Day 27 MCAT Practice Question

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.

Two ribose molecules undergo an aldol addition reaction. Which of the

following are true about the product after the initial addition step prior to the

addition of heat?

A) A. A β-hydroxy aldehyde with an empirical formula of C5H9O5

B) B. A β-hydroxy ketone with an empirical formula of C5H9O5

C) C. A β-hydroxy aldehyde with an empirical formula of C5H10O5

D) D. A β-hydroxy ketone with an empirical formula of C5H10O5