Carbohidratos

Carbohidratos IntroducKon to Carbohydrates •  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? •  Carbohydrates (sugars) are polyhydroxy aldehydes or ketones. – Consider glucose, which is made by plants: –  Is glucose a polyhydroxy aldehyde or ketone? ClassificaKon of Monosaccharides •  Saccharides have multiple chiral centers, and they are often drawn as Fischer projecKons. – Designate each chirality center in glucose as either R or S. •  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. ClassificaKon of Monosaccharides •  Naturally occurring larger sugars can be broken down into glyceraldehyde by degradaKon. •  Such sugars are often called D-­‐sugars. ClassificaKon of Monosaccharides •  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. ConfiguraKon of Aldoses •  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. ConfiguraKon of Aldoses •  Ribose is a key building block of RNA. – WHAT is RNA? •  Arabinose is found in plants. •  Xylose is found in wood. ConfiguraKon of Aldoses • 
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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. ConfiguraKon of Ketoses •  Relevant ketoses have between three and six carbons. •  For each naturally occurring D isomer, there is an L enanKomer. ConfiguraKon of Ketoses Cyclic Structures of Monosaccharides •  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. Cyclic Structures of Monosaccharides •  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? Rosanoff Fischer silla Haworth ¿cómo converKr proyecciones de Fischer en proyecciones de Haworth? CHO
H
OH
H
OH
H
HO
HO
∗
H
CH2OH
CHO
H
OH
H
HO
H
∗
OH
CH2OH
¿cómo converKr proyecciones de Fischer en proyecciones de Haworth? Dibuje las estructuras del carbohidrato ¿cómo converKr proyecciones de Fischer en proyecciones de Haworth? 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. O
CH2OH
CH2OH
O
¿cómo converKr proyecciones de Fischer en proyecciones de Haworth? 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
CH2OH
CH2OH
O
OH
OH
carbohidrato L-α
carbohidrato D-β
¿cómo converKr proyecciones de Fischer en proyecciones de Haworth? Los grupos OH a la derecha (Fischer) van abajo del anillo (Haworth), y los grupos OH a la izquierda, van arriba. H
OH
O
H
CH2OH
H
H
OH
OH
H
OH
OH
carbohidrato L-α
CH2OH
OH
O
H
H
H
OH
H
carbohidrato D-β
Anomeric effect •  Which would you predict to be more stable? – Beta 67% , alpha 33%, open 0.01% 9
El efecto anomérico Cyclic Structures of Monosaccharides •  Ketoses form both furanose (5-­‐membered) and pyranose (6-­‐membered) rings: Cyclic Structures of Monosaccharides 70% β
2% α
0.7%
23%-β
5%-α
•  The equilibrium concentrations in water are above. ¿de donde provienen los términos furanosa y piranosa? O
pirano
O
furano
Cyclic Structures of Monosaccharides •  The furanose form takes part in most biochemical reacKons. ReacKons of Monosaccharides •  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? El efecto anomérico ReacKons of Monosaccharides •  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. El efecto anomérico ReacKons of Monosaccharides •  Under strongly basic conditions, glucose and mannose interconvert. •  Mannose and glucose are epimers because they only differ in the configuraKon of one carbon center. ReacKons of Monosaccharides •  Monosaccharides can be reduced to ALDITOLs shijing the equilibrium to the right. HOW? – D-­‐sorbitol or D-­‐glucitol are sugar subsKtutes. Reducing sugars •  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 Reducción de carbohidratos ReacKons of monosaccharides oxidaKon ReacKons of monosaccharides oxidaKon Reducing sugars OxidaKve cleavage of sugars OR1
H Nu
H
OH
hemiacetal
R
R
OH
HO
HO
OR1
+
H
Nu
H
OH
O
OH
H Nu
OH
HO
HO
OH
OH
O
OH
+
Nu
HO
HO
O
Nu
OH
ReacKons of Monosaccharides •  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? Disaccharides •  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? Polysaccharides •  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 1à6-­‐ α-­‐glycoside branches. We can eat corn and potatoes, but not grass or trees. WHY? Amino Sugars •  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? Glicósidos O-­‐glicósidos S-­‐glicósidos N-­‐Glycosides •  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. Chain Lengthening: The Kiliani-­‐Fischer Synthesis Ruff degradaKon of sugars Resumen de reacciones