Dynamic equilibrium of Girard T-derivatized oxysterols
Posted: Sat Apr 08, 2023 3:27 am
Hi, I posted this over on stackexchange a couple of days ago, but haven't gotten any response. Perhaps someone here might be able to share some thoughts.
I am running LC-MS on various hydroxycholesterols (HC) and dihydroxycholesterols (diHC). To increase ionization efficiency with the electrospray, I derivatize my oxysterols with Girard T reagent. The picture below shows the two forms, syn and anti, that the derivatized oxysterols take (here showing derivatized 7α,24S-dihydroxycholesterol).
It is common knowledge that these isomers often have different retention times through an LC column, leading to split peaks. Some of my oxysterols do show splitting, but my group believes this not to be caused by the syn/anti-situation, as they claim the stationary phase I am using (phenylhexyl from ACE) does not show this. Rather, they say it is probably contamination of our standards (perhaps of some precursor).
I have observed that the split peaks converge when column temperature is increased. Below is a chromatogram of 7α,24S-diHC running in SRM mode (530.432->471.358), and splitting at 40 C, but becomes a single broad peak at 70 C. This is consistently true for all my diHCs and one of my HCs (22R-HC, the rest show no splitting at 40 C).
I am using an AFFL system (automatic filtration and backflushing) where I first load my sample onto an SPE, diverting polar compounds (like excess Girard T reagent) to waste, before switching and eluting the oxysterols onto my column with a higher %B (~67% water, 26.7% acetonitrile and 6.3 % methanol. pH is 2.8 via 0.1 % formic acid).
My question is this: Could there be a dynamic/active equilibrium ongoing between the syn and anti conformations, or is this unlikely given the presumably rigid hydrazone bond? This could, perhaps, explain the "bridge" between the two split peaks, and how an equilibrium might favor a certain isomer at a higher temperature.
I am running LC-MS on various hydroxycholesterols (HC) and dihydroxycholesterols (diHC). To increase ionization efficiency with the electrospray, I derivatize my oxysterols with Girard T reagent. The picture below shows the two forms, syn and anti, that the derivatized oxysterols take (here showing derivatized 7α,24S-dihydroxycholesterol).
It is common knowledge that these isomers often have different retention times through an LC column, leading to split peaks. Some of my oxysterols do show splitting, but my group believes this not to be caused by the syn/anti-situation, as they claim the stationary phase I am using (phenylhexyl from ACE) does not show this. Rather, they say it is probably contamination of our standards (perhaps of some precursor).
I have observed that the split peaks converge when column temperature is increased. Below is a chromatogram of 7α,24S-diHC running in SRM mode (530.432->471.358), and splitting at 40 C, but becomes a single broad peak at 70 C. This is consistently true for all my diHCs and one of my HCs (22R-HC, the rest show no splitting at 40 C).
I am using an AFFL system (automatic filtration and backflushing) where I first load my sample onto an SPE, diverting polar compounds (like excess Girard T reagent) to waste, before switching and eluting the oxysterols onto my column with a higher %B (~67% water, 26.7% acetonitrile and 6.3 % methanol. pH is 2.8 via 0.1 % formic acid).
My question is this: Could there be a dynamic/active equilibrium ongoing between the syn and anti conformations, or is this unlikely given the presumably rigid hydrazone bond? This could, perhaps, explain the "bridge" between the two split peaks, and how an equilibrium might favor a certain isomer at a higher temperature.