Hello, D-β-hydroxybutyric and L-β-hydroxybutyric acids difference that they enantiomers and occur widely in nature.
In humans, D-β-hydroxybutyric acid is one of two primary endogenous agonists of hydroxycarboxylic acid receptor 2 HCA2 and G protein-coupled receptor GPCR.
Dear Joana d'Avila
The D-L system corresponds to the configuration of the molecule: spatial arrangement of its atoms around the chirality center.
While (+) and (-) notation corresponds to the optical activity of the substance, whether it rotates the plane of polarized light clockwise (+) or counterclockwise (-).
D-L system tells us about the relative configuration of the molecule, compared to the enantiomers of glyceraldehyde as the standard compound. Compounds with the same relative configuration as (+)-glyceraldehyde are assigned the D prefix, and those with the relative configuration of (-)-glyceraldehyde are given the L prefix.
It's kind of another way to tell the configuration of molecules beside the Cahn–Ingold–Prelog convention (R/S system), with little difference. (D-L system labels the whole molecule, while R/S system labels the absolute configuration of each chirality center.)
In short, the D-L system doesn't have direct connection to (+)/(-) notation. It only relates the stereochemistry of the compound with that of glyceraldehyde, but says nothing about its optical activity. We may have compound with same relative configuration as (+)-glyceraldehyde (thus, it's given the D prefix), yet it rotates the polarized light counterclockwise (-), such as D-(-)-ribose.
And also, don't confuse the D-L system with d- and l- naming. d- and l- is the exact same with (+) and (-) notation.
Good luck!
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