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Option D. D.8 drug action. Some aspects of molecular structure that affect the action of a drug . stereo isomerism geometric ( cis /trans) optical isomerism ( enantiomers ) s train in ring structure molecular polarity ( solubility and uptake).

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Option d

Option D

D.8 drug action


Some aspects of molecular structure that affect the action of a drug

Some aspects of molecular structure that affect the action of a drug

  • stereo isomerism

    • geometric (cis/trans)

    • optical isomerism (enantiomers)

  • strain in ring structure

  • molecular polarity (solubility and uptake)


Example geometric isomerism in diaminedichloro platinum ii complex ion used in a drug

Example: Geometric isomerism in diaminedichloro platinum (II) complex ion used in a drug

cisplatin

transplatin

cannot act as an antic-cancer drug as nitrogen atoms are too far apart to react with DNA – no pharmaceutical effect – actually toxic to body

Inorganic transition complex with a square planar shape, 900 bond angles and dative covalent bonds

  • can act as an anti-cancer drug because of its orientation, it can bond with DNA in the nucleus of cancer cells and cause the cell to die.


Optical isomerism

Optical isomerism

  • Synthesis of chiral drug molecules in laboratories produces racemic mixtures i.e. equimolar solution of both enantiomers.

  • These are known as “in vitro” drugs.

  • Such drugs were often prescribed as racemic mixtures or racemates.

  • Each enantiomer also has different biological activity as they behave differently with other chiralmolecules in the body.

  • Pharmaceutical companies must analyze effects of each isomer, and determine if it should be marketed as a racemate or if they need to then produce the single enantiomer.

  • Some enantiomers are harmful to humans.


Optical isomerism 2 3 of drugs on the market are chiral

Optical isomerism(2/3 of drugs on the market are chiral!)

Carbon atoms in ring structures (in which there is no delocalization to have 4 different atoms) can also be chiral if the part of the ring on one side of the carbon atom is different from the part of the ring on the other side of the carbon atom.

Each side of a ring is classified as a different group if, when moving around the ring from the chiral centre or carbon, the order of atoms is different each way. It is said the chiral atom has two different views around the ring.


Thalidomide

thalidomide

Identify the chiral carbon in the structure below

  • One enantiomer (R) alleviates morning sickness in pregnant women and induced sleep and calmness

  • Other enantiomer (S) is a teratogenic - causes deformities in the fetus

  • Later discovered that administering just the R enantiomer was no good either. It is able to turn into S inside the body.


Thalidomide1

thalidomide

Identify the chiral carbon in the structure below

This carbon has the N-ring, two

different sides of the benzene

structure, and a H (not shown)


How many chiral centres in fluoxetine

How many chiralcentres in fluoxetine?


More examples of chiral drugs

More examples of chiral drugs

  • Ibuprofen: one enantiomer is more effective than the other – identify the chiral centre in your data booklet (in vitro)

  • DOPA: this is a drug which is used in the treatment of Parkinson disease; one isomer is effective whilst the other isomer has no physiological effect

  • Fluoxetine (Prozac) – used to treat depression (in vitro)

  • Morphine/codeine/heroin: there are 5 chiralcentres in each of these molecules; only one enantiomer is analgesic. (in vivo – only one enantiomerb/c harvested naturally)


Beta lactam ring in penicillin

beta-lactam ring in penicillin

a four-membered square ring structure which contains an amide group

(-CONH-) and consists of one nitrogen atom and three carbon atoms (and two hydrogen atoms).


Beta lactam ring

beta-lactam ring

  • For each carbon and the nitrogen atom, identify the type of hybridization and preferred bond angles around each atom.

  • Referring to bond angles say why the ring is a strained structure


What causes the strain

What causes the strain?

  • As a result of the sp3hybridisation of two of the three carbon atoms and the single nitrogen atom and the sp2hybridisation of the third carbon atom, the preferred bond angles are 109 and 120.

  • However, the bond angles in the beta-lactam ring are reduced to only 90° and this puts the beta-lactam ring structure under strain weakening the bonds.


Beta lactam ring1

beta-lactam ring

(between

the N and the

sp2 C)

  • Strains makes ring reactive.

  • Ring easily breaks open in the amide group,(inthe presence of an enzyme such as transpeptidase)to form covalent bonds with the transpeptidase.

  • This deactivates the transpeptidase that synthesizes the bacterial cell walls, thus inhibiting the growth of bacterial cell walls.

  • As a result water enters the bacteria cell causing it to burst due to increased osmotic pressure.


Beta lactam ring2

beta-lactam ring

Sad 

Happy 


Drug structure and solubility

Drug Structure and Solubility

  • Drugs need to efficiently reach target receptors in the body.

  • Body is water soluble environment so water soluble drugs are good and can reach blood stream.

  • If CNS is the target, drug needs to reach brain.

    • Brain is surrounded by a “blood-brain” barrier which protects it from restricting what can enter from the blood. It is hydrophobic, and non-polar so polar molecules cannot easily cross.


Why is heroin more potent than morphine

Why is Heroin more potent than morphine?

morphine

heroin

Both hydroxyl groups replaced by ester groups

  • Two hydroxyl groups


Heroin more potent than morphine

Heroin more potent than morphine

  • Diamorphine is less polar than morphine as it does not have the polar hydroxyl groups anymore but two less polar ester groups.

  • Diamorphine (heroin) cannot form any hydrogen bonds and is therefore less soluble in polar substances such as water but more soluble in non-polar fatty tissue, which makes up the central nervous system.

  • Heroin can therefore cross the blood-brain barrier faster/more easily than morphine.


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