D8 Drug Action Stereochemistry: Geometrical isomerism Stereochemistry: Chirality Ring strain: The  -lactam ring in penicillin Polarity: The increased potency of heroin

Geometrical isomerism Carbon to carbon double bond Full name? A ring where there is restricted rotation. The two different geometric isomers may have very biological different effects. Diamine dichloroplatinium (II), Pt(NH 3 ) 2 Cl 2 has a cis- and a trans- isomer. The cis- isomer of Pt(NH 3 ) 2 Cl 2 is highly effective in the treatment of testicular and ovarian cancers, as well as other forms of cancer. The trans-isomer trans- platin is not an effective anti-cancer drug.

Chirality Chiral carbons have four different groups and may exist as mirror images, e.g. amino acids Drug molecules that process a chiral carbon atom will exist in two different enantiomeric forms. For example thalidomide (neurosedyn) Where is the chiral carbon?

Thalidomide – One of the thalidomide enantiomers alleviates the symptoms of morning sickness in pregnant woman; – The other enantiomer causes severe deformities in the limbs of the baby.

Ring strain All penicillins contain a 4-membered beta-lactam ring. The ring contains 2 carbon atoms that are sp 3 hybridized, a nitrogen atom that is sp 3 hybridized, and a carbon atom that is sp 2 hybridized. This structure is similar to the combination of two amino acids cysteine and valine. Penicillin Cysteine Valine Because of the restrictions of the ring, the normal bond angles of and 120 are not able to be obtained, and so the ring becomes strained. This makes the amide highly reactive. The ring opens so that the penicillin can become covalently bonded to the enzyme that synthesizes the cell walls of the bacterium, thus blocking its action.

Polarity- the increased potency of diamorphine (heroin) compared to morphine Morphine molecules contain two polar hydroxyl groups; in heroin, these molecules are replaced by ethanoate (acetate) groups. These two ester groups make heroin much less polar than morphine and more soluble in lipids, which are non-polar. Heroin can thus easily penetrate the lipid-based blood/brain barrier. MorphineHeroin Polar groups Much less polar groups

The blood-brain barrier A mechanism that controls the passage of substances from the blood into theblood cerobrospinal fluidcerobrospinal fluid and thus into the brainbrain and spinal cord. The blood-brain barrier (BBB)spinal cord lets essential metabolites, such as oxygenoxygen and glucose, pass from the blood to theglucose brain and central nervous system (CNS) butcentral nervous system blocks most molecules that are more massive than about 500 daltons. This is a low mass in biomolecular terms and means that everything from hormones andhormones neurotransmittersneurotransmitters to viruses and bacteriavirusesbacteria are refused access to the brain by the BBB. It also means that many drugs, which would otherwise be capable of treating disorders of the CNS, are denied access to the very regions where they would be affective.