Why is RNA more susceptible to alkaline hydrolysis than DNA? 3. Provide the chemical nature or composition of the compound formed in a positive result for the following: a. Test for ribose b. Test for phosphate c. Test for purines d. Test for pyrimidines
The correct answer and explanation is:
Answer:
RNA is more susceptible to alkaline hydrolysis than DNA because RNA contains a hydroxyl group (-OH) on the 2′ carbon of its ribose sugar, whereas DNA lacks this group and only has a hydrogen atom at the same position.
Positive test results:
a. Test for ribose: Forms a furfural derivative that reacts with orcinol in the Bial’s test, giving a green color.
b. Test for phosphate: In the presence of ammonium molybdate under acidic conditions, phosphate ions form a yellow precipitate of ammonium phosphomolybdate.
c. Test for purines: Positive results produce a red color when treated with concentrated nitric acid followed by alkali (Murexide test).
d. Test for pyrimidines: When subjected to the Morgan–Elson test, pyrimidines may give a colored complex depending on the specific structure, but many tests show no strong color change unless further derivatized.
Explanation (300 words):
RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) differ structurally in one key aspect: the sugar component. RNA contains ribose, which has a hydroxyl (-OH) group attached to the 2′ carbon atom. DNA, in contrast, has deoxyribose, which lacks the 2′-OH group, having only a hydrogen atom at that position. Under alkaline conditions, the 2′-OH in RNA acts as a nucleophile and attacks the adjacent phosphate group. This leads to cleavage of the phosphodiester bond in the RNA backbone, effectively breaking down the RNA molecule. DNA does not have this reactive 2′-OH, so it remains more stable and resistant to alkaline hydrolysis.
In biochemical identification tests, specific functional groups in nucleic acid components yield characteristic reactions. Ribose, being a pentose sugar, reacts with orcinol in Bial’s test. When heated with concentrated hydrochloric acid and orcinol, ribose forms a green complex, indicating a positive result.
Phosphate groups are detected using ammonium molybdate in acidic medium. The reaction forms a yellow precipitate of ammonium phosphomolybdate, signifying the presence of phosphate ions.
Purines (adenine and guanine) are detected by the Murexide test. They react with concentrated nitric acid, followed by exposure to alkali, forming a reddish or purple-colored complex due to the formation of murexide (purpurate).
Pyrimidines (cytosine, thymine, uracil) may give less vivid reactions. While there is no universal color test for pyrimidines alone, certain reactions such as the Morgan–Elson test can help detect related structures based on the presence of N-acetylated sugars or similar derivatives, though the results are more subtle and often depend on specific modifications.