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75 terms

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.