Chemistry

Session

Session Objectives

1. The cell and energy cycle

2. Introduction to carbohydrates

3. Classification of carbohydrates

4. Preparation of glucose

5. Chemical properties of glucose

6. Proteins

7. Structure of protein

Chemical Structure of Living Matter:An Overview

Cellular Organization

Photosynthesis

Plants convert carbon dioxide and water into carbohydrates via photosynthesis.

– 100 sequential steps convert six moles of CO2 to one mole of glucose.

– Carbon-14 radiolabelling helped to identify individual steps.

Conversion of solar energy to chemical energy.

– Light reactions.

Synthesis of carbohydrates.

– Dark reactions.

Carbohydrate Metabolism

Energy Relationships in Metabolism

Carbohydrates

Hydrates of carbon: Cx(H2O)y.

Monosaccharides

The simplest carbohydrates.

Oligosaccharides

Two to ten monosaccharides attached together.

Polysaccharides

Starch and cellulose.

Classification

Monosaccharides

Sixteen possible aldohexoses.

Three occur widely.

D-glucose, D-mannose and D-galactose.

Predominantly in the cyclic form.

Reducing sugars.

Reduce Cu2+ to Cu2O and form a brick red precipitate.

Fehling’s solution (tartrate) or Benedict’s solution (citrate).

Preparation of glucose

From sucrose (cane sugar): sucrose is boile with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts,

H12 22 11 2 6 12 6 6 12 6C H O H O C H O C H O

Sucrose Glucose Fructose

Form starch: Hydrolysis of starch

H6 12 5 n 2 6 12 6 n(C H O ) nH O (C H O )

Properties of glucose

1. Acetylation of glucose with acetic acid anhydride gives a pentaacetate confirming the presence of five hydroxyl groups in glucose.

2. Glucose reacts with hydroxlamine to give monoxime and adds a molecule of hydrogen cyanide to give a cynohydrin.

3. Glucose reduces ammoniacal silver nitrate solution (Tollen’s reagent) to metallic silver and also Fehling solution to reddish brown cuprous oxide and itself gets oxidised to gluconic acid.

4. On oxidation with nitric acid glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic group in glucose.

Properties of glucose

5. Glucose on prolonged heating with HI forms n-hexane, suggesting that all the six carbon atoms in glucose are linked linearly.

6. D-Glucose reacts with phenyl hydrazine to give glucose phenylhydrazone which is soluble. In excess of phenylhydrazine gives osazone.

7. On heating with conc. Solution of sodium hydroxide glucose first turns yellow, then brown and finally resignifies. With dilute NaOH glucose undergoes a reversible isomerization and is converted into a mixture of D-glucose, D-mannose and D-fructose.(Lobry de Bruyn-van Ekenstein reaarangement)

Ring Closure in Glucose

- and -D-glucose

Mutarotation: the spontaneous change in specific rotation of an optically active compound is called mutarotation.

Glucose doesn’t give shiff’s test and it doesn’t react with sodium bisulphite and ammonia.

Pentaacetate of glucose doesn’t doesn’t react with hydroxylamine indicating absence of —CHO group.

o

D ( ) Equilibrium D ( )

glucose mixture

52.5

Disaccharides

Made of two monosaccharides (same or different).Hydrolysed to giving two monosaccharides.

Two Common Polysaccharides

Main sources are wheat maize, rice, potatoes, barley etc.

Chief constituent of the cell walls of plants.

Proteins – Amides from Amino Acids

Amino acids contain a basic amino group and an acidic carboxyl group.

Joined as amides between the NH2 of one amino acid and the CO2H the next.

Chains with fewer than 50 units are called peptides.

Protein: large chains that have structural or catalytic functions in biology.

Structures of Amino Acids

In neutral solution, the COOH is ionized and the NH2 is protonated.

The resulting structures have “+” and “-” charges (a dipolar ion, or zwitterion).

They are like ionic salts in solution.

The Common Amino Acids

20 amino acids form amides in proteins

All are -amino acids - the amino and carboxyl are connected to the same C

They differ by the other substituent attached to the carbon, called the side chain, with H as the fourth substituent except for proline

Proline, is a five-membered secondary amine, with N and the C part of a five-membered ring.

Abbreviations and Codes

Alanine A, Ala

Arginine R, Arg

Asparagine N, Asn

Aspartic acid D, Asp

Cysteine C, Cys

Glutamine Q, Gln

Glutamic Acid E, Glu

Glycine G, Gly

Histidine H, His

Isoleucine I, Ile

Leucine L, Leu

Lysine K, Lys

Methionine M, Met

Phenylalanine F, Phe

Proline P, Pro

Serine S, Ser

Threonine T, Thr

Tryptophan W, Trp

Tyrosine Y, Tyr

Valine V, Val

Zwitterionic Form of an Amino Acid

The pH at with amino acid doesn’t move to any electrode, called isoelectric point.

Peptides

A tripeptide

Gly-Ala-Ser

N-terminal C-terminal

Amino Acid Sequence in Beef Insulin

Protein Classification

1. Simple proteins yield only amino acids on hydrolysis.

2. Conjugated proteins, which are much more common than simple proteins, yield other compounds such as carbohydrates, fats, or nucleic acids in addition to amino acids on hydrolysis.

3. Fibrous proteins consist of polypeptide chains arranged side by side in long filaments

4. Globular proteins are coiled into compact, roughly spherical shapes.

Most enzymes are globular proteins.

Some Common Fibrous and Globular Proteins

Protein Structure

1. The primary structure of a protein is simply the amino acid sequence.

2. The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern.

3. The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape.

4. The quaternary structure describes how different protein molecules come together to yield large aggregate structures

Structure of Proteins: -Helix

-Keratin

A fibrous structural protein coiled into a right-handed helical secondary structure, -helix stabilized by H-bonding between amide N–H groups and C=O groups four residues away a-helical segments in their chains.

Structure of Proteins: -Sheet

Linkages Contributing to Tertiary Structure

Fibroin

Fibroin has a secondary structure called a -pleated sheet in which polypeptide chains line up in a parallel arrangement held together by hydrogen bonds between chains.

Myoglobin

Myoglobin is a small globular protein containing 153 amino acid residues in a single chain.

8 helical segments connected by bends to form a compact, nearly spherical, tertiary structure.

Internal and External Forces

Acidic or basic amino acids with charged side chains congregate on the exterior of the protein where they can be solvated by water

Amino acids with neutral, nonpolar side chains congregate on the hydrocarbon-like interior of a protein molecule

Also important for stabilizing a protein's tertiary structure are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the development of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein

Protein Denaturation

The tertiary structure of a globular protein is the result of many intramolecular attractions that can be disrupted by a change of the environment, causing the protein to become denatured

Solubility is drastically decreased as in heating egg white, where the albumins unfold and coagulate

Enzymes also lose all catalytic activity when denatured

Thank you