Carbohydrates+(sugars)+are+polyhydroxy+ aldehydes+or+ketones.+depa.fquim.unam.mx/amyd/archivero/Carbohidratos_24660.pdf · • Carbohydrates+(sugars)+are+abundant+in nature: – They+arehigh+en

Carbohidratos

•  Carbohydrates (sugars) are abundant in nature: –  They are high energy biomolecules. –  They provide structural rigidity for organisms (plants,

crustaceans, etc.). –  The polymer backbone on which DNA and RNA are

assembled contains sugars.

•  The term, carbohydrate, evolved to describe the formula for such molecules: Cx(H2O)x.

•  Carbohydrates are NOT true hydrates. WHY?

IntroducKon to Carbohydrates

•  Carbohydrates (sugars) are polyhydroxy aldehydes or ketones. – Consider glucose, which is made by plants:

–  Is glucose a polyhydroxy aldehyde or ketone?

•  Saccharides have multiple chiral centers, and they are often drawn as Fischer projecKons.

– Designate each chirality center in glucose as either R or S.

ClassificaKon of Monosaccharides

•  Saccharides have mulKple chiral centers, and they are often drawn as Fischer projecKons.

•  What does the suffix, “ose” mean? •  Define the following terms:

–  Aldose and ketose –  Pentose and hexose

•  Glyceraldehyde is a monosaccharide with one chirality center.

– Natural glyceraldehyde is dextrorotatory (D): it rotates plane polarized light in the clockwise direcKon.

•  Naturally occurring larger sugars can be broken down into glyceraldehyde by degradaKon.

•  Such sugars are often called D-‐sugars.


•  Recall that dextrorotatory versus levorotatory rotation cannot be predicted by the R or S configuraKon.

•  Here, D no longer refers to dextrorotatory. Rather it refers to the R configuraKon at the chiral carbon farthest from the carbonyl.


•  There are four aldotetroses. Two are shown below.

•  What are the other two structures?

ConfiguraKon of Aldoses

•  Aldopentoses have three chirality centers. The number of isomers will be 23.

•  Recall the 2n rule.

•  The D-‐sugars are naturally occurring.


•  Ribose is a key building block of RNA. – WHAT is RNA?

•  Arabinose is found in plants. •  Xylose is found in wood.


•  Based on the 2n rule, how many aldohexoses are there? •  How many of the aldohexoses are D isomers. •  Glucose is the most common aldohexose. •  Mannose and galactose are also common.


•  Relevant ketoses have between three and six carbons. •  For each naturally occurring D isomer, there is an L enanKomer.

ConfiguraKon of Ketoses

ConfiguraKon of Ketoses

•  Carbonyls can be a[acked by alcohols to form hemiacetals.

– The intramolecular reacKon is generally favored for 5 and 6-‐ membered rings. WHY?

Cyclic Structures of Monosaccharides

•  For the following compound, draw the mechanism and resulKng product that results from acid catalyzed ring-‐ closing hemiacetal formaKon.


•  Monosaccharides, like glucose, can also undergo ring-‐closing hemiacetal formaKon.

•  The equilibrium greatly favors the closed form called pyranose.


¿cómo converKr proyecciones de Fischer en proyecciones de Haworth?

Fischer Rosanoff Haworth

silla

CHOOHHOHHHHOHHO

CH2OH

CHOOHHHHO

∗∗ OHHCH2OH

∗∗



Dibuje las estructuras del carbohidrato


Si se trata de un carbohidrato D. Colocar CH2OH arriba del anillo, a la izquierda de O. Si se trata de un carbohidrato L, colocar CH2OH debajo del anillo.

OCH2OH

CH2OH O


Para un carbohidrato α, colocar OH abajo del anillo, en el C a la derecha de O. Para un carbohidrato β, colocar OH arriba del anillo, en el C a la derecha de O.

O

OH

CH2OHOHCH2OH O

carbohidrato L-α carbohidrato D-β

OOH

H

H

OHH

OHOH

H

H

CH2OH

H

OH

H

CH2OH

H OH

OH HO

carbohidrato L-α carbohidrato D-β


Los grupos OH a la derecha (Fischer) van abajo del anillo (Haworth), y los grupos OH a la izquierda, van arriba.

Anomeric effect

•  Which would you predict to be more stable?

9

– Beta 67% , alpha 33%, open 0.01%

El efecto anomérico

•  Ketoses form both furanose (5-‐membered) and pyranose (6-‐membered) rings:



•  The equilibrium concentrations in water are above.

70% β 0.7% 23%-β 2% α 5%-α

¿de donde provienen los términos furanosa y piranosa?

O Opirano furano

•  The furanose form takes part in most biochemical reacKons.


•  Monosaccharides are generally soluble in water. WHY? •  To improve their solubility in organic solvents, the hydroxyl groups can be acetylated.

•  WHY is pyridine added to the reacKon?

•  How might acetylaKon help in purificaKon efforts.

ReacKons of Monosaccharides

•  Monosaccharides can also be converted to ethers via the Williamson ether synthesis.

•  Ether linkages are more robust than ester linkages. WHY?



•  When treated with an excess of an alcohol, the hemiacetal equilibrium can be shifted to give an acetal.

•  When a sugar is used, alpha and beta glycosides are formed.


•  The mechanism of glycoside formaKon is analogous to the acetal formaKon mechanism.

•  Only the anomeric hydroxyl group is replaced.


•  The mechanism of glycoside formation is analogous to the acetal formaKon mechanism.

What factors would you consider when trying to predict whether the alpha or beta anomer will be the major product?


•  Under strongly basic conditions, glucose and mannose interconvert.

•  Mannose and glucose are epimers because they only differ in the configuraKon of one carbon center.


•  Monosaccharides can be reduced to ALDITOLs shijing the equilibrium to the right. HOW?

– D-‐sorbitol or D-‐glucitol are sugar subsKtutes.


•  If the sugar has an –OH a[ached to the anomeric carbon, then the sugar is a reducing sugar

•  If it has –OR, then it is not a reducing sugar

Reducing sugars

Reducción de carbohidratos

ReacKons of nonosaccharides oxidaKon

•  Disaccharides form when two sugars connect through a glycosidic linkage.

–  The 1 à 4 glycosidic linkage is most common.

–  The bottom ring is capable of mutarotaKon at its anomeric posiKon.

–  Because the anomeric posiKon of the bottom ring is a HEMIACETAL rather than an acetal, it is in equilibrium with the open form. Thus, maltose is a reducing sugar.

Disaccharides

•  Cellobiose is similar to maltose. WHAT are the differences?

•  Will cellobiose be a reducing sugar?

Disaccharides

•  Lactose is another disaccharide.

•  Some people have trouble digesKng lactose.

Disaccharides

•  Sucrose (table sugar) is also a disaccharide.

–  Honey bees can convert sucrose into a mixture of sucrose, fructose, and glucose.

–  Fructose is very sweet. •  Sucrose is not a reducing sugar. WHY?

Disaccharides

•  Cellulose is a polysaccharide containing 7000–12000 glucose units connected through glycosidic bonds.

•  How is cellulose capable of giving plants like trees their rigidity and strength?

Polysaccharides

•  Starch is a major components of grains and other foods, like potatoes. What is the difference between molecules of starch and molecules of cellulose? Starch is made of amylose and amylopecKn.

•

Polysaccharides

•  Amylopectin has some 1à6-‐ α-‐glycoside branches.

Polysaccharides

We can eat corn and potatoes, but not grass or trees. WHY?

•  Amino sugars like glucosamine are important biomolecules.

•  Acetylated glucosamine can form an

important polysaccharide called chitin.

Amino Sugars

•  The carbonyl groups in chitin allow for even stronger H-‐ bonding between neighboring chains.

•  ChiKn is used in insect and arthropod exoskeletons. WHY?

Amino Sugars

Glicósidos

O-‐glicósidos

S-‐glicósidos

•  N-‐glycosides can be formed when sugars are treated with an amine and an acid catalyst.

•  RNA and DNA incorporate important N-‐glycosides called nucleosides.

N-‐Glycosides

•  Ribose forms ribonucleosides in RNA. •  Deoxyribose forms deoxyribonucleosides in DNA.

N-‐Glycosides

•  There are four different heterocyclic amines that a[ach to deoxyribose molecules to form DNA nucleosides.

N-‐Glycosides

•  In DNA, the nucleosides are a[ached to phosphate groups forming nucleoKdes.

N-‐Glycosides

•  The phosphate groups of the nucleoKdes are connected together to make the DNA strand or POLYNUCLEOTIDE.

N-‐Glycosides

•  The nucleotides in DNA can attract one another through H-‐bonding of the DNA base pairs.

N-‐Glycosides

•  WHY does DNA form a double helix?

N-‐Glycosides

•  RNA is structurally different from DNA : –  The sugar in RNA is ribose. WHAT is the sugar in DNA? –  RNA contains uracil instead of thymine.

•  RNA translates the informaKon stored in DNA into working molecules (proteins and enzymes).

N-‐Glycosides

•  RNA strands generally do not form double helices like DNA.

•  RNA strands can fold into many different shapes, and some even act as catalysts called ribozymes.

•  It is possible that RNA evolved self-‐replicaKon as an early step in the evoluKon of life from small molecules.

N-‐Glycosides

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Carbohydrates+(sugars)+are+polyhydroxy+ aldehydes+or+ketones.+depa.fquim.unam.mx/amyd/archivero/Carbohidratos_24660.pdf · • Carbohydrates+(sugars)+are+abundant+in nature: – They+arehigh+en