CHEM 16-2 - Meso Compounds Meso compounds(example is...

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Meso Compounds Meso compounds (example is 2,3-butanediol) are optically inactive, as they have at least 2 chiral centers plus an internal symmetry plane. The meso molecule as a whole is not chiral, although particular carbons in the molecule may be chiral centers. For 2,3-butanediol with 2 chiral centers, although there are possibly a maximum of 4 stereoisomers, it has only 3 stereoisomers since one is a meso one. When does one observe no optical activity? 1) An achiral compound, one with no chiral center(s). 2) A meso compound, which has chiral centers but also an internal symmetry (mirror) plane which renders the whole molecule achiral. 3) A racemic mixture, which contains chiral molecules, but there are equimolar amounts of dextrorotatory and levorotatory molecules, so they exactly cancel each other out. Examples of biochemical significance for a particular stereoisomer: Epinephrine (one of the catecholamine neurotransmitters) is levorotatory, 15-20 times more active than the dextrorotatory form. The (+) form of Amphetamine (CNS stimulant), called Dextroamphetamine (in Dexedrine), is a stronger stimulant, and if pure, 2-4 times more active. Stereochemistry of Monosaccharides The simplest monosaccharides are the trioses glyceraldehyde (with 1 chiral center) and dihydroxyacetone (with no chiral centers). Fischer, a German chemist, proposed a convention for drawing the formulas for monosaccharides, where he wrote the aldehyde group at the top, and then the carbon chain in vertical order with the –H or –OH written to the right or left. Glyceraldehyde with the –OH group on the right of the penultimate chiral carbon atom (chiral carbon furthest from aldehyde or
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ketone group) is assigned the letter D as its prefix, the enantiomer with the –OH on the left of that carbon atom is assigned the letter L as its prefix. These letters do NOT refer to the optical rotation! This means the D enantiomer does not necessarily give a (+) optical rotation, or the L isomer gives a (-) one. Monosaccharides that are tetroses, pentoses, hexoses which terminate in the same Fischer projections as D-glyceraldehyde are D sugars, while those from L-glyceraldehyde are L sugars. Our human bodies can only metabolize D sugars, because of the enzymes used.
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