Fischer projection

Meaning: A Fischer projection is a flat drawing of a chiral molecule. The carbon chain runs vertically with the most oxidized carbon at the top. At each stereocenter, horizontal bonds point toward the viewer and vertical bonds point away. D/L is read from it: in a sugar, the OH on the stereocenter farthest from the carbonyl is on the right for D and on the left for L. In an amino acid with COOH on top, NH₂ on the left is L, the form found in proteins.

On the MCAT: Questions ask you to name D or L, to classify structures as enantiomers (every center reversed), diastereomers (some centers reversed) or epimers (exactly one center different, like glucose and galactose at C4), or to assign R/S. Rotating the drawing 180° in the plane keeps the same molecule; a 90° turn or swapping two groups on one center gives the opposite configuration. For R/S, if the lowest-priority group is on a horizontal bond, reverse the direction you traced.

Common confusion: D/L is not R/S and not d/l (+/−). D-glyceraldehyde is R, but L-cysteine is R while most other L-amino acids are S. The sign of optical rotation must be measured; it cannot be read from the drawing.