= Retinyl ester
A <retinyl ester> is an <ester> formed at the alcohol group of <retinol>. The retinyl portion is attached through oxygen to an <acyl group>; different attached groups produce different members of the class. The name therefore does not identify one unique molecular formula or <molar mass>.<All-trans-retinyl ester in ChEBI>[CHEBI:63410, generic structure and retinyl-ester classification]{ref}
Write a retinol alcohol schematically as $\mathrm{Ret{-}OH}$, where Ret represents its carbon framework. The corresponding ester has the arrangement $\mathrm{Ret{-}O{-}C(=O){-}R}$. Varying $R$ changes the attached acyl group without requiring a change in the retinyl framework. This is a structural comparison, not a claim that a free carboxylic acid supplies every biological esterification reaction.
For example, <lecithin retinol acyltransferase> uses <phosphatidylcholine> as an acyl donor. Its products include <retinyl esters> that can be stored or used in subsequent <retinoid> pathways.<Lecithin retinol acyltransferase in IUBMB Enzyme Nomenclature>[reaction and comments, donor and storage products]{ref} An assay of total ester therefore needs an additional identity measurement if the question concerns a particular ester, geometric isomer or downstream substrate.
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