Step 1: Understanding the Concept:
Adrenaline has one chiral centre, so it can exist as two mirror-image forms, R and S. The body makes and uses only the R-form, which fits tightly and strongly onto adrenergic receptors.
Step 2: Key Formula or Approach:
Synthetic adrenaline, made without a step that selects for one mirror-image form, comes out as a racemate: a 50:50 mix of R and S. Since the S-form binds adrenergic receptors very weakly, its contribution to the overall effect is small, so the biological activity of the mixture is set mostly by the R-half.
Step 3: Detailed Explanation:
If pure natural R-adrenaline is taken as 100% active, then a sample that is only half R-adrenaline, diluted with an essentially inactive S-half, gives about half the overall biological response, roughly 50%. This is why synthetic (racemic) adrenaline is markedly weaker in practice than an equal weight of the pure natural hormone.
Step 4: Final Answer:
The biological activity of synthetic adrenaline is almost 50% of natural R-adrenaline.