Ribose (source code)
= Ribose
{title2=$\mathrm C_5\mathrm H_{10}\mathrm O_5$}
{wiki}
<Ribose> is a <monosaccharide> with <molecular formula> $\mathrm C_5\mathrm H_{10}\mathrm O_5$. Its biologically familiar form is D-ribose, an <aldopentose> whose open-chain <Fischer projection> has $\mathrm{CHO}$ at the top, $\mathrm{CH}_2\mathrm{OH}$ at the bottom, and <hydroxy groups> on the right at C2, C3, and C4. The opposite horizontal substituents are <hydrogen>. See https://openstax.org/books/organic-chemistry/pages/25-3-d-l-sugars[McMurry, Organic Chemistry, 25.3: D-ribose and D-glucose configurations].
\Image[/pentose-sugar-fischer-comparison.png]
{title=Open-chain <Fischer projections> of D-<ribose> and 2-deoxy-D-<ribose>}
{description=At each cross, horizontal <chemical bonds> point toward the viewer and vertical <chemical bonds> point away. Both drawings number the free <sugar> from its <aldehyde> carbon, C1. In <deoxyribose>, C2 has two <hydrogen> substituents and is not a <stereocenter>. These are open-chain reference structures, not a depiction of the predominant <chemical species> in <water>.}
Read the drawing by counting groups: $\mathrm{CHO}$ contributes $\mathrm C\mathrm H\mathrm O$, three $\mathrm{CHOH}$ groups contribute $\mathrm C_3\mathrm H_6\mathrm O_3$, and $\mathrm{CH}_2\mathrm{OH}$ contributes $\mathrm C\mathrm H_3\mathrm O$. Their sum is $\mathrm C_5\mathrm H_{10}\mathrm O_5$. The count checks composition; the right/right/right pattern additionally specifies the <stereochemistry> of D-<ribose>.
In <RNA>, <ribose> occurs as a ring within each <nucleotide>, joining a <nucleobase> to the <phosphate>-linked backbone. Its C2 position retains a <hydroxy group>, unlike <deoxyribose> in <DNA>. In a <nucleotide>, the <sugar> positions carry primes, so this is C2′. See https://www.ncbi.nlm.nih.gov/books/NBK26883/[Alberts et al.: nucleotide sugars in RNA and DNA]. Free <ribose> in <water> is also predominantly cyclic, with several ring forms in <chemical equilibrium> and very little open-chain form; the <Fischer projection> is valuable for comparing configurations, not for estimating their abundances. See https://pmc.ncbi.nlm.nih.gov/articles/PMC3819725/[Mason et al., The Conformation of a Ribose Derivative in Aqueous Solution: A Neutron Scattering and Molecular Dynamics Study (2013), introduction].
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