The best explanation of stability of nucleobases in DNA with respect to UV-light

Question

The best explanation of stability of nucleobases in DNA with respect to UV-light caused chemical damage is:

Select one:

a. The double helical structure of DNA ensures that nucleobases are safely inside and any possibly damaging light is absorbed by the phosphate backbone.

b. The excited state of a nucleobase decays rapidly to the nearby triplet state via intersystem crossing. The nucleotide spends considerable time in the triplet state before emitting phosphorescent light.

c. The excited state of a nucleobase decays rapidly to the ground state by emitting a photon of fluorescent light.

d. The excited state of a nucleobase reacts with a the ground state of a nearby nucleobase via electrocyclic cyclobutane ring formation. The light energy is convered simply into chemical bond energy.

e. The excited state of a nucleobase decays rapidly back to the ground state via conical intersection at a point where the potential energy surface of the excited state and geometrically strained ground state meet.

Explanation / Answer

This is an important question with respect to the importance of harmful effects of UV.

DNA comprises of nucleobases like adenine, guanine, thymine, cytosine. When treated to UV, the nucleobases can return to the ground state in ultrafast picosecond level, thereby avoiding photochemical reactions in excited state. Ultrafast deactivation occurs through internal conversion mechanisms, in which the molecule transfers the energy stored in the electronic system to vibrational degrees of freedom. This is different from what happens in fluoresnce or phosphoresence.

It happens via a complex mechanism known as Conical Intersection (CI) where two potential energy surfaces intersect. They are also known as Molecular funnels and Diabolic points. It is responsible for non-radiative de-excitation transitions from excited state to ground state of molecules. The excited molecular photon follows the potential energy surface and comes down. As it reaches the CI, large vibronic coupling enables the non-radiative transition and the molecule comes to GS. These are often those points where excited state and geometrically strained GS meets.

Hence option (e) is most suitable answer

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.