Opioids Opiate potency/chemistry question

[Branggen]

Hello everyone, what I was wondering was : What is the property of the various opiates that makes them differ in potency so much, considering 100mg of codeine is about equal to .1mg of fentanyl. I have a limited knowledge of opiate chemistry in the brain but would most likely be able to understand a full explanation, the only guess I had was that perhaps the stronger opiates just had more 'endorphin like'(opiod) molecules per unit of volume. Please enlighten me as this doesn't seem very plausible.

 

[sekio]

Some molecules fit better in an opioid receptor than others.

 

[Branggen]

This has got me wondering what exactly it takes for a molecule to be opiate receptor compatible, and if anything other than chemical structure effects the potency/compatibility. I have also heard that certain opioids bind to specific receptors (or multiple ones) and can either be an agonist, antagonist or both. If anyone has any info, or a link to something similar I would appreciate it... Thanks

And here's a pic of some opioids