Chemical substances of natural or synthetic origin which are used for curing disease and reducing suffering from pain are called medicines or drugs. The branch of chemistry which deals with the treatment of diseases using suitable chemicals is known as chemotherapy.
Drugs or Medicines
A medicine is a chemical substance which cures the disease, is safe to use, has negligible toxicity and does not cause addiction. In contrast, a drug is a chemical substance which also cures the disease but is habit forming, causes addiction and has serious side effects.
Thus , both penicillin and heroin are medicines because penicillin is an effective antibacterial agent and heroin is a powerful pain killer but penicillin is called a medicine since it does not have addictive properties while heroin is called a drug because of its pronounced habit forming and addictive properties .
Classification of Drugs
On the basis of pharmacological effect : It is useful for doctors because it provides them the whole range of drugs available for treatment of a particular disease or infection. For e.g. Analgesics reduce or kill pain while antiseptics either kill or arrest the growth of microorganisms.
On the basis of drug action : This classification is based on the action of drug on a particular biochemical process. For e.g. All antihistamines inhibit the action of histamines.
On the basis of chemical structure : Drugs have also been classified on this basis since drugs have common structural features often have similar pharmacological activity. For e.g. All sulphonamides have common structural feature and mostly are antibacterial.
On the basis of molecular targets : Drugs possessing some common structural features may have the same mechanism of action on a specific drug target molecule. This classification is most useful for medicinal chemists.
Drug Target Interaction
a) Catalytic Action of Enzymes
The first action of an enzyme is to hold the substrate molecule for a chemical reaction . The active site of enzyme hold the substrate in a suitable position so that it can be attacked by reagent effectively . The substrate molecule bind to amino acid residue present on active site of enzyme through a variety of interaction such as hydrogen bonding , dipole dipole interaction and vanderwaal interaction .
These binding forces should be strong enough to hold the substrate for long so that enzyme can catalyse the reaction , but weak enough to allow the product to depart after the formation .
The second function of enzyme is to provide the functional group which will attack the substrate to carry out the chemical reaction .
b) Drug Enzyme Interaction
Drugs which inhibit any of the two activities of the enzyme discussed above are called enzyme inhibitors . Drug inhibit the attachment of natural substrate on active sites of enzymes in two different ways as explained below :
Drugs which compete with natural substrate for their attachment on the active site of enzymes are called competitive inhibitors .
There are some drugs which do not bind to the active site but bind to a different site of the enzyme which is called allosteric site . This binding of drug at allosteric site changes the shape of active site of the enzyme in such a way that natural substrate cannot recognize it . Such enzymes are called non – competitive inhibitors.
Receptors As Drug Targets
Receptors are proteins which are crucial to the communication system in the body . Majority of these are embedded in the cell membrane in such a way that their small part possessing the active site projects out of the surface of membrane and opens on the outside region of the cell membrane .
How do receptors transfer message into the cell ?
In the body , the message between two neurons or that between neurons and muscles is communicated through certain chemical substances called chemical messengers . These chemical messengers are received at the binding site of the receptor protein . To accommodate these chemical messengers , the shape of receptor protein changes a little and the messenger gives the message to the cell without entering the cell . After the transfer of message , the chemical messenger departs and the active site of the receptor protein returns to its original shape.
Therapeutic Action of Different Classes of Drugs
Over production of hydrochloric acid in the stomach cause irritation and pain . In severe cases , ulcers are produced in stomach . Thus , substances which neutralize the excess acid and raise the pH to an appropriate level in stomach are called antacids ,e.g. ranitidine (zantac) , cimetidine (tegamet) .
Recently , omeprazole and lansoprazole have also been used as antacid . These also prevent the formation of acid in stomach .
The hypersensitivity of some persons to some drugs , dust , pollen grains , cat fur , a particular type of food or fabric is called allergy and is due to release of substance called histamine in the body .
The drugs which interfere with the natural action of histamine by competing with it for binding sites of receptor where histamine exert its effect are called antihistamines or anti-allergic drugs .
These are widely used for treatment of hay fever, conjunctivitis, sneezing, nasal discharge, itching of eyes.
The antihistamine drug which are widely used are brompheniramine and terfenadine.
3) Neurologically Active Drugs
Drugs which are used for treatment of stress , fatigue , mild and severe mental disease are called tranquilizers. They are also essential component of sleeping pills .
Noradernaline is one of the neurotransmitter which play an important role in mood changes . If the level of noradernaline in body is low , then message transfer process become slow and person suffers from depression . In such cases antidepressant drugs are used .
Chlordiazepoxide and meprobamate are relatively mild tranquilizers and are used for relieving tension .
Equanil is used in controlling depression and hypertension . Some other substances used as tranquilizers are valium and serotonin .
Neurologically active drugs which reduce or abolish pain without causing impairment of consiousness , mental confusion , incoordination or paralysis or some other disturbance or disorder of the nervous system are called analgesics .
These are classified into following two categories :
Non – narcotic (non -addictive) analgesics : Aspirin (2-acetoxybenzoic acid ) and paracetamol (4-acetamidophenol) are the most important examples of non addictive analgesics . These drugs are quite effective in relieving skeletal pain . Beside this , these drugs have many other effect such as reducing fever (antipyretic) and preventing blood platelet coagulation.
Narcotic (addictive) analgesics : Drugs which when administered in small doses relieve pain and produce sleep are called narcotics . Morphine diacetate commonly known as heroin is the most widely used analgesic .
Drugs which are used to cure diseases caused by microbes or microorganisms such as bacteria , viruses , fungi are called antimicrobials . These are of following types :
Antibiotics are defined as chemical substance (produced wholly or partially by chemical synthesis ) , which in low concentration either kill or inhibit the growth of microorganisms by intervening in their metabolic processes .
Types of Antibiotics : The antibiotics can either be bactericidal (that kills microorganisms , e.g, penicillin ) or bacteriostatic (that either inhibit or arrest the growth of microorganism , e.g, Chloramphenicol)
Penicillin is a very effective drug for pneumonia , bronchitis , sore throat .
The full range of microorganism attacked by an antibiotic is called its spectrum .
Broad spectrum antibiotic are effective against several different types of harmful bacteria and kill a wide range of gram positive and gram negative bacteria . Example : tetracyclin , ofloxacin
Narrow spectrum antibiotic are effective against a short range of gram positive or gram negative bacteria . Example : Penicillin
If an antibiotic is effective against a simple microorganism , it is referred to as limited spectrum antibiotics .
Certain antibiotics are specific for certain disease . For example streptomycin for tuberculosis , chloramphenicol for typhoid and pneumonia and dysidazirine is supposed to be toxic towards certain strains of cancer cells .
b)Antiseptics and Disinfectants
Antiseptics are the chemical substances (i.e antibacterial) which prevent the growth of microorganism and may even kill them . They are safe to be applied to living tissues . They are generally applied on wounds , ulcer and diseased skin surface in form of antiseptic cream like furacin , soframycin .
Disinfectant are chemical substances which kill microorganism but are not safe to be applied to the living tissue .
The same substance can act as an antiseptic as well as disinfectant depending upon the concentration of solution used . For example : 0.2% solution of phenol acts as antiseptic whereas 1% solution acts as disinfectant .
Dettol which is a well known antiseptic is a mixture of chloroxylenol and α-terpineol in a suitable solvent .
Savlon is a solution of chlorhexidine gluconate in a suitable solvent .
Salol (phenyl salicylate) is used as an intestinal antiseptic for throat ailments .
Chemicals In Food
All those chemicals which are added to food to improve its keeping qualities, appearance, taste, odour and nutritive value are called food additives . Some important food additives are :
Flavours and sweeteners
Fat Emulsifiers and stabilising agent
Nutritional supplements such as minerals , vitamins and amino acids
Artificial Sweetening Agents
Artificial sweeteners are the food additives that provides a sweet taste like that of sugar while containing significantly less calories than sugar based sweeteners .
Aspartame It is one of the most significantly and widely used artificial sweetener . It is methyl ester of the dipeptide derived from phenylalanine and aspartic acid . It is roughly 100 times as sweet as cane sugar . It decomposes at baking or cooking temperatures and hence can be used only in cold food .
Alitame : It is similar to aspartame . However , it is more stable than aspartame . It is about 2000 times as sweet as sucrose . It is difficult to control the sweetness of food to which it is added .
Sucrolose : It is a trichloro-derivative of sucrose . It looks and taste like sucrose and is stable at temperatures used for cooking and baking . It is about 600 times sweeter than sucrose. It neither provide calories nor causes tooth decay .
Chemical substances which are used to protect food against bacteria , yeasts and moulds are called preservatives . Example : Sodium benzoate , Epoxides , Sodium metabisulphite etc.
Chemicals which are used to prevent oxidation of fats in processed foods such as potato chips , biscuits , breakfast cereals , crackers are called antioxidants . These may also be regarded as sacrificial materials since they are more reactive towards oxygen than material they are protecting .
Chemical substances which concentrate at the surface of the solution or interfaces , form surface films , reduce surface tension of the solution and help in removing dirt and dust by emulsifying grease are called surface active agents or surfactants .
These are of two types : Soaps and Synthetic detergents.
Soaps are sodium or potassium salt of long chain fatty acid represented as RCOO– Na+ .
Soaps are formed by heating fat or oil with aqueous sodium hydroxide solution . This reaction is called saponification .
Advantage: Soap is a good cleansing agent and is 100% biodegradable, i.e., microorganism present in sewage water can completely oxidise soap to CO2. As a result , it does not create pollution problem.
Question : Why soap do not work in hard water ?
Answer : Soaps do not work well with hard water because Mg2+ and Ca2+ ions present in hard water react with soap to form magnesium and calcium salt of fatty acid which are insoluble in water and form scum , which causes hindrance to good washing .
Question : Why soap cannot be used in acidic solution ?
Answer : This is because acid present in solution precipitate the insoluble free fatty acid which adhere to the fabrics and thus reduce the ability of soap to remove oil and grease from fabrics.
Types of Soaps
Toilet Soaps are prepared by using better grades of fats and oils and care is taken to remove the excess alkali . Colour and perfumes are added to make them attractive .
Floating Soaps can be prepared by beating tiny air bubbles into the product before hardening .
Transparent Soaps are prepared by dissolving soap in ethanol and then evaporating excess solvent .
Shaving Soaps contain glycerol to prevent rapid drying . While preparing these soaps , a gum called rosin is added to them .
Soap granules are dried miniature soap bubbles.
Detergents are also called soapless soap because they have all the properties of soap but do not contain any soap like sodium salt of higher fatty acids. Since these are synthetic substances , therefore they are called synthetic detergents or syndets or simply detergents.
Unlike soap , synthetic detergent can be conveniently used even in hard water . This is due to the reason that calcium and magnesium salt of detergents are also soluble in water . Hence , synthetic detergent do not form scum with hard water .
They can also be used in acidic solution because unlike fatty acid , sulphonic acid are soluble in water .
Disadvantage : They are not completely biodegredable and cause water pollution .
Classification of Detergents
a) Anionic Detergents
They are so called because a large part of their molecules are anions and it is the anionic part which is involved in their cleansing action .
These are also used in toothpaste .
They are of two types :
Alkyl hydrogen Sulphates : These are obtained from long chain alcohols by treatment with concentrated sulphuric acid followed by neutralisation with NaOH . These are 100% biodegradable . Example : sodium lauryl sulphate or sodium stearyl sulphate.
Alkylbenzene sulphonates : These are obtained by friedel craft alkylation of benzene with a long chain alkene or an alcohol followed by sulphonation and neutralisation with NaOH .
b) Cationic Detergent
These are also called invert soap. These are quaternary ammonium salt containing one or more long chain alkyl group . Example : cetyltrimethylammonium bromide . These detergent possess germicidal properties and hence are quite extensively used as germicides .
c) Non-Ionic Detergents
They do not contain any ions. These are actually esters of higher molecular mass alcohols obtained either by reaction between polyethylene glycol and stearic acid . Liquid dishwashing detergents contain non-ionic detergents.