Kaplan MCAT OChem Ch. 14: Carbs

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gen formula of carbs

Cn (H2O)n

carbs

form of polyhydroxylated aldehydes or ketones

monosac, disac,

number of carb units

oligsac

short carb chains

polysacs

long carb chains

monosacs

prefix = #
suffix = -ose

monosacs

simplest is glyceraldehyde

glyceraldehyde

http://upload.wikimedia.org/wikipedia/commons/a/a1/D-glyceraldehyde-2D-Fischer.png

aldose

glyceraldehyde is polyhydroxylated aldehyde

dihydroxacetone

simplest ketone sugar (ketose)

dihydroxacetone

http://www.bmrb.wisc.edu/metabolomics/standards/dihydroxyacetone/lit/3484.png

designation of (glyceraldehyde) monosacs

D and L(-)

(glyceraldehyde)

D or L config of other monosacs based on relationship to this

molecule's highest numbered chiral center (farthest from carbonyl) as D+ => then D sugar

three types of stereoisomers (monosacs)

1. enantiomers
2. disasteromers
3. epimers

enatiomers (monsacs)

same sugars, in diff. optical families

diastereomers (monosacs)

nonidentical sugars w/in same family, as long as both are ketoses/aldoses, and have same number of carbons

epimers

diastereomers that only differ at only one chiral center

D-fructose (monosac)

http://www.uspto.gov/web/patents/classification/uspc536/c536s1-11-4.gif

D-glucose (monosac)

http://www.uspto.gov/web/patents/classification/uspc536/c536s1-11-2.gif

D-galactose (monosac)

http://upload.wikimedia.org/wikipedia/commons/5/58/D-galactose.png

D-mannose (monosac)

http://upload.wikimedia.org/wikipedia/commons/2/23/D-mannose.png

ring prop of monosacs

OH = Nu
carbonyl = Electrophile

=> intramolecular rxn => cyclc hemiacetal/hemiketal (from aldoses/ketoses) and O becomes part of ring

ring prop of monosacs

due to ring strain, only cyclic molecule that are stable in solution are pyranose and furanose

pyranose

6 member ring of monosac

furanose

5 membered ring of monosac

pyranose

chairlike

and sub assume axial and equatorial => min. steric hindrance

right of Fischer

down on on Haworth

left of Fishcer

up on Haworth

Nu acyl sub

intramolecular rxn of monosacs

six membered rings formed from

six carbon aldoses or seven carbon ketoses, since O on highest-numbered chiral group acts as Nu

5 membered rings formed from

5 carbon aldoses or 6 carbon ketoses, since O on highest-numbered chiral group acts as Nu

ring prop of monosacs

straight monosac => cyclic => carbonyl becomes chiral

anomers

cyclic stereoisomers that differ about new chiral carbon

anomeric carbon

carbon that becomes chiral in ring

anomeric carbon

to identify:
find it to both O of ring and OH gorup

in glucose, alpha anomer

has OH group of C-1 trans to CH2OH sub (pointing down)

in glucose,beta anomer

OH group of C-1 cis to CH2OH sub (pointing up)

mutarotation

hemiacetal rings to water => spon open and reform => since C1 and C2 can rotate freely => both anomer α and β at eq. conc.

mutarotation

occurs more rapidly when we catalyze hemiacetal w/ acid or base

mutarotation

process of one anomer changing into other anomer by opening and reclosing

anomerization

forming of one anomer or another from the straight-chain sugar

type of Monosac Rxns

ester formation

oxidation of monosacs

glycosidic rxns

reaction types of monosacs

are determined by func. groups present, think alcohols and carbonyls

ester formation (monosac)

same rxn as simple alcohols

ester formation (monosac)

monosacs to this using acid anhydride and base

ester formation (monosac)

all OH gorups will be esterified

oxidation of monosacs (monosac)

switching between anomeric configs => hemiacetal rings spend short time in openc hain aldehyde form => can be oxidized to carb acids (these oxidized aldoses)

aldonic acids

oxidized aldoses called this, in oxidation of monosacs (monosac)

reducing sugars

since aldoses can be oxidized, they are reducing agents, so called this

Tollen's reagent and Benedict's reagent

detect presence of reducing sugars, in oxidation of monosacs (monosac)

Tollen's reagent

positive involves reduction of Ag+ to form metallic silver in oxidation of monosacs (monosac)

Benedict's reagent

a red ppt of Cu2O indicate presence of reducing sugar in oxidation of monosacs (monosac)

ketose sugars

also reducing sugars and use both reagents, however, since ketones can't be oxidized to carb acids -> isomerize to aldoses via keto-enol shifts => react w/ reagents => carb acids

dilute nitric acid (powerful oxidizing agent)

oxidizes both aldehyde and primary alcohols (c-6) to carb acisd

glycosidic rxns

hemiacetal + alcohol => acetal

under acidic conditions

glycosidic rxns

anomeric OH group transformed into alkoxy group => mix of α and β acetal (h2o as LG)

glycosidic linkage

resulting C-O bond called this

glycoside

acetal in glycosidic rxn known as this

glycosidic rxns

sn2, sugar as nu

Benedict's reagent

can be used to test glucose presence in urine

disacs

monosac can react OH as alcohol w/ other monosacs => two react => this happens

1,4'-linkage

most common glycosidic linkage between C-1 of first sugar and C-4 of second

maltose

1,4'α -linkage

cellobiose

1,4;β-linkage

glycosidic linkage

can be cleaved in presence of aq. acid

lactose intolerance

person missing enzyme lactase, which breaks disac lactose into glu and galactose molecules

polysacs

large chains of monosacs, linked together by glycosidic bonds

3 most important bio polysacs

cellulose, starch, glycogen

3 most important bio polysacs

have diff functions, but all cmposed of same monosac, D-glu

cellulose (polysacs)

1,4'-β glycosidic bonds

cellulose (polysacs)

components of plants and not digestible by humans

starch (polysacs)

more digestible by humans

starch (polysacs)

plants store energy as these molecules by linking glucose molecules primariy in 1,4'α glycosidic bond, although occasional 1,6'α glycosidic bonds form branches off chain

glycogen (polysacs)

similar to starch, except more 1,6'α glycosidic bond (approx 1 for every 12 gluc molecules) => highly branced compound (dendrimer)

3 most important bio polysacs

are composed of gluc sub units, but diff. in config about anomeric carbon and position of glycosidic bonds => bio diff.

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