Question

Give reasons for the following

a. Cytosine and guanine form three hydrogen bonds

b. Addition of acid to a DNA solution causes a change in its absorbance.

c. Adenosine is more soluble than adenine even though the latter is smaller

d. Guanosine monophosphate is acidic

e. The base pairing of DNA becomes disrupted at high pH.

Answer 1

Answer a:

In a double stranded DNA, Cytosine (C) makes hydrogen bonds with Guanine (G) and Adenine (A) bonds with Thymine (T). There are three hydrogen bonds between C and G as compared to 2 hydrogen bods between A and T.

Hydrogen atoms attached to highly electronegative atoms like O and N make hydrogen bonds with other atoms. In C-G pairing the exocyclic NH2 at 2nd carbon of Guanine can make bonds with carbonyl at second C of Cytosine

Answer b:

Addition of acid to a DNA solution increases its absorbance. The purine and pyrimidine bases can absorb UV light. In a double stranded DNA there is stacking of these bases and hence these are less available to absorb. However on addition of acid, DNA melting occurs and the hydrogen bonds between purines and pyrimidines weaken and break leading to their more availability and hence an increase in absorbance of UV light. This increase is absorbance of DNA is called hyperchromic shift.

Answer c.:

Adenine is one of the four aromatic bases of nucleic acids. It is poorly soluble in water dur to it's aromatic nature. even at low concentration of the compound, it forms oligomers and this is hydrophobic and it in turn reduces solubility of adenine. A large molecule adenosine is formed after the addition of sugar moiety. This presence of sugar reduces the aromatic nature of the nitrogenous base and increases its solubility in the water.

Answer d:

Purines are both weak acids and even weaker bases. Guanosine monophosphate (GMP) is an ester of phosphoric acid with the nucleoside guanosine. Due to addition of phosphoric acid the negative charge on GMP increase making it more acidic in nature. It can make salts with Na+ and K+ which are called Disodium guanylate or Dipotassium guanylate.

Answer e:

In the pH around neutral, (>5 but <9) the DNA double helix is stable. All it's hydrogen bonds in place, none of the functional groups will titrate in this pH range.

However, when pH is raised beyond 9, there are lots of hydroxyl ions in the basic solution that attracts and takes the H atoms off the DNA base pairs. This disrupts the hydrogen bonds and DNA is disrupted at high pH.