Day 31 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.

The researchers performing the experiments in the passage (Group A) forgot to

label the test tubes containing their reagents and products. Another group of

researchers (Group B) working in the same lab was conducting spectroscopic

analysis on derivatives of amino acids, and also forgot to label their test tubes.

Which pair of amino acids, if mixed with Group A’s test tubes, would be the

most difficult to distinguish from the products outlined in the passage via UVVis spectroscopy?

A) F and W

B) T and H

C) D and E

D) A and L