Cholesterol molecular structure
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Cholesterol Molecular Structure: Core Features
Cholesterol is a steroid molecule that is a major component of animal cell membranes. Its structure consists of a rigid four-ring steroid core, a hydrocarbon tail, and a single polar hydroxyl group at the C(3) position. This unique combination of a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) body allows cholesterol to position itself at the interface between polar and nonpolar regions in biological membranes, influencing membrane structure and function 234.
Cholesterol Crystal Structures: Anhydrous and Monohydrate Forms
Cholesterol can crystallize in different forms, most notably as anhydrous cholesterol and cholesterol monohydrate. Both forms have complex triclinic crystal structures with multiple independent molecules per unit cell, arranged in double-layered or bilayer-like stacks. These arrangements mimic the way cholesterol packs in biological membranes, providing insight into its preferred molecular associations and conformations 23. In the monohydrate form, water molecules are incorporated into the crystal lattice, further stabilizing the structure .
Cholesterol Self-Association and Clustering in Membranes
Within lipid bilayers, cholesterol molecules tend to self-associate, forming small clusters such as dimers and tetramers. At lower concentrations, dimers are the most common, while higher concentrations (similar to those found in certain cell membranes) lead to the presence of both dimers and tetramers. These clusters are stabilized by face-to-face interactions between cholesterol molecules and are observed in both ordered and disordered membrane environments . Molecular dynamics simulations confirm the stability and lifetimes of these clusters, suggesting that dimers are the basic structural unit of cholesterol in membranes .
Cholesterol Conformation and Interactions in Membranes
Cholesterol’s rigid steroid ring system and flexible hydrocarbon tail allow it to interact closely with phospholipid molecules in membranes. The hydroxyl group can form hydrogen bonds with water or with the oxygen atoms of lipid head groups, while the tail aligns with the fatty acid chains of phospholipids. This interaction increases the order and rigidity of nearby lipid chains, reducing their flexibility and promoting a more ordered membrane structure 45. The iso-octyl tail of cholesterol is particularly rigid at the top, similar to the ring system, and its conformation can shift depending on the saturation of surrounding phospholipid chains .
Cholesterol in Complexes and Model Membranes
Cholesterol can also form stable inclusion complexes with molecules like β-cyclodextrin, where it is encapsulated within a dimeric structure stabilized by water molecules. These complexes have been structurally characterized and remain stable in aqueous environments, providing models for cholesterol’s behavior in biological and industrial contexts . In model membranes, cholesterol’s presence leads to increased membrane thickness and the formation of distinct domains or “rafts,” especially at critical concentrations, which are important for membrane stability and function .
Conclusion
Cholesterol’s molecular structure—featuring a rigid steroid core, a flexible tail, and a single polar group—enables it to self-associate, interact with membrane lipids, and form complex crystal and membrane structures. Its ability to cluster, form hydrogen bonds, and influence membrane order is central to its biological roles in cell membranes and its involvement in health and disease 1234567.
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