Reaction of ionised steryl esters with ozone in the gas phase

Abstract

Cholesterol is an ubiquitous membrane lipid, that also serves as a precursor to many steroid hormones. The 5,6 carbon-carbon double bond on the tetracyclic carbon backbone of cholesterol is an attractive target for ozone with the reaction giving rise to a wide range of possibly bioactive molecules. Despite this, little is known about the ozonolysis of cholesterol esters, which often possess an additional double bond(s) on the fatty acyl chain. Understanding the intrinsic gas phase reaction of ozone with the two disparate double bond positions on cholesteryl esters can inform our understanding of these processes in vivo, particularly reactions occurring at the air-water interface (e.g., tear film lipid layer) and on the surfaces of the body where these cholesterol and cholesteryl esters may be present (e.g., sebum). In the present work we describe the gas phase ozonolysis of lithium and sodium cations formed from three steryl esters: two isomeric for double bond position (cholestanyl oleate and cholesteryl stearate), and a third with carbon-carbon double bonds present in both the sterol ring system and fatty acyl chain (cholesteryl oleate). We confirm the enhanced reactivity of the endocyclic carbon-carbon double bond with ozone over double bonds present in the acyl chain, and elucidate competitive interactions between the two double bond positions during ozonolysis. Elucidation of the mechanisms underlying this interaction is important for both understanding these processes in vivo and for deploying ozonolysis chemistry in analytical strategies for lipidomics.

Ozone-Induced Cleavage of Endocyclic C═C Double Bonds within Steroid Epimers Produces Unique Gas-Phase Conformations

“…24 In this work, we show that ozone-induced cleavage of steroid endocyclic CC double bonds results in a ring opening that produces unique, energetically stable gas-phase conformations. Hancock et al recently demonstrated that OzID could be used to react endocylic CC double bonds in steryl esters; 25 however, the analytical utility of that observation was limited due to a lack of ion mobility separation. In this work, we demonstrate efficient IMS separation in a model steroid epimer pair, testosterone and epitestosterone, following ozone-induced cleavage of their endocyclic CC double bond.…”

Analytical separations for lipids in complex, nonpolar lipidomes using differential mobility spectrometry

“…Atomistic modeling has shown that liquid-phase cholesteryl oleate exists on a spectrum of folded to unfolded ensembles, with the greatest flexibility being present between carbons 3 and 5 on the acyl chain (67). In previous work, we determined that an interaction exists between the site of charge localization on the carbonyl and both the endocyclic and acyl carbon-carbon double bonds in cholesteryl oleate that potentially aids folding of the structure in the gas phase, resulting in a smaller CCS and enhanced ion mobility (68). Therefore, it stands to reason that the distance between the sterol head group and double bond may also affect the folding of the steryl ester in the gas phase, with elongated distances between the sterol and double bond resulting in compact structures when the double bond is located >15 carbons from the carboxylate moiety.…”

Ion‐molecule reactions of mass‐selected ions

“…While IMR remain a fairly “niche” technique, undoubtedly due to the requirements for often‐undesirable instrument modifications and reaction time penalty compared to other tandem MS techniques, some areas have clearly benefited from the technique. On the analytical side, the OzID technique pioneered by the Blanksby group for fatty acids and lipid analysis has been very successful (Bhujel et al, 2020; Hancock et al, 2019; Khairallah et al, 2015; Marshall et al, 2014, 2016, 2019, Paine et al, 2018; Pham et al, 2014; Poad et al, 2011, 2017; Thomas et al, 2007), especially with the rapid development of lipidomics. Recent advances include incorporation of liquid chromatography, ion mobility and imaging creating very powerful applications with unmatched capabilities.…”

“…Work done by Blanksby and coworkers explored the potential use of OzID on steryl esters. It revealed that endocyclic double bonds are far more reactive to those on the unsaturated acyl chain (Hancock et al, 2019).…”