A rotatable bond is defined as any single non-ring bond, attached to a non-termi

Question

A rotatable bond is defined as any single non-ring bond, attached to a non-terminal, non-hydrogen atom. Amide C-N bonds are typically not counted because of their high barrier to rotation. Remember Delta G = Delta H - T Delta S! A molecule will have to adopt a fixed conformation to bind to an enzyme or receptor, or to pass through a membrane (bioavailability). This involves a loss in entropy, so if the molecule is more rigid to start with, less entropy is lost. Therefore, a limited number of rotatable bonds can lead to better bioavailability. Determine the number of rotatable bonds in the beta-blockers shown below. Which drug would you expect to have better bioavailability? Based on the compounds below, explain why rigid analogous may not always be more potent or efficacious than the more flexible analogue.

Explanation / Answer

3. In the given two structures,

the number of rotatablesbonds in the moelcule are,

first molecule : 15

second moelcule : 9

Thus of the two, the more bioavailable drug would be propranolol

Rigid analogus may not always be more potent as they cannot rearrange themselves to the binding site very easily due to fixed shape and rotation and thus, are less possible in interacting with al types of potential sites.

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