An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 52

HOCH 2

HO

OH

-O-

OH

O

OH

O

C

HO

OH

D-enantiomer β-anomer

FIGURE 2.18 b - D -glucose. The D -enantiomer is observed based on the chiral center furthest away from the
reducing end. The C-5 chiral center follows the right-hand rule and thus is of the D -form. The anomeric OH follows
also the right-hand rule. Therefore, it is b - D -glucose.

ring (left-hand gesture). Figure 2.18 shows the configuration of b - D -glucose, where both
right-hand rule applied for the D -enantiomer and b -anomer (OH). When the enantiomer
and anomeric OH follow the same hand rule, the anomer is a b -anomer, whereas when
different hand rules apply the anomer is an a -anomer. One can also interpret it with the
anomeric proton (H-atom), which is omitted from the ring presentations. a -anomer has
the anomeric proton attached from above the ring (H on the thumb of the right hand
with ring being the circle form by the hand d right hand gesture), whereas b -anomer
has anomeric H attached from below the ring (left-hand gesture). These rules apply for
D -form sugars. For L -form sugars, it is just reversed ( a versus b )because L -enantiomer

follows the left-hand rule, rather than the right-hand rule.

b. Pentoses

Pentoses are five carbon sugars, they all have a stoichiometry of C 5 H 10 O 5 . Some common
pentoses in nature are shown below:

CHO
HCOH

CHO
HOCH

CHO
HCOH

CHO
HCOH

CHO
HOCH

HCOH
HCOH

HCOH
HCOH

HOCH
HOCH

HOCH
HCOH

HOCH
HCOH

CH2OH CH2OH CH2OH CH2OH CH2OH

D -ribose D -arabinose L -arabinose D -xylose D -lyxose

D -arabinose and L -arabinose are mirror image of one another. These are known as enantio-
mers. D -form sugars are more common in nature. There are only a few L -form sugars in nature
and L -arabinose is one of them.