# How many cis-trans isomers does 1-O-stearoyl-2-O-arachidonyl-phosphatidyl-inositol-4',5'-bisphosphate have?

I saw an exam that had a question asking the number of cis-trans-isomers of 1-O-stearoyl-2-O-arakidonyl-phosphatidyl-inositol-4',5'-bisphosphate:

The "correct" answer in the exam is 1023, but this is clearly not correct, because the stereoisomerism of the inositol ring is not called cis-trans-isomerism.

So, obviously only the arachidonyl chain has cis-trans isomerism. But if an isomer has several of the double bonds in trans orientation (not just one), is is still correct to call it a cis-trans isomer of the original molecule. If it is, I would say that this molecule has 15 cis-trans isomers. If it isn't I would say that it has only 4 cis-trans isomers.

Which one is correct?

• There can be cis and trans in the cyclic rings. The substituents can be axial or equatorial. Also, 15 is too low a number for the number of stereo isomers. Just consider the double bonds, you have four of them, each has two choices, cis or trans. That gives $2^4$ isomers on its own. Just 16 isomers from the double bond chain, there's way more possible from the ring. Keep trying, you'll get it soon. – Pritt says Reinstate Monica Jun 2 '17 at 9:44
• The isomerism in the ring is not called cis-trans-isomerism in this case. (See my other question at chemistry.stackexchange.com/questions/74853/…) And I would not say that the molecule is a cis-trans isomer of itself. Therefore it can't have more than 15 cis-trans isomers. – ttsc Jul 3 '17 at 18:27