Understanding Reactive Oxygen Species Inter-conversion

The question asks to identify the intermediates marked as '?' in the sequence of reactive oxygen species (ROS) transformations: $O_2 \rightarrow ? \rightarrow H_2O_2 \rightarrow ? \rightarrow H_2O$. We need to identify the species corresponding to the two '?' marks based on chemical pathways.

Identifying the First Intermediate ('?')

The initial step involves the conversion of molecular oxygen ($O_2$) to the first unknown intermediate. Molecular oxygen can accept an electron ($e^-$) to form the superoxide radical anion, $\mathrm{O}_2^{\bullet -}$.

• Reaction: $O_2 + e^- \rightarrow \mathrm{O}_2^{\bullet -}$
• Option P represents $\mathrm{O}_2^{\bullet -}$.
• $\mathrm{O}_2^{\bullet -}$ is recognized as a free radical and a primary ROS.

Therefore, the first '?' corresponds to P ($\mathrm{O}_2^{\bullet -}$).

Identifying the Second Intermediate ('?')

The sequence progresses from hydrogen peroxide ($H_2O_2$) to the second unknown intermediate, eventually leading to water ($H_2O$). Hydrogen peroxide ($H_2O_2$) reduction pathways are crucial in ROS chemistry. A common step in these pathways involves the generation of the hydroxide ion ($OH^-$) and the highly reactive hydroxyl radical ($^{\bullet}OH$).

• Reaction Example (e.g., Fenton chemistry): $H_2O_2 + e^- \rightarrow OH^- + ^{\bullet}OH$
• Option Q represents $OH^-$.
• Although $OH^-$ is technically an ion rather than a free radical, it is a significant product formed during the reduction of $H_2O_2$ on the pathway towards forming water ($H_2O$).

Considering the options and common ROS pathways, the second '?' corresponds to Q ($OH^-$).

Conclusion: ROS Inter-conversion Sequence

The identified species for the two '?' marks are P ($\mathrm{O}_2^{\bullet -}$) and Q ($OH^-$).

This sequence represents $O_2 \rightarrow \mathrm{O}_2^{\bullet -} \rightarrow H_2O_2 \rightarrow OH^- \rightarrow H_2O$. The corresponding option is P, Q.