Passage 5: Ribose
Ribose (C5H10O5) is biologically important as the pentose component of
ribonucleotides. It primarily exists in its aldose form, making it highly reactive in
oxidative and reductive conditions.
Ribose’s oxidation to ribonic acid and reduction to ribitol are widely known
transformations of this pentose. In bromine water (Br2 and H2O), ribose is converted
to ribonic acid, and in the presence of sodium borohydride (NaBH4), ribose is
converted to ribitol.
Experiment 1:
Researchers performed the following reactions with ribose under controlled conditions:
Reaction 1:
Ribose + Br2 + H2O → Ribonic acid + 2HBr
Reaction 2:
Ribose + NaBH4 + H2O → Ribitol + NaBO2
Observations:
Following these procedures, the researchers used pH indicators and TLC to monitor
reaction progress/success.
Experiment 2:
In order to map cellular processes that synthesize purines and pyrimidines, ribose
was converted to 5-phosphoribosyl-1-pyrophosphate (PRPP), a precursor for these
nitrogenous bases. This intermediate was then used to synthesize a compound that
is structurally similar to xanthine, a derivative of a purine:
Reaction 3:
Ribose-5-Phosphate + ATP → PRPP + AMP
Reaction 4:
PRPP + Amine → Xanthine Derivative
Experiment 3:
In an effort to further explore the reactivity of xanthine derivatives and extrapolate
to biological systems and biochemical reactions, they used their synthesized
xanthine derivative and reacted with alcohol to form an ether. Reaction progress
was monitored via TLC and confirmation of desired product was determined via
NMR. UV-Vis spectroscopy was conducted before NMR to ensure that a chemical
reaction had indeed occurred, because the former analytical technique is
significantly less expensive.
How many chiral centers does ribose have, and how does this change during
the conversion to ribitol?
A) 3; formation of ribitol introduces a new chiral center
B) 4; formation of ribitol does not change the number of chiral centers
C) 3; formation of ribitol reduces the number of chiral centers by 1
D) 4; formation of ribitol introduces a new chiral center
Correct answer: C. Ribose has 3 chiral centers (carbons 2, 3, and
4) because these three carbons are each bonded to four different substituents.
However, reduction of the aldehyde group of carbon 1 creates an internal plane of
symmetry, removing carbon 3’s chirality. A breakdown of each carbon’s substituents
is as follows:
Carbon 1: (CHO) This carbon is part of an aldehyde group (sp2 hybridized) and
cannot be chiral.
Carbon 2: (CHOH) This is attached to four different groups (hydrogen, hydroxyl,
CH2OH, and CHOH), making it a chiral center.
Carbon 3: (CHOH) This carbon is attached to four different groups (hydrogen
hydroxyl, CHOH, and CHOH), making it a chiral center. Note that the two CHOH
groups that carbon 3 is bonded to are different due to being on opposite halves
of the molecule that are different (the end that includes Carbon 1 has an aldehyde
group, while carbon 5 is an alcohol).
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Carbon 4 (CHOH) This carbon is attached to four different groups as well (hydrogen,
hydroxyl, CHOH, and CH2OH), meeting criteria for chirality.
Carbon 5: (CH2OH) This carbon is bonded to two hydrogen atoms (which are the
same) and cannot meet criteria for chirality.
Due to this, ribose has 3 chiral centers.
Ribitol:
Carbon 1: (CH2OH) This is attached to two hydrogen atoms, and therefore cannot
meet criteria for chirality.
Carbon 2: (CHOH) This is attached to four different groups (hydrogen, hydroxyl,
CH2OH, and CHOH), making it a chiral center.
Carbon 3: (CHOH) Two of this carbon’s groups are identical after reduction of the
aldehyde (hydrogen hydroxyl, CHOH, and CHOH), making it achiral. Note that the
two CHOH groups that carbon 3 is bonded to are chemically the same due to being
on opposite halves of the pentose that are identical (the end that includes Carbon 1
which is now an alcohol is identical to carbon 5’s half).
Carbon 4 (CHOH) This carbon is attached to four different groups (hydrogen,
hydroxyl, CHOH, and CH2OH), meeting criteria for chirality.
Carbon 5: (CH2OH) This carbon is bonded to two hydrogen atoms (which are the
same) and cannot meet criteria for chirality.
Therefore, ribitol has 2 chiral centers, so conversion of ribose to ribitol reduces the
number of chiral centers by 1.
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