Rules of the Day CH320M/328M

Featured Golden Rules of Chemistry: 3. There are two possible arrangements of four different groups around a tetrahedral atom. The two different arrangements are mirror images of each other, a property referred to as chirality and often compared to handedness.

1. Enantiomers have identical properties measured in an achiral way, but diastereomers have different properties measured in any fashion.

2. Samples of enantiomers rotate the plane of plane polarized light to an equal extent, but in opposite directions. Rotation is denoted as either clockwise ("+") or counterclockwise ("-"), which replaced the older designations "D" and "L", respectively.

3. There is no absolute link between "R" and "S" and "+" and "-". Sometimes the R enantiomer rotates plane polarized light in the "+" direction, and for other molecules, the S enantiomer rotates plane polarized light in the "+" direction.

4. We are chiral because our proteins, carbohydrates, DNA etc. are chiral, so different enantiomers of the same constitutional isomer may taste/smell differently to us. Also, for most chiral drugs, only one enantiomer usually has the desired effect.

5. The biochemical literature uses the classic designations of "D" and "L" for key molecules such as amino acids. The common 19 chiral amino acids are all "L". Rather than a reference to rotation of plane polarized light by these structures, this designation refers to a structural connection to L-glyceraldehyde.

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