Carbs Flashcards
monosaccharides
- trioses, tetroses, pentoses, and hexoses
carbs that contain aldehyde group as their most oxidized functional group
aldoses
carbs that contain ketone group as their most oxidized functional group
ketoses
what would a 6C sugar with an aldehyde group be called
aldohexose
simplest aldose
glyceraldehyde
O=CH
|
HCOH
|
CH2OH
The aldehyde carbon, C1, can participate in…
- glycosidic linkages
- sugars acting as substituents via this linkage are called glycosyl residues
simplest ketose
HOCH2C(=O)CH2OH
carbonyl carbon of ketose
- C2
- can participate in glycosidic bonds
D-fructose
know structure
D-glucose
know structure
D-galactose
know structure
D-mannose
know structure
number of stereoisomers w common backbone
2^n
epimers
- special subtype of diastereomers that differ in configuration at exactly one chiral center
monosaccharides contain…
- hydroxyl group nucleophile
- carbonyl group electrophile
- therefore aldoses can become hemiacetals and ketoses can become hemiketals
only cyclic molecules stable in solution due to ring strain
pyranose (6 membered)
furanose (5 membered)
mechanism of hemiacetal or hemiketal formation
1) nu attack of second to last hydroxyl group upon electrophilic carbonyl carbon
2) anomeric carbon becomes chiral and the oxygen is protonated
alpha and beta anomers
Fischer ~> Haworth
- when we convert monosaccharides from straight-chain Fischer projection to Haworth projection, any group on the right in the Fischer projection will point down
mutarotation
exposing hemiacetal rings to water cause cause cycling between open and closed form
C1-C2 substituents can rotate freely, either alpha or beta anomer can be formed
unequal proportions at equilibrium bc axial position for hydroxyl groups is unfavored and not stable
monosaccharides contain…
alcohols and either aldehydes or ketones
hemiacetal open ring oxidation
when hemiacetal rings are open they can be oxidized to form aldonic acids (the CA version)
aldoses are considered reducing agents since they can be oxidized
reducing sugar
any monosaccharide with a hemiacetal ring
hemiacetal closed ring oxidation
yields a lactone, a cyclic ester w a carbonyl group persisting on the anomeric carbon
which reagents can be used to detect the presence of reducing sugars?
Tollen’s reagent and Benedict’s reagent
Tollen’s reagent
must be freshly prepared
start with AgNO3, which is mixed w NaOH to produce Ag2O
this is dissolved in ammonia to produce [Ag(NH3)2]+ which is the actual Tollen’s reagent
Benedict’s reagent
aldehyde of aldose sugar is readily oxidized
indicated by red Cu2O precipitate
to test specifically for glucose…
use glucose oxidase
why do ketose sugars produce positive Tollen’s and Benedict’s tests?
- although ketones cannot be oxidized directly to CAs, they can tautomerize to form aldoses under basic conditions via keto-enol shifts
- while in aldose form they can react w Tollens or Benedict’s reagents to form carboxylic acid
alditol
aldehyde group of aldol reduced to alcohol
deoxy sugar
hydrogen that replaces hydroxyl group on the sugar (deoxyribose)
phosphorylation of glucose
phosphate group is transferred from ATP to glucose
esterification of glucose
carb reacts w pyridine and (CH3CO)2O at 0°C to form ester
glycoside formation
- hemiacetal react w alcohols to form acetals
- the anomeric hydroxyl group is transformed into an alkoxy group yielding a mixture of alpha and beta-acetals with H2O as an LG
- the resulting C-O bonds are called glycosidic bonds and the acetals formed are glycosides
- react sugar w ethanol and HCl to form glycosides and H2O
breaking a glycosidic bond…
requires hydrolysis
disaccharides and polysaccharides forms as a result of…
glycosidic bonds between monosaccharides
disaccharides
- glycosidic linkage is nonspecific in that the anomeric carbon of a cyclic sugar can react w any hydroxyl group on any other sugar molecule
sucrose
know structure
lactose
know structure
maltose
know structure
polysaccharides
long monosaccharide chains linked together by glycosidic bonds
a polysaccharide composed entirely of one monosaccharide…
homopolysaccharide
a polymer made up of more than one type of monosaccharide…
heteropolysaccharide
cellulose, starch, and glycogen…
- all composed of D-glucose
- differ in configuration about the anomeric carbon and the position of glycosidic bonds
cellulose
- main structural component of plants
- homopolysaccharide of D-glucose
- linked by B-1,4 glycosidic bonds
- we do not have cellulase and therefore cannot digest cellulose
- hence why fruits and veggies are good fiber
starches
- polysaccharides more digestible by humans
- linked a-D-glucose monomers
- plants predominantly store starch as amylose (a-1,4 glycosidic polymer)
- amylopectin, begins as a-1,4 glycosidic polymer and has a-1,6 glycosidic bonded branches
iodine starch testing
- Iodine fits into the helical structure of amylose to test for starch
B-amylase
- cleaves amylose at the nonreducing end of the polymer (end w acetal) to yield maltose
a-amylase
- cleaves randomly along the chain to yield shorter polysaccharide chains, maltose, and glucose
amylopectin is broken down by…
- debranching enzymes
glycogen
- carb storage unit in animals
- a-1,6 glycosidic bonds (1 for every 10 glucose molecules whereas amylopectin is 1 in every 25)
- highly branched
- branching optimizes the energy efficiency of glycogen and makes it more soluble in solution, thereby allowing more glucose to be stored in the body
- allows enzymes that cleave glucose from glycogen like glycogen phosphorylase to work on many sites within the molecule simultaneously
glycogen phosphorylase
- cleave glucose from nonreducing end of glycogen and phosphorylating it, producing G1-P
