercially from sugar beet or sugar cane – Only one isomer present in solution – Easily crystallized + • Lactose – Also known as milk sugar – Primary sugar in mammalian milk (human > cow) Fischer projection Haworth projection – Also is a by-product of cheese manufacture – Not very sweet (relative intensity ≤0.3 cf sucrose) • Conversion of either anomer into the equilibrium mixture is called mutarotation – Hydrolyzed by enzyme β-galactosidase. Many adults lack this enzyme and are therefore lactose intolerant

Glycosidic linkages Invert sugars • Disaccharides are formed by linking two monosaccharides • Invert sugar is produced by acid hydrolysis of sucrose via a glycosidic link (acetal). • Produces a mixture of glucose and fructose H2O + sucrose →glucose + fructose [α]D +66.5º +52.7 º – 92.4º SUCROSE LACTOSE – Rotation of solution goes from dextrorotatory to laevorotatory overall – Process is called “inversion” of sucrose – producing “invert sugar” 13 15 Tri- and tetrasaccharides Polysaccharides Raffinose • Digestible α-D-galactopyranosyl-(1J6)-α-D-glucopyranosyl-(1J2)- – Starches (glycogen / amylose / amylopectin) β-D-fructofuranose • Indigestible (dietary fibre / roughage) Stachyose – Cellulose α-D-galactopyranosyl-(1J6)- α-D-galactopyranosyl- (1J6)-α-D-glucopyranosyl-(1J2)-β-D-fructofuranose – Pectin – Seaweed polysaccharides (agarose / carageenan) • Both found in legumes (peas, beans, soya) – Xanthan and gellan (gums) • But neither hydrolysed nor digested by humans…… • providing “a feast” for bacteria (e.g. E. coli) in the gut which produce H2 and CO2 causing flatulence.

Glycogen Cellulose • Glycogen is the major carbohydrate storage • Cellulose is a a linear glucan with β-(1,4)-glycosidic molecule in animals. linkages – Humans lack cellulase enzymes, so cellulose is indigestible for us • It is a homopolymer of glucose (a glucan) with α-(1,4)-linkages. • Different stereochemistry of glycosidic bond compared to starches [i.e. β−(1,4)- vs. α-(1,4)-] – results in very different molecular shape in solution • It is also highly branched, with α-(1,6) branch linkages occurring every 8-10 residues. 17 19 Starches: amylose and amylopectin Amylose vs. Cellulose • Starch is the major form of stored carbohydrate in plant cells e.g. grains, potatoes etc. • Unbranched starch is called amylose and branched starch is called amylopectin. – Amylose : linear glucan with α-(1,4)-glycosidic linkages and ~104 subunits – Amylopectin : branched glucan with α−(1,4) and α-(1,6) linkages with ~106 subunits: more compact than amylose for same MW • Structure is identical to glycogen, except for a much lower degree of branching (about every 20-30 Amylose helix Cellulose sheets residues). 18 (Solomons and Fryhle, 2000; © John Wiley & Sons, Inc.)

Pectin • Polysaccharide of D-galacturonic acid (and derivatives) joined by α-(1,4)-linkages – can contain up to 20% neutral sugars – found naturally in fruits, vegetables – charged at higher pH – can form gels – used in the making of jams / preserves 21 Other food polysaccharides Polysaccharide Subunits Linkages Characteristics Alginate D-mannuronic α−(1,4) + Charged, linear + L-guluronic β−(1,4) Agarose D-galactose α−(1,3) + Neutral, linear β−(1,4) Carrageenan sulphonated α−(1,3) + Charged (-SO3-), D-galactose β-(1,4) linear Guar Gum D-mannose & α−(1,6) + Neutral, branched D-galactose β−(1,4)