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?
Perhaps the answer is correct. If by cis/trans isomers the question includes the four stereogenic double bonds of the arachadonic acid chain and the six stereogenic carbons of the inositol ring, then there is a total of ten or 210=1024 isomers. But the answer is 1023 stereoisomers. That is because the question says "cis-trans-isomers OF...", which means NOT including the one shown. [210-1=1023.] Had the question asked "How many stereoisomers OF ...", then the lone stereogenic center of the glycerol moiety would have to be included. Then the answer would be 211-1=2047.