Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Appendix 1 273
Prochiral conWguration is obtained by replacing the substituent of interest
with the next highest available isotope. Commonly, prochiral hydrogens are of
interest in enzyme-catalysed reactions, in which case the hydrogen is replaced
with a deuterium atom, and the conWguration of the resulting chiral centre
analysed as above. Thus, in the labelled ethanol molecule above the substituent
replaced with deuterium is the proR hydrogen.
Finally, the direction of attack onto a double bond can be deWned in similar
fashion, by designating the three substituents around the sp 2 carbon of interest
as 1,2 and 3 as above. Look down onto one face of the double bond, and if
the substituents are arranged in clockwise fashion then you are looking at the
re-face. If they are arranged in anti-clockwise fashion then you are looking at
the si-face. For example, the two faces of the carbonyl group of glyceraldehyde-
3-phosphate are indicated below.
1
O
O
H
3
2
OH
OPO 3
2−
looking at the re-face
of the C-1 carbonyl of
glyceraldehyde-3-phosphate
attack on
si-face
H
R
attack on
re-face