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