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
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 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 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*
