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 do you expect the IR results for ribose to change when placed under
oxidative conditions described in the passage?
A) A strong absorption peak would shift from 1725 cm-1 to 1700 cm-1 under
oxidative conditions. A broad stretch between 3200 cm-1 to 3500 cm-1
would still be observed in both conditions
B) A strong absorption peak would shift from 1650 cm-1 to 2100 cm-1. An
initially observed broad stretch would disappear under oxidative conditions
C) A weak stretch around 1725 cm-1 would shift to 1700 cm-1 under oxidative
conditions. A broad stretch between 3200 cm-1 to 3500 cm-1 would still
be observed in both conditions
D) A strong absorption peak would shift from 1725 cm-1 to 1600cm-1 under
oxidative conditions. The broad stretch between 3200 cm-1 to 3500 cm-1
would disappear under oxidative conditions
Correct answer: A. Aldehydes (such as ribose) display a strong
absorption peak around 1725 cm-1, while carboxylic acids (such as ribonic acid, the
product of placing ribose in oxidative conditions) have a strong absorption peak
around 1700 cm-1. Both groups still have an alcohol group, so the broad stretch
from 3200 cm-1 to 3500 cm-1 should be conserved. The aldehyde C=O stretch
shifts to a slightly lower frequency upon oxidation to a carboxylic acid due to
increased conjugation and resonance stabilization.
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