MCAT ORG #2 - s3. · PDF fileCLUTCH MCAT / ORG 2: ... 11 | P a g e CONCEPT: CARBOHYDRATE...

CLUTCH MCAT / ORG 2: Carbohydrates

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MCAT ORG #2 Carbohydrates



CONCEPT: ISOMETRIC RELATIONSHIPS

PRACTICE: What is the relationship between the sets of molecules shown?



CONCEPT: OPTICAL ACTIVITY—SPECIFIC AND OBSERVED ROTATION

One of the special features of chiral molecules is that they are able to ____________ plane-polarized light.

● Clockwise rotation = dextrorotary (d) or ________ ● Counterclockwise rotation = levorotatory (l) or _______

□ These random names have ____________ to do with the chirality of a molecule

Enantiomeric Excess:

● Specific rotation [α] is the rotation that 100% pure enantiomers produce. Opposite enantiomer = _____________ rotation.

● A perfect 1:1 ratio of enantiomers is called _________________

The enantiomeric excess: Observed Rotation:

EXAMPLE: Calculate the ee and observed rotation for the following chiral mixture where S-enantiomer has [α] = +20.



PRACTICE: A compound is 30% optically pure R. Which is true?

a. 65% R; 35%S

b. 70% R; 30% S

c. 90% R; 10%S

d. 60% S; 40%R

e. 85% S; 15%R

PRACTICE: Which is true of the S-enantiomer?

a. It is dextrorotary

b. It is levorotary

c. It will rotate light (+)

d. It will rotate light (-)

e. None of these



CONCEPT: INTRODUCTION TO CARBOHYDRATE MONOSACCARIDES

Carbo-“hydrate” implies that a carbon has been combined with _______________

● All monosaccharides have the same basic formula: _________________

● C : H : O ratio = _______________

● Monosaccharides can be represented as straight chains OR rings

□ One oxygen attached to each and every carbon atom, and 1 IHD regardless of form

● Monosaccharides can either be _________________ or _______________

□ Aldehyde sugar = _______________

□ Ketone sugar = _______________

CONCEPT: MONOSACCARIDES — GENERAL FEATURES

Monosaccharides can be described by both generic names and specific names. Generic naming involves:

● Carbonyl type (i.e. aldo— )

● Carbohydrate Chain Length (i.e. –pentose)

□ The total number of stereoisomers possible is described by _________

□ Epimers are stereoisomers of monosaccharaides differing at only one chiral carbon

EXAMPLE: Epimers of aldohexose



EXAMPLE: Stereoisomers of aldotetrose

CONCEPT: D and L-ISOMERISM

All monosaccharides come in dextrorotary (D) and levorotary (L) forms. These are enantiomers of each other.

● Monosaccharide stereochemistry is determined by the “penultimate carbon” or ________ chiral carbon.

□ D = _____ - configuration (-OH pointing RIGHT)

□ L = _____ - configuration (-OH pointing LEFT)



CONCEPT: MONOSACCHARIDES – COMMON STRUCTURES

Pentoses and hexoses are highly prevalent in biological systems. We only need to know a few of these.

● Biological systems tend to recognize D-aldoses more than L-aldoses

● The other important monosaccharides can all be related back to these basic structures.



CONCEPT: MONOSACCHARIDES – CYCLIZATION

When monosaccharides cyclize, two different diastereomers are possible. These are known as anomers.

● Sugars are most stable in a cyclic form

□ Furanose = ___-carbon cyclic sugar

□ Pyranose = ___-carbon cyclic sugar

● Nucleophilic attack of the penultimate alcohol to the electrophilic carbonyl carbon leads to cyclization

□ The carbonyl carbon is _________________, so it can be attacked from either the top or bottom

- The α-anomer = anomeric oxygen is __________ with the last carbon

- The β-anomer = anomeric oxygen is __________ with the last carbon (most common)

□ The OH’s on the _______ in the straight chain point ______ in the ring

□ The OH’s on the _______ in the straight chain point ______ in the ring

PRACTICE: Draw the most abundant anomer predicted through the cyclization of D-mannose.



CONCEPT: MONOSACCHARIDES – HAWORTH PROJECTIONS

Simplified drawings of sugars in their cyclic form, imagining that the ringed structure is planar.

● NOTE: The anomeric carbon is always drawn at the far ___________ in Haworth Projections.

□ This is true for both pyranose and furanose rings

PRACTICE: Properly number the following Haworth Projections. Identify each anomer and predict anomeric equilibrium.

CONCEPT: DISACCHARIDES

Two monosaccharides bound by an O-glycosidic linkage (α or β)

● Two alcohols react in a dehydration mechanism to release water and form an ether (acetal) linkage

● NOTE: Humans can’t digest carbohydrates with O-(β-glycosidic) linkages, with one important exception

Other Disaccharides:



CONCEPT: POLYSACCHARIDES

● Trisaccharides = ____________ monosaccharides

● Oligosaccharides = ____________ monosaccharides

● Polysaccharides = More than ____________ monosaccharides

□ Polysaccharides that are created by plants for use as an energy store are known as starches

□ Polysaccharides can have straight vs. branched linkages

- 1,4-linkages = _____________ Chains

- 1,6 linkages = _____________ Chains

EXAMPLE: Common Polysaccharides

● Amylopectin is the “plant version” of glycogen, but with branches every _______ monosaccharides apart



CONCEPT: CARBOHYDRATE TESTS

Sugars with an anomeric carbon that reacts with oxidizing agents are known as “reducing sugars”.

● The sugar is the reducing agent, hence the name.

● Sugars with all of their anomeric carbons forming O-glycosidic linkages are not reducing sugars

□ This includes all polysaccharides, and some disaccharides like sucrose.

Tollen’s Test

Ag2O is used as a strong oxidizing agent to react with reducing sugars, particularly aldoses.

● Aldoses are oxidized quickly, whereas alcohols (such as those on ketoses) react more slowly

● The rapid formation of a silver mirror (Ag(s)) on the beaker wall is positive for aldose

Benedict’s Test

A chemical reagent used to test for the presence of reducing sugars. Positive results (color change) in the following:

● All monosaccharides

● Most disaccharides, sucrose being an exception.

MCAT ORG #2 - s3. · PDF fileCLUTCH MCAT / ORG 2: ... 11 | P a g e CONCEPT: CARBOHYDRATE...

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