All-trans-retinol is the form of retinol whose four acyclic double bonds have trans, or , geometry. Its molecular formula is , and it retains the alcohol group characteristic of retinol. The prefix fixes double-bond geometry; it does not specify one frozen conformation about every single bond.[ref]
The distinction from 11-cis-retinol illustrates why elemental composition is incomplete structural information. Both have the same formula, while one specified double bond has different geometry. A mass measurement consistent with that formula cannot on its own identify which isomer is present.
A second distinction concerns the alcohol group. Lecithin retinol acyltransferase can convert retinol into a retinyl ester by transferring an acyl group. Changing ester status and changing cis–trans geometry are separate chemical operations.[ref]
For example, an assay that pools all vitamin-A-related signals could register material before and after esterification without resolving the conversion. An interpretable experiment names the chemical form and the structural distinction being measured, rather than inferring identity solely from a shared retinoid classification.
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