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BIOC 2580 > Carbohydrates > Flashcards

Carbohydrates Flashcards

1
Q

chemical properties of monosaccharides

A
  • very water soluble
  • poorly soluble in organic solvents
  • colourless
    formula: (CH2O)n
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2
Q

Monosaccharides

A

combine a carbonyl group which is either an aldehyde or ketone AND at least 2 carbons bearing hydroxyl groups (alcohol)
- can contain 3-7 carbon atoms
- hexoses are the most common

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3
Q

Emil Fischer

A

studied analysis, synthesis and stereochemistry of simple sugars and developed Fischer projections

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4
Q

Fischer projection and perspective formulas

A

Fischer projection: vertical bonds project behind the plane horizontal bonds to project out of the plane
Perspective formula: solid wedge shaped bonds project in front, dashed bonds point away

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5
Q

asymmetric carbon atoms

A
  • all monosaccharides except dihydroxyacetone contain one or more chiral carbon atoms
  • gives rise to occurrence of optically active isomeric forms
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6
Q

enantiomers

A
  • mirror images, differ in configuration at every chiral carbon
  • “left-handed” or “right-handed”
  • have identical chemical properties
  • differ in optical activity: plane of polarization of polarized light is bent in opposite directions
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7
Q

diasteromers

A
  • monosaccharides with more than one chiral carbon that differ in handedness at only some carbons
  • do not have identical properties because of different spatial relationships
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8
Q

D-sugars

A

chiral carbon furthest away from the carbonyl group has the same configuration as D-glyceraldehyde (OH on right)
- most common in nature

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9
Q

L-sugars

A

chiral carbon furthest away from the carbonyl group has the same configuration as L-glyceraldehyde (OH on left)

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10
Q

Epimers

A

pair of sugars that are identical except for the configuration at one carbon atom
- special case of diastereomers

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11
Q

how many stereoisomers will a sugar have?

A

2^n
n= number of chiral centres
- every time a carbon is added the number of isomers doubles

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12
Q

Hemiacetals ands Hemiketals

A

hemiacetal: aldehyde + alcohol
hemiketal: ketone + alcohol
- the original carbonyl carbon becomes chiral upon formation

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13
Q

cyclization of sugars - basic info

A
  • sugars have BOTH alcohol and aldehyde or ketone functional groups
  • hemiacetal or hemiketal formation is intramolecular
  • hemiacetal and hemiketal formations turn into ring structures
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14
Q

cyclization of glucose

A
  • the OH group at C5 reacts with the carbonyl carbon of the aldehyde group to form a stable ring
  • this renders C1 asymmetric, giving rise to alpha and beta forms
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15
Q

anomers

A

isomeric form of a molecule that differs only in configuration around hemiacetal or hemiketal carbon
- alpha and beta forms

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16
Q

cyclization chemistry for a ketose

A
  • same as for an aldose, except the OH group of C5(furanose) or C6(pyranose) reacts with the ketone group (usually C2)
  • the electrophilic carbonyl carbon reacts with the nucleophilic O of an OH group
17
Q

mutarotation

A
  • when dissolved in water a sugar slowly converts into an equilibrium mixture of the alpha, beta and linear forms
  • a solution of alpha glucose and one of beta glucose will form identical equilibrium mixtures over time (1/3 alpha, 2/3 beta and trace amounts of linear)
18
Q

what forms can sugars take on when they cyclize

A

pyranose: 6-membered
furanose: 5-membered
- size of the ring formed depends on relative thermodynamic stabilities of possible ring structures

19
Q

Haworth projections

A

for D sugars the anomeric OH is above in the beta form and below in the alpha form
- OH groups that are left in the fisher formula are above in Haworth

20
Q

converting Fischer structures to Haworth projections

A
  • carbonyl carbon is always the electrophile in the reaction
  • any OH group can act as the nucleophile
  • OH group that reacts depends on if the ring is pyranose or furanose
  • fructose can cyclize to a pyranose or furanose
  • if the OH of C6 is the nucleophile then fructose will be a 6 membered ring
21
Q

reducing sugars

A

carbonyl carbon can be oxidized to a carboxyl group by oxidizing agents (ex: Cu2+)
- Cu2+ gets reduced to Cu+ which forms red precipitate
- sugars that react like this are known as reducing sugars

22
Q

non-reducing sugars

A

sugars that can’t be oxidized by oxidants such as Cu2+
- lack a free aldehyde or ketone group

23
Q

conditions for sugars to be oxidized by Cu2+

A

can only occur in the linear form (with a carbonyl carbon; the aldehyde or ketone form) which exists in equilibrium with cyclic forms

24
Q

glycosides

A

the anomeric carbon of a sugar is electrophilic - its at this position most reactions involving ring sugars take place
- condensation of the anomeric carbon with the nucleophilic OH or NH is the most important sugar rxn
- resulting molecule is a glycoside
- bond formed is glycosidic (C-O) or glycosilic (C-N)

25
Q

What happens if the anomeric carbon is involved in a glycosidic bond?

A
  • it becomes a non-reducing sugar
  • if the anomeric carbon is in a glycosidic bond the sugar can no longer open up from ring to linear form
  • since only open chain forms can undergo oxidation by Cu2+ the sugar becomes non-reducing
  • when dissolved remains in ring structure
26
Q

disaccharides

A
  • formed when 2 monosaccharides are linked through a glycosidic bond
  • the aromatic carbon (electrophile) of one sugar reacts with the hydroxyl group (nucleophile) of another
  • disaccharides are glycosides
27
Q

naming disaccharides

A

state configuration the sugar with the anomeric carbon reacting followed by an arrow and which C the hydroxyl group of the other sugar reacting is on
Lactose: gal(B1->4)glc

28
Q

reducing and non-reducing disaccharides

A
  • the end of a chain with a free anomeric carbon is called the reducing end
  • anomeric carbon of galactose is unable to undergo maturation since it is in a glycosidic bond
  • the anomeric carbon of glucose is free and able to undergo maturation
  • lactose is a reducing sugar
29
Q

basic info on disaccharides

A
  • formation of a disaccharide must involve the anomeric C of one sugar as its the only electrophilic carbon
  • the OH on any carbon of the sugar could be the nucleophile
  • galactose and glucose could form lots of disaccharides that would be structural isomers of lactose
30
Q

sucrose

A
  • disaccharide of glucose and fructose
  • ## anomeric carbons of both glucose and fructose are involved in the glycosidic bond, therefore is non-reducing (no reducing end)
31
Q

2 of many glucose disaccharides

A

maltose: glc(a1->4)glc
trehalose: glc(a1<->a1)glc
- maltose has a reducing end, trehalose does not

32
Q

polysaccharides

A
  • often highly branched
  • branched since sugars have many OH groups that can act as a nucleophile in forming glycosidic bonds
  • differ from each other in types of sugars that are linked, length of the chains, type of bonds and the degree of branching
33
Q

chiral carbon centers

A

carbon atoms that are attached to 4 different substituents
- not involved in double bonds

34
Q

anomeric carbon

A
  • the carbon of a cyclic sugar bearing a hemiacetal or hemiketal
  • C1 in aldoses C2 in ketoses
35
Q

lactose

A
  • disaccharide of galactose and glucose gal(B1->4)glc
  • able to act as a reducing sugar because it has a free reducing end

BIOC 2580 (28 decks)

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