Carbohydrates Flashcards
What is the ficher projection formula
- represent 3 dimensional sugar strucutres on paper
- vertical binds project beind plans
- horiozontal bonds project out of plane
What is a perspective formula
- used dashes and wedges
What is significant about the carbon atoms in monosaccharides
- all monosaccharides except dihydroxyacetone contain one or more chiral carbon atoms
- gives rise to the occurence of optically active isomeric forms
What are enantiomers
- differ in configuration at every chiral carbon
- have identical chemical properities
- differ in optical activity
What are diastereomers
- monosaccharies with more than one chiral carbon atom have stereoisomers that differ in handedness at some carbon atoms but not at others
- diastereomers do NOT have identical chemical properties, because spactial relationship among the atoms making up the moleucle are different
How do you assign D and L to sugars with more than 3 carbon atoms
- D sugar if chiral carbon atom furthest away from the corbonyl group has same configuration as D-glyeraldehyde
- those with configuration similar to L-glyeraldegyde care called L sugars
- most usgars are D sugars
What are epimers
pair of sugars that are identical except for the configuration at one carbon atom
- special case of diastereomers
How many stereoisomers does a sugar was n charial carbons have
2^n
- an aldose with 5 carbons, 3 chiral centers 2^3 = 8 sterioisomers
- hlaf of the stereoisomers are D sugars, other half are L
- every time a C is added, the number of isomers double
What are hemiacetals and hemiketals
- aldehydes and ketones can react wtih alcholhols to produce hemiacetals and hemikatals
*original carbonyl carbon becomes chiral upon formation of hemiacetals and hemiketals
Explain the cyclization of glucose
- OH group at C-5 reacts with the Carbonyl carbon of the aldehyde group to form a stable 6-membered ring
- this renders C-1 asymmetric giving rise to 2 stereoisomers alpha and beta
- these isomeric form differ only in thier configuration around the meniacetal or hemiketal carbon (anomers)
Explain the cyclization chemistry for a Ketose
- same theme as for an aldose
- electrophile carbnoyl carbon atom reacts with the nucleophilic O of an OH group
What is mutarotation
- solid glucose found either in the alpha or beta form for the hemiacetal
- when dissoled in water, slowly converts into an equilibrium mixture of the alpha beta and linear form
- called mutarotation
- with time a soution of a-D-glucose and a solution of b-D-glucose form identical equilibrium mixtures and identical optical properties
- approx 1.2 alpha and 1/2 beta with trace amount linear
What are the two forms of rings that crystalized sugars can form
- pyranose (6-membered)
- furanose (5-membered)
- size of ring formed depends on relative thermodynamic stabilities of the various possible ring structures, this depends on geometry of the molecule
Explain the stereochemistry of ring forms: Haworth projections
for D sugars:
- OH groups are
- LEFT in Fischer formula
- ABOVE in Haworth
- Anomeric OH
- B form = above ring
- a form = below ring
*LAB Ledt=above=beta
How do you convert Fischer structures to Haworth Porjections
- carbonyl carbon always the electrophile in the reaction (will form anomeric carbon)
- any of the OH groups that is present can act as a nuceleophile
- which OH reacts with the carbonyl C depends on the type of ring that is formed (pyranose or furanose)
- if the OH of carbon 6 is a nucleophile, then fructose will form a 6-membered pyranose ring
- once you have drawn the ring apply “left above beta above”
How are sugars reduced
- the carbonl carbon of sugars can be oxidized to a corboxyl group by oxidizing agents such as a cupric ion
- the Cu 2+ gets reduced to cuprous ion (Cu +) which forms a red precipitate (clear colour change)
- any sugar that reacts like this is called a reducing sugar
*must contain a corbonyl carbon
- some sugars dont react with oxidants: they are “non-reducing sugars”
- this chemical property provides a simple basis for detecting the presence and concentration of a sugar like glucose
*for oxidation of sugars by Cu2+ only occurs with the linear form which exists in equilibrium with cyclic forms
What are glycosides
- anomeric carbon atom of a sugar (only carbon attached to two oxygen atoms) is electrophilic
- this makes it reactive it this position that most reactions involving ring forms of sugars take place
- the condensation of anomeric carbon with the nucleophilic -OH of an alochol or the NH of an amine is the most important reaction of sugars
- this resulting molecule is called a glycoside, and the bond formed is known as a glycosidic C-O or a glycosilic C-N
- anomeric carbon is identified as the one with two oxygens attached
If the anomeric carbon is involved in a glycosidic bond with sugar becomes a ________
- non reducing sugar
- if the anomeric carbon is associated in a glycosidic bond, the sugar (in ring form) can no longer open up to assume the linear form
- since only the open chain forms of sugars are able to undergo oxidation by cupric ions (reducing sugars), the sugar residue become non-reducing sugar
* in solution cannot go through mutarotation, cant open up bc no free hydroxyl at anomeric carbon
How are disaccharides formed
- formed when two monosaccharides are linked thorugh a glycosidic bond
- the reaction occurs when the *anomeric carbon* electrophile of one sugar reacts with a *hydroxyl group nucleophile of the other
- disaccharides are glycosides
ex: lactose is a disaccharide formed by the condensation of glucose with galactose
How are disaccharides named
- since anomeric carbon of galactose is involved in glycosidic bond, it is unable to undergo muration and remains in B configuration
- hence the glycosidic bond is described as B1 ->4 : going from C-1 of galactose in the B configuration to C-4 of glucose
- the anomeric carbon of glucose is free and is able to undergo murotation
either existing in alpha or beta or inlear forms
* glycosidic bond in lactose is still B1 ->4 regardless of anomeric form at glucose C1
- lactose is a reguing sugar
What is reducing end
-in disaccharides and polysaccharides the end of a chain with a free anomeric carbon (one not involve in the glycosific bond) is called the reducing end
*note: formation of glycosidic bond of a disaccharide must involve the anomeric C of one sugar, because that is the only electrophilic C atom in a sugar
- the PH group (the nucelophile) could be the OH on the anomeric C of a second sugar - or, any other -OH group in the sugar
So galactose and glucose could form lots of different disaccharides besides lactose: they would be structural isomers of lactose
What is sucrose
- glucose and fructose
- amoneric carbons of both glucose and fructose are involved in the glycosidic bond: sucrose therfore is a non reducing sugar
- Double headed arrow: used to show the involvement of both anomeric carbon atoms in the glycosidic linkage
What is trehalose
- a non reducing sugar
What are polysaccharides
- highly brached
- branching is possible bc sugars have several PH grups, each of which ca act as a nucleophile in forming a glycosidic bond, bonding a single subunit to two (or more) others
- polysaccharides differ from each other in the identity of the sugar units linkes, length of chains in type of bonds lnking units, and degree of branching
What are homopolysaccharides and heterpolysaccharides
- made of single type of sugar monomer
- glucans: glucose homopolymers ex: starch (plants), glycogen (animals), cellluse, chtiin
- mannans: mannose homopolymers
Heteropoly saccharides
- made from two or more kinda of suar subunits
Wat are nucleic acids
- RNA= ribonucleic acid
- DNA= deoxyribonucleic acid
What is the primary structure of nucleic acids
- are linear polymers made by phosphodiester bonds
- repeating unit is the nucelotide
- consists of sugar, base and phosphate group
= sugars linked together by the phosphates from the backbone of nucleic acids (sugar-hposphate backbone)
What are the two types of pentose sugars of nucleic aicds
RNA contains B ribose
- DNA contains D-2-deoxyribose where the -OH of carbon 2 is replaced by -H. (hence deoxyribonucleotide)
- both sugars occur in their B-furanose form
*carbon atoms of pentoses are numbered with a ‘ to distingosh them from wither bases
What are the basic structures for pyrimidines and purines
What are the pyrimidines
*thymine is really just 5-methyluracil
What are the tautomeric forms of cytosine
- isomers that differ by the shift of a H atom and has a double bond
- the OH group undergoes keto/enol tautomerism
- the NH2 group undergoes amino/imino tautomerism
*middle form predominaties
What are the basic form of the purine ring
- the purine ring is a fused (joined together) bicyclic (two rings) heterocycle
What is a nucleoside
- Base + sugar
- base is joined to the sugar through a glycosidic bond to form a nucleoside
- nucleoside is a special type of glycoside found in nuceic acids
- the glycosidic bond is sometimes called a glycosylic bond to designnate the C-N linkage
How are purine and pyrimidine bases linked to sugars
- the NH at _____ of purines and the NH at position 1 of pyrimidines link to the anomeric carbon of ribose/deoxyribose
What are the corresponding nucleosides to the bases
What are nucleotides
base + sugar + phosphate
- are phosphorylated nucleosides
* ntoe if question asks what bond it is could be phosphoester OR phosphodiester
How are the nucleotides lnked together with phosphodiester bonds
- phosphate group bridges between the 5’ OH of one nuckeotide unit and the 3” PH of another forming a phosphodiester linkage
- the linkage pattern is the sme for both DNA and RNA
- eahc linear nucelic acid stand has specific 5’ (lacking nucleotide at 5’ position) and a 3’ end (lacking a nucleotide at 3’ position)
- phosphate groups are completly ionized and negatively charged at pH 7
* always written n- c term 5’ to 3’
What happens to RNA and DNA under alkaline conditions
- RNA is rapidly hydrolyzed (due to presence of extra hydroxyl group)
- DNA is much more stable
- the 2’ Hydroxyl of RNA, which is absent frmo DNA acts as a nucleophile in intramolecular displacement, breaking phosphodiester linkage
What are other names for sugars
- saccharides
- Monosaccharides: consist of single sugar unit
- Oligosaccharides:
- short chains of monosaccharide units
- Disaccharides: consist of two monosaccharide units
- Polysaccharides
- Polymers fo 20 or more sugar units
What are monosaccharides
- combine two organic chemical functional groups
- Carbonyl (C=O) which is either an aldehyde or ketone
- at least two carbonds bearing a hydroxyl (alcohol) -C-OH groups
What are the simplest monosaccharides
- contain 3 carbons (trioses
- glyceraldehyde (aldose), dihydrozyacetone (ketose), Glycerol and acetone
*memorize*
in purines the ___ atom binds with the ____ of the sugar, in prymidines the _____ atom binds with the _____ of the sugar
Purines: NH at position 9 binds w/ anomeric carbon (C-1)
Prymidines: N at position 1 binds w/ anomeric carbon (C-1)
