Question

Define D- and L- isomers.
Topic : Monosaccharides​

Answer 1

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Define D- and L- isomers.

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L and D isomerism is most commonly used with sugar molecules.

The main difference between L and D isomers is in the position of –OH group in the penultimate carbon atom.

• In D isomer, the OH- group of the penultimate carbon is positioned on the right side whereas,
• In L isomer, the OH- group of the penultimate carbon is positioned on the left side.

Answer 2

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» The D- and L- isomers are mirror image of each other.

» The spatial orientation of -H and -OH group on the carbon atom ($\sf{C_5}$ for glucose) that is adjacent to the terminal primary alcohol carbon determines whether the sugar is D- or L- isomer.

» If the -OH group is on the right side, the sugar is of D-series, and if on the left side, it belongs to L-series.

» The strictures of D- and L- glucose based on the reference mono-saccharide, D- and L- glyceraldehyde are attached. (↑)

• It may be noted that the naturally occurring monosaccharides in the mammalian tissues are mostly of D-configuration.
• The enzyme machinery of cells is specific to metabolise D-series of monosaccharides.