In stereochemistry, prochiral molecules are those that can be converted from achiral to

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

in a single

step Step(s) or STEP may refer to: Common meanings * Stairs#Step, Steps, making a staircase * Walking * Dance move * Military step, or march ** Marching Arts Films and television * Steps (TV series), ''Steps'' (TV series), Hong Kong * Step (film), ' ...

. An achiral species which can be converted to a chiral in two steps is called proprochiral. If two identical

substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...

s are attached to a sp³-hybridized

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

, the descriptors ''pro''-R and ''pro''-S are used to distinguish between the two. Promoting the ''pro''-R substituent to higher priority than the other identical substituent results in an ''R'' chirality center at the original sp³-hybridized atom, and analogously for the ''pro''-S substituent. A trigonal planar sp²-hybridized atom can be converted to a chiral center when a substituent is added to the ''re'' or ''si'' () face of the molecule. A face is labeled ''re'' if, when looking at that face, the substituents at the trigonal atom are arranged in increasing Cahn-Ingold-Prelog priority order (1 to 2 to 3) in a clockwise order, and ''si'' if the priorities increase in anti-clockwise order; note that the designation of the resulting chiral center as ''S'' or ''R'' depends on the priority of the incoming group. The concept of prochirality is necessary for understanding some aspects of enzyme

stereospecificity In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers."Overlap Co ...

Alexander Ogston Sir Alexander Ogston MD CM LLD (19 April 1844 – 1 February 1929) was a British surgeon, famous for his discovery of ''Staphylococcus''. Life Ogston was the eldest son of Amelia Cadenhead and her husband Prof. Francis Ogston (1803– ...

pointed out that when a symmetrical molecule is placed in an asymmetric environment, such as the surface of an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

, supposedly identically placed groups become distinguishable. In this way he showed that earlier exclusion of non-chiral

citrate Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the ...

as a possible intermediate in the tricarboxylate cycle was mistaken.

References

{{Reflist Stereochemistry