Monosaccharide reactions, oligosaccharides and polysaccharides

hydroxy functional group reactions

can be converted into esters or ethers or can be oxidised to aldehydes, ketones or carboxylic acids

carbonyl functional group reactions

can react with nucleophiles or can be reduced or oxidised

reactivity of C1 (anomeric carbon) in cyclic sugars

Anomeric carbon is the most reactive, as it has two C-O single bonds. It’s prone to nucleophilic attack because of the oxygens’ electron withdrawing effect.

glucose acetal formation

hemiacetal monosaccharides react with alcohols and acid to give acetals at the anomeric centre

naming monosaccharide acetals

glycosides. Exact name determined by the monosaccharide they’re derived from eg glucose acetals are *glucosides*

glycoside hydrolysis

Presence of 2 O at anomeric centre means glycosides undergo hydrolysis in presence of acid to give a hemiacetal sugar and alcohol (aglycone).

What are reducing sugars?

sugars with an OH group at the anomeric centre (hemiacetals). The hemiacetal group is in equilibrium with the open chain form (aldehyde or ketone).

Tollens reagent

Solution of ammoniacal silver nitrate (contains Ag(I)). Adding a reducing sugar forms silver metal which forms a silver mirror

Benedict’s reagent

Aqueous solution containing Cu(II). On reaction with a reducing agent, copper (I) oxide (Cu2O) is formed as a brick-red precipitate

How was Benedict’s reagent used in medicine?

can be used to detect elevated glucose levels in urine to diagnose diabetes

Non reducing sugars and oxidising agents

Don’t react. They are acetals at the anomeric centre (glycosides)

oxidation of sugars

Oxidation of primary alcohol in an aldose gives -uronic acid eg oxidation of D-glucose at C6 gives D-glucuronic acid. These undergo intramolecular esterification to give lactones eg ascorbic acid.

oligosaccharides

polysaccharides that hydrolyse to give 2 - 10 monosaccharides. Often associated with proteins and lipids.

Most common oligosaccharides

Disaccharides - the two component monosaccharides can be the same or different. Disaccharides contain an  O-glycosidic (acetal) bond between C-1 on one sugar and any hydroxy group on the second sugar.

starch hydrolysis

Diastase (enzyme) hydrolysis of starch gives the disaccharide maltose.

cellulose hydrolysis

Partial chemical hydrolysis of cellulose gives the isomeric cellobiose.

structure of maltose and cellobiose

composed of two glucose units linked by a 1-4’ glycosidic linkage. The anomeric carbon C-1 is an alpha-anomer (maltose) or beta-anomer (cellobiose).

lactose

Disaccharide which only naturally occurs in milk, concentration species dependent from 0 to 7%. Made up of D glucose and D galactose

lactose intolerance

Many adults have low levels of intestinal enzyme beta-D-galactosidase enzyme. Ingested lactose moves to the colon, where bacterial fermentation produces CO2, H2 and organic acids.

sucrose

Most naturally abundant disaccharide, used as table sugar. Not a reducing sugar and doesn’t undergo mutarotation as there’s no hemiacetals

sucrose hydrolysis

gives a mixture of D-glucose and D-fructose called ‘invert sugar’. Sucrose  \[a\]D = + 66.6. Invert sugar  \[a\]D = - 22. Enzyme catalysing the process =beta-D-fructofuranoside (‘invertase’)

producing invert sugar in cooking

heating sucrose with a little lemon juice/cream of tartar

what are polysaccharides?

*glycans* are carbohydrates with 10’s, 100’s or 1000’s of simple sugars linked by glycosidic bonds.

heteropolysaccharides

composed of more than one type of monosaccharide unit

homopolysaccharides

Most common. Composed of just one type of monosaccharide unit.

glucans

glucose homopolysaccharides

galactans

galactose homopolysaccharides

cellulose

Linear structural polysaccharide, the most abundant organic molecule in the biosphere. Composed of D glucose units joined by 1, 4 beta glycosidic linkages

cellulase

Higher animals eg humans don’t have the enzymes that hydrolyse 1,4’-β-linkages, so cellulose can’t be used ‘directly’ as a food source.

starch

Starch and its derivatives are the second most abundant polysaccharide, found in plants and animals. Found in microscopic plant granules

amylose

has between 10^2 and 10^3 D-glucose monomers linked by 1,4’-a-glycosidic bonds.

digestion of starch

It’s hydrolysed by glycosidase enzymes which hydrolyse alpha-glycosidic links.

Why is D glucose stored as a polymer rather than as a large number of monomer units?

To avoid large osmotic pressures. Osmotic pressure from 1000 aqueous glucose monomers would be 1000 times 1 amylose molecule with 1000 glucose units linked together which would lead to cell membranes rupturing.

amylopectin

branched polysaccharide. Contains 1,4’-alpha and 1,6’-alpha-glycosidic linkages (responsible for the branching). Distance between branch points is 24 to 30 glucose units.

glycogen

Energy storage glucose polysaccharide in animals containing 1,4 and 1,6 alpha glycosidic links. Highly branched with 1,6 glycosidic links every 6 to 8 glucose units.

how is glycogen degraded for metabolic use?

Glucose phosphorylase hydrolyses the terminating group and adds a phosphate → glucose-1-phosphate. Glycogen phosphorylase can’t cleave linkages closer than four glucoses from a branch point.

glycogen metabolism

Human metabolism consumes 160g carbs/day. 75% is consumed as glucose by the brain, and is the brain’s only fuel except upon prolonged starvation. Most of carbohydrate intake is via starch

glycogen debranching enzyme

Transfers 1,4’-alpha-linked trisaccharide unit from a ‘limit branch’ to the non-reducing end of another branch. Remaining 1,6’-alpha-linked glucose hydrolysed by the same enzyme

polysaccharide secondary structure

Results from local conformational variety due to rotations about the single bonds involved in glycosidic linkages. Glycosidic bond nature important as certain angle combinations are more stable

angles of 1,4 glycosides in alpha links

have 2 torsional angles to consider, phi and theta. In alpha links this results in a gentle turn that produces a helix when extended.

benefit of sugar units rotating 180 to the next in beta chains

reduces steric repulsion and maximises number of H bonds able to form

1,6’ glycoside torsional angles

three angles to consider, phi, theta and w : there’s a much wider range of conformations available

polysaccharide tertiary structure

concerns the way the entire polysaccharide backbone is arranged in three dimensional space.

polysaccharide quaternary structure

concerns the way polysaccharide chains aggregate with other polysaccharide chains eg ribbons forming sheets and sheets stacking on top of each other.

What dictates how the chain folds and packs together?

non-covalent , long-range interactions between the functional groups present in the monosaccharide units eg hydrogen bonding or charge-charge interactions

ribbons

linear arrangement of beta linked glucose units in cellulose/N-acetylglucosamine units in chitin. Uniform distribution of hydroxy groups on outside of chains.

cellulose ribbons and hydrogen bonding

Ribbbon stabilised by intramolecular H bonds between OH groups on adjacent glucoses. They associate laterally to form sheets stabilised by intermolecular H bonds. Inter-sheet hydrogen bonding → further association.

flexible helices

Alpha-glycosidic bond results in chains with wide hollow helix stabilised by H bonding

How does the iodine-starch test work?

Small molecules can be accommodated into polysaccharide helices’ central cavities to give inclusion complexes. Aqueous solution of I2 and I- forms a blue-violet colour starch inclusion complex.

why is the blue-black colour produced in the starch-iodine test?

arises from charge-transfer interactions between the rows of triiodide anions, \[I3\]-, arranged end-to-end in the amylose cavity.

composition of starch

amylose (insoluble in cold water, 20 % of starch) and amylopectin (soluble in cold water, 80 % of starch)

Angles of 1,4 glycosides in beta links

2 torsional angles to consider, phi and theta. For beta links it results in a zig zag pattern with sugars rotated by 180 degrees in relation to the next. forming ribbons

advantage of beta ribbons

When two or more cellulose/chitin chains make contact, OH groups are ideally arranged to make hydrogen bonds.