Interdisciplinary Note (4 of 15)
Biochemists generally do not employ the Cahn-Ingold-Prelog system to represent carbohydrate stereochemistry, relying instead on Fischer projection formulas and the D,L system of notation. This system relates the stereochemistry of carbohydrate carbons to the configuration of (+)-glyceraldehyde or (-)-glyceraldehyde, which are dextrorotatory and levorotatory, respectively. If the configuration of the stereocenter furthest from the carbonyl matches the single stereocenter of (+)-glyceraldehyde, it's big D. The hydroxyl group will be on the right side in the Fischer projection. If the hydroxyl group is on the left side, the sugar is big L. You can't tell from this whether the sugar is going to be dextro- or levorotatory. It just tells you the absolute configuration of the carbon furthest from the carbonyl group.
It seems like an insane system unless you understand the historical context. Fischer's laboratory, way back in 1886, developed a benchtop method of lengthening sugars, the Kilani-Fischer synthesis. It uses cyanohydrin formation, adding one carbon at a time. From one triose, it yields two tetroses, then four pentoses, then eight hexoses. Basically, all of the sugars that began with (+)-glyceraldehyde in this synthesis got named big D, because they knew they had to agree with (+)-glyceraldehyde in that single stereocenter. Likewise, the sugars synthesized starting with (-)-glyceraldehyde got named big L. Now we're stuck with it.
As far as the MCAT goes, one of the biggest things to watch out for are questions trying to get you to confuse big D with "dextrarotatory". Dextrarotatory is designated either "d", "D" (small capital D!!!), or with (+). You can tell if a structure is big D from the Fischer projection. To determine if substance is dextrarotatory, on the other hand, you would need to put it on the polarimeter and determine its optical rotation.
