dextrorotatory and levorotatory
/ DEKS-troh / LEE-voh /
When a chiral compound twists plane-polarized light, it can twist it one of two ways — clockwise or counterclockwise as you look toward the oncoming beam. Chemists gave these two directions tidy names borrowed from the Latin for right and left, and a pair of plus and minus signs to match.
A substance is dextrorotatory if it rotates the plane of polarized light clockwise; this is marked with a plus sign, (+), and sometimes the lowercase prefix d-. A substance is levorotatory if it rotates the light counterclockwise, marked with a minus sign, (-), or the lowercase prefix l-. The two enantiomers of any chiral compound always fall on opposite sides of this divide: if one is (+), its mirror image is (-), rotating the light by exactly the same magnitude in the opposite sense. These labels are purely experimental — they record what the polarimeter measured, nothing more.
The crucial caution is that (+)/(-) is an observed optical property, while R/S and D/L are structural conventions, and the three do NOT reliably match up. You cannot look at a structure, decide it is R, and conclude it must be (+); the sign of rotation can only be measured, not deduced from the drawing. Historically this caused real confusion, which is exactly why the absolute R/S system was invented to describe geometry unambiguously, separate from whatever the light happens to do.
Natural alanine is (S)-(+)-alanine: structurally S, and experimentally dextrorotatory. Notice the labels are independent — the S center happens to give a (+) rotation here, but in other amino acids an S center can be (-). Only measurement tells you the sign.
(+) and (-) are measured, not predicted from R/S or from the structure.
Do not confuse lowercase d-/l- (dextro/levo, about rotation direction) with uppercase D/L (the Fischer-projection configuration system for sugars and amino acids). They look similar but mean completely different things; modern usage prefers (+)/(-) for rotation to avoid the clash.