Comparison of Polypeptide and Glycopeptide Antibiotics

 

Below are two different diagrams of bacitracin. Not sure which one has more accurate bond angles. I rarely see molecules form big circles.

Wikipedia says:

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Cyclic peptides are polypeptide chains which contain a circular sequence of bonds.[1] This can be through a connection between the amino and carboxyl ends of the peptide, for example in cyclosporin; a connection between the amino end and a side chain, for example in bacitracin; the carboxyl end and a side chain, for example in colistin...

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Colistin is a polymyxin, but since it was mentioned as being a cyclic peptide too, I'll add it below.

Amanitin has an indole core. That's first one I've seen so far. 

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Teicoplanin is an antibiotic used in the prophylaxis and treatment of serious infections caused by Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and Enterococcus faecalis. It is a semisynthetic glycopeptide antibiotic with a spectrum of activity similar to vancomycin. Its mechanism of action is to inhibit bacterial cell wall synthesis.[2]

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Its strength is considered to be due to the length of the hydrocarbon chain.[4]

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I'm not sure why the length of the hydrocarbon chain would matter. Then again, I only had two semesters of organic chemistry in college. Checking the reference, I got this:

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Teicoplanin is the only glycopeptide antibiotic not to dimerise, but its potency is believed to come from its long hydrocarbon chain. This anchors the antibiotic to the cell membrane from where it can more effectively inhibit cell wall synthesis.

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I wonder how that hypothesis about the length of the carbon chain could be tested. If it could be made to dimerize, would that molecule also be medicinal?

This one has a good breakdown of what are effectively the antibiotic's functional groups. 

Like teicloplanin, telavancin has a long hydrocarbon chain coming off a nitrogen.  

Ramoplanin also has a hydrocarbon chain coming off a nitrogen, though the chain has carbon-carbon double bonds and a keto group like teicoplanin.  

These last three are chemotherapy drugs.

Pingyangmycin is interesting because it is also a glycopeptide but is used to treat cancer rather than bacterial infections.