Stereochemistry of Carbohydrates

What does the 2ⁿ rule determine?

The maximum possible number of stereoisomers.

What is the formula for the maximum number of stereoisomers?

2ⁿ.

What does n represent in the 2ⁿ rule?

The number of chiral centers.

Why does each chiral center double the number of stereoisomers?

Each chiral center can have two configurations: R or S.

How many configurations can one chiral center have?

Two: R and S.

How many stereoisomers are possible with one chiral center?

2¹ = 2.

How many stereoisomers are possible with two chiral centers?

2² = 4.

How many stereoisomers are possible with three chiral centers?

2³ = 8.

How many stereoisomers are possible with four chiral centers?

2⁴ = 16.

What is a chiral carbon?

A carbon bonded to four different groups.

Why is the carbon bonded to H, Br, Cl, and F chiral?

It is bonded to four different substituents.

How many chiral centers are present in the first structure?

One.

How many possible stereoisomers does the first structure have?

Two.

How many chiral centers are present in the second structure?

Two.

How many possible stereoisomers does the second structure have?

Four.

What are stereoisomers?

Compounds with the same connectivity but different three-dimensional arrangements.

What relationship exists between the two stereoisomers of a molecule with one chiral center?

They are enantiomers.

What are enantiomers?

Nonsuperimposable mirror-image stereoisomers.

What are diastereomers?

Stereoisomers that are not mirror images.

Does 2ⁿ always give the actual number of stereoisomers?

No; it gives the maximum possible number.

When can the actual number be less than 2ⁿ?

When meso forms or molecular symmetry are present.

What is a meso compound?

An achiral molecule with chiral centers and an internal plane of symmetry.

Why can a meso compound reduce the number predicted by 2ⁿ?

Two configurations may represent the same achiral structure.

Does the second structure have a meso form?

No; its two ends are different, so it lacks internal symmetry.

What should be identified before applying the 2ⁿ rule?

The number of true chiral centers.

Can a carbon with two identical substituents be chiral?

No.

How is a carbohydrate classified as D or L?

By the OH position on the chiral carbon farthest from the carbonyl group.

Which chiral carbon determines D/L configuration?

The highest-numbered chiral carbon.

Where is the carbonyl group placed in a Fischer projection?

At the top.

What indicates a D-sugar in a Fischer projection?

The determining OH is on the right.

What indicates an L-sugar in a Fischer projection?

The determining OH is on the left.

Does every OH group need to be on the right for a D-sugar?

No; only the OH farthest from the carbonyl determines D/L.

Does every OH group need to be on the left for an L-sugar?

No; only the OH farthest from the carbonyl determines D/L.

Which carbon determines D/L in an aldohexose?

C5.

Which carbon determines D/L in an aldopentose?

C4.

Which carbon determines D/L in an aldotetrose?

C3.

Which carbon determines D/L in a ketohexose?

C5.

Why is the highest-numbered chiral carbon used?

Its configuration is compared with glyceraldehyde.

What molecule is the reference for D/L classification?

Glyceraldehyde.

What is D-erythrose?

An aldotetrose whose determining OH is on the right.

What is L-erythrose?

An aldotetrose whose determining OH is on the left.

What is the OH pattern of D-erythrose?

Both OH groups are on the right.

What is the OH pattern of L-erythrose?

Both OH groups are on the left.

How are D-erythrose and L-erythrose related?

They are enantiomers.

What are enantiomers?

Nonsuperimposable mirror-image stereoisomers.

Are all chiral centers inverted between D- and L-erythrose?

Yes.

Does D mean dextrorotatory or positive rotation?

No; D indicates configuration only.

Does L mean levorotatory or negative rotation?

No; L indicates configuration only.

Can a D-sugar rotate light to the left?

Yes.

Can an L-sugar rotate light to the right?

Yes.

Can an open-chain sugar be classified as D or L?

Yes, using its Fischer configuration.

Are D/L and α/β the same classification?

No.

What does D/L describe?

The configuration relative to glyceraldehyde.

What does α/β describe?

The configuration at the anomeric carbon in a cyclic sugar.

What is optical activity?

The ability of a substance to rotate plane-polarized light.

What is a levorotatory substance?

A substance that rotates plane-polarized light to the left.

What symbol indicates levorotation?

(−).

What lowercase symbol may indicate levorotation?

l, although (−) is clearer.

What is a dextrorotatory substance?

A substance that rotates plane-polarized light to the right.

What symbol indicates dextrorotation?

(+).

What lowercase symbol may indicate dextrorotation?

d, although (+) is clearer.

What does clockwise rotation indicate?

Dextrorotatory or (+).

What does counterclockwise rotation indicate?

Levorotatory or (−).

What are molecules that rotate plane-polarized light called?

Optically active.

What instrument measures optical rotation?

A polarimeter.

What type of molecules are commonly optically active?

Chiral molecules.

Is every sample containing chiral molecules optically active?

No; a racemic mixture has no net optical rotation.

Why is a racemic mixture optically inactive?

Its two enantiomers rotate light equally in opposite directions.

How do enantiomers affect plane-polarized light?

They rotate it equally but in opposite directions.

What is a dextrorotatory glucose written as?

(+)-Glucose.

What is a levorotatory glucose written as?

(−)-Glucose.

Is D the same as dextrorotatory (+)?

No.

Is L the same as levorotatory (−)?

No.

What do D and L describe?

Configuration relative to glyceraldehyde.

What do (+) and (−) describe?

The experimentally observed direction of optical rotation.

Can a D-compound be levorotatory?

Yes.

Can an L-compound be dextrorotatory?

Yes.

How is optical rotation determined?

Experimentally using a polarimeter.

Can optical rotation be predicted solely from D/L configuration?

No.

Why are many biological molecules optically active?

Many are chiral and occur mainly as one stereoisomer.

What is biological stereospecificity?

The preference of living systems for one stereoisomer.

Why do biological systems distinguish stereoisomers?

Enzymes and receptors have specific three-dimensional shapes.

Can two enantiomers have different biological effects?

Yes.

Where is L-lactic acid commonly formed?

In muscles during anaerobic metabolism.

Which lactic acid form may be produced by some microorganisms in sour milk?

D-Lactic acid.

Which monosaccharide configuration is most common in human metabolism?

D-Monosaccharides.

Can humans efficiently metabolize most L-monosaccharides?

Usually not as efficiently as corresponding D-sugars.

Is it absolute that humans cannot metabolize any L-sugar?

No; this is an oversimplification.

Which amino-acid configuration is used in ribosomal protein synthesis?

L-amino acids.

What is the main exception to amino-acid chirality?

Glycine is achiral.

Are D-amino acids completely absent from nature?

No; they occur in bacteria and some specialized peptides.

What does chirality mean?

A molecule is not superimposable on its mirror image.

What is an enantiomer?

One of two nonsuperimposable mirror-image stereoisomers.

Can one enantiomer be more beneficial or harmful than another?

Yes, because biological targets are stereospecific.

Why is the thalidomide example more complex than “one good, one poisonous”?

Its enantiomers can interconvert in the body, and their effects are not completely separable.

What factors affect observed optical rotation?

Concentration, path length, temperature, wavelength, and solvent.