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I've seen people arguing for some questionable "miracle cure-all" alternative medicines not scientifically, but by their own logic. A common claim is, that if a compound looks very similar to an important compound found in our body, it must be very beneficial. By looks very similar I mean the drawings of their structures look similar, as in, similar as images.

I found this reasoning alone to be unscientific. (correct me if I'm wrong). I would like to explain it to people, but it's hard to do if they lack even basic understandings in chemistry, and have claims like "I think it might happen to be like this, so it's a proof that it's like this".

So, I'm looking for a counter-example. Are there famous and well-known (organic) compounds which look similar on a structure drawing, but have vastly different properties, for example one is either beneficial or a necessary component of our body, and the other is highly toxic?

I'm sorry for the possible unclear terminology, English is not my first language and I didn't find any better term to say "structure drawing" for something like this

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trb456 and Mad Scientist have raised the point that enantiomeric forms of a bioactive molecule may have markedly different activities, however there's more to it than that. A molecule that has a similar shape and surface charge distribution to another biologically important molecule is liable to bind to the same protein receptor sites, but in different ways. This may be a good or a bad thing and is a crucial observation of modern drug design.

As an example, the potent hallucinogen DMT (all image credits go to wikipedia):

DMT

and the fairly pedestrian neurotransmitter serotonin:

enter image description here

Are strikingly similar in the presence of both an indole and ethylamine group in both molecules.

Perhaps the ur-example of two extremely similar molecules with tremendously different effects in the body are:

Estradiol (female sex hormone):

enter image description here

Testosterone (male sex hormone):

enter image description here

The reasoning that a molecule that looks similar to a biologically important species must be beneficial is flawed. A molecule that looks similar to a biologically important species may just as easily strongly bind to and jam up the active site of something really important. This is how carbon monoxide, sarin, disulfiram and glyphosate work.

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    $\begingroup$ this is a great answer with terrific diagrams! Yours was the type of answer I was expecting to this question, so I'm glad it's here. My answer was just to point the even more extreme conclusion that sometimes shape is the only thing that matters. For people without much knowledge of chemistry, that's likely a very surprising result. $\endgroup$ – user467 Nov 28 '12 at 13:37
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One example of disastrously different effects of very similar molecules is thalidomide. There are two isomers, only differing in their stereochemistry. The (R)-isomer is responsible for the sedating effect it was initially used for. The (S)-isomer lead to horrifying birth defects in the children of woman that took that drug.

enter image description here

It is somewhat more complicated, as the isomers are racemized quickly in vivo, so even if you administer only the (S)-isomer, it will still be converted to the (R)-isomer in the body.

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  • $\begingroup$ @vsz: I'd say the examples by Mad Scientist and F'x are even better than you asked for. These compounds are not just similar, they have the same atomic structure. Only the spacial geometry differs, to disastrous results. Hopefully this gives you good educational material for the people you have in mind! $\endgroup$ – user467 Nov 28 '12 at 2:01
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Recall that carbon in an organic molecule has 4 bonds in a tetrahedral shape. If each of those bonds connects to a different group, then there are two configurations of the bonds which have the same formula but a mirror image geometric relationship in space. Such a carbon (or molecule) is called chiral. Chiral molecules have the same formula but different spacial structures and thus are different molecules.

For example, carvone has two enantiomers (mirror images): R-(-)-carvone smells like spearmint, while S-(+)-carvone, smells like caraway. This is because different olfactory receptors in the nose fit the molecules differently.

Here's a video about carvone and chirality from the good folks at Periodic Videos.

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    $\begingroup$ Good example. Maybe even more remarkable is Naproxen: one enantiomer is used to treat arthritis pain, but the other causes liver poisoning with no analgesic effect. (source) $\endgroup$ – F'x Nov 27 '12 at 21:10
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In biochemistry branch searching for new drugs, it was common to search for new active substances using chemical similarity. It does have reasoning behind it.

Most drugs acts, binding themselves to some protein site, in the place, where normal ligand of the protein will bind. However, chemical similarity is not enough to be a drug. Sure, a substance must bind to given protein strongly, and imitating typical protein ligand is wonderful for it. But to be a drug, substance must differ from natural substance to modify protein behavior, usually blocking its activity or activating in heavily.

Because of it, drugs are often searched in modified natural protein ligands or their structural analogs, but usually modified. For example, there is well known group of beta-blockers, that mimics noradrenaline. However, they have to indestructible in vivo and bind strongly, preferably to some, but not all types of Adrenergic receptor, so all of them has structural twist.

In addition, theoretically it is possible to find a drug in chemically unrelated structures, if they exhibit required groups in right positions or can bind to target protein in some other way.

As for your question, the most infamous example is previously reported thalidomide. However, it is not the only one. For example, most aminoacides has to enantiomers, but only one of them is usable for our bodies

To get basic understanding of the subject I recommend to dwell the wiki starting from this article

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1-propanol and 2-propanol are isomers but have different boiling points: 370 K and 356K. As I remember my undergraduate studies correctly, my distillation was to separate a mixture of these two alcohols. The metabolism of the two alcohols are different.

Further readings:
http://en.wikipedia.org/wiki/Propan-1-ol
http://en.wikipedia.org/wiki/2-propanol

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