3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.1. Pharmacology principles
                      3.1.3.1.2. Pharmacodynamics
 3.1.3.1.2.1. Stereochemistry 

Stereochemistry

[SH4:p3]

Stereochemistry is the study of molecule structure in 3D.

Terminology

Chiral

  • Chiral describes a molecule that has a centre or centres of 3D asymetry.

Enantiomer

  • Enantiomer is a pair of molecules that exist in 2 forms that are mirror images of one another, but cannot be superimposed.

Effects on polarised light

When dissolved in solutions, they rotate polarised light

  • Clockwise
    = dextrorotatory
    = d
    = [+]
    = (???) Rectus
  • Anticlockwise
    = levorotatory
    = l
    = [-]
    = (???) Sinister

Properties of enantiomers

  • Enantiomers have the same physical and chemical properties
  • Biological effects are not the same
    --> Can even antagonise each other

Racemic mixture

  • Racemic mixtures are solutions where the two enantiomers are present in equal proportions
    --> Actions cancel out
    --> Does NOT rotate polarised light

Examples

Examples of racemic mixture

Common drugs administered as racemic mixture

  • Thiopentone
  • Methohexital
  • Ketamine

Examples of enantiomers

Ketamine

S(+) enantiomer of ketamine is

  • More potent
  • Less likely to cause emergence delirium

Ropivacaine

...is S(-) enantiomer of a bupivacaine homolog

  • Has less cardiac toxicity than the R(+) enantiomer

(???) (Is ropivacaine administered as a racemic mixture or not?) I think it is administered as S(-) --> Ref???

Levobupivacine

... is S(-) enantiomer of bupivacaine

  • Has less cardiotoxicity than the R(+) enantiomer

Cisatracurium

... is one of the ten isomers of atracurium

  • Less histamine-releasing potential

Inhalational agents

  • Most inhalational agents are chiral (except for SEVO)
  • Enantiomer-selective effects for inhalational agents are not as strong as it is for IV anaesthetics

Others

  • Dextrose - dextrorotatory