BIOS 301: L4 Carbs

Carbohydrates

C_n(H₂O)_n

Produced in plants from CO2 and H2O via photosynthesis

Functions: energy source and energy storage, structural components of cell walls and exoskeletons (chitin), informational molecules in cell to cell signaling

Can be covalently linked with proteins and lipids

Oxidation of sugars is the central energy-yielding pathway in most non-photosynthetic cells!

Fischer Projections – Chiral Compounds

• Purpose: Represent chiral carbohydrates in 2D

• Perspective rules:

  • Horizontal bonds: pointing toward you

  • Vertical bonds: pointing away from you

• Useful for quickly visualizing stereochemistry of sugars

Carbohydrate Nomenclature

• Basic naming: number of carbons + -ose

  • 3C → triose, 4C → tetrose, 5C → pentose, etc.

• Functional groups:

  • Aldehyde → aldose

  • Ketone → ketose

• Carbohydrates can undergo extensive isomerization:

  • Structural isomers (different connectivity)

  • Stereoisomers (different spatial arrangement)

Structural Isomers – Aldose vs Ketose

• Aldose: carbohydrate with an aldehyde group

• Ketose: carbohydrate with a ketone group

• Structural isomers: same molecular formula, different functional group placement

Stereoisomers – Diastereomers & Epimers

• Diastereomers: stereoisomers that are not mirror images (≠ enantiomers)

• Epimers: a type of diastereomer differing at only one chiral center

  • Epimers ≠ enantiomers

  • Have different physical properties

• Examples:

  • D-Mannose and D-Glucose → epimers (differ at 1 chiral center)

  • D-Galactose and D-Glucose → epimers

  • D-Mannose and D-Galactose → diastereomers (differ at >1 chiral center, not epimers)

D-Glyceraldehyde Structure

D-Ribose (Rib) Structure

D-Glucose (Glc) Structure

D-Mannose (man) Structure

D-Galactose (Gal) Structure

Dihydroxyacetone

a d ketoses

D-Erythrulose

D ketoses

Aldoses

• Carbohydrates with an aldehyde functional group (–CHO)

• Carbonyl is at the end of the molecule (C1)

• Named based on carbon number:

  • 3C → aldotriose

  • 6C → aldohexose

• Examples: glucose, ribose

D-Fructose

Ketoses

• Carbohydrates with a ketone functional group (C=O)

• Carbonyl is usually at C2 (middle of the chain)

• Named based on carbon number:

  • 3C → ketotriose

  • 6C → ketohexose

• Examples: fructose, ribulose

Reducing Sugars

• Definition: sugars with a free aldehyde at the anomeric carbon (C1)

• Can reduce metal ions:

  • Cu²⁺ → Cu⁺ (Fehling’s test)

  • Ag⁺ → Ag⁰ (Tollens’ test)

• Allows detection of reducing sugars like glucose

• Modern methods: colorimetric or electrochemical tests

Reducing sugar assay

Reducing sugars have a free aldehyde (or sometimes ketone) group that can donate electrons to metal ions like Cu²⁺ or Ag⁺, reducing them to Cu⁺ (forms a brick-red precipitate) or Ag⁰ (silver mirror). The color change shows sugar is present.

Nelson’s Test (Color Development)

To make the result easier to measure with a lab instrument (spectrophotometer), a second step is added:

• The Cu₂O produced in the first step reacts with Arsenomolybdic acid.

• This converts the clear acid into Arsenomolybdous acid, which has a distinct blue color. The intensity of this blue color tells you how much sugar was originally in the sample.

Why is reducing sugar assay a slow reaction?

Cyclization In an aqueous solution (like your blood or a lab beaker), glucose doesn't mostly exist in that straight-chain "linear" form shown on the left. Instead:

• Over 99% of glucose molecules fold into a ring structure (pyranose).

• The aldehyde group is "hidden" or "locked" inside the ring (as a hemiacetal).

• Only the small fraction (~1%) that is in the open-chain form at any given moment can react. As that 1% is used up, the rings slowly open to provide more, but the "instant" availability of the reactive group is very low.

Hemiacetals & Hemiketals – carbohydrate reactivity

Aldehydes/ketones are electrophilic. Alcohols are nucleophilic.

• Aldehyde + alcohol → hemiacetal

• Ketone + alcohol → hemiketal

• These reactions let sugars cyclize into rings.

Formation of pyranose ring (hemiacetal)

A sugar’s aldehyde reacts with an internal alcohol (usually on C5) → forms a hemiacetal → sugar cyclizes into a 6-membered pyranose ring. The anomeric carbon becomes newly chiral.

pyranose

A 6-membered sugar ring that contains an oxygen atom. Highlights that sugars can cyclize into 6-membered rings.

furanose

A 5-membered sugar ring that contains an oxygen atom. Highlights that sugars can cyclize into 5-membered rings.

anomer

The new chiral carbon formed from the former carbonyl carbon during ring formation. Highlights that cyclization creates a new stereocenter at the anomeric carbon.

Colorimetric Glucose Analysis

Enzymatic methods are used to

quantify reducing sugars such as

glucose.

– The enzyme glucose oxidase

catalyzes the conversion of

glucose to glucono-

-lactone and

hydrogen peroxide.

– Hydrogen peroxide oxidizes

organic molecules into highly

colored compounds.

– Concentrations of such

compounds are measured

colorimetrically.

Pyranose rings favor chair or boat conformations?

A pyranose is a monosaccharide (sugar) that exists as a six-membered cyclic hemiacetal ring, comprising five carbon atoms and one oxygen atom.

Favors chair

chair confirmations require energy to flip.

Types of Isomers

Types of Diastereomers

The Glycosidic Bond

• Two sugars join via a glycosidic bond: anomeric carbon + hydroxyl group

• The bond is an acetal, more stable & less reactive than a hemiacetal

• Second sugar’s hemiacetal = reducing end

• Anomeric carbon in glycosidic bond = nonreducing end

• Disaccharides named by linkage (e.g., α-D-glucopyranosyl-(1→4)-D-glucopyranose = maltose)

Nonreducing Disaccharides

• Formed when two anomeric carbons join via a glycosidic bond

• Product has two acetal groups, no hemiacetals

• No reducing ends → nonreducing sugar

• The orientation of the sugars can be switched