Exercise 3 on the following pages is referring to a disputed 2010 study claiming that bacteria isolated from Mono Lake ([As] = 200 M, more than 1000x the legal limit in drinking water) can substitute As instead of P into its DNA.

Skeptical of the textbooks framing on this, you find the following data in which the free energy profiles for hydrolysis of As and P esters through two transition states are predicted by computational methods. Based on your knowledge of chemical kinetics and DNA structure, why do you think it is unlikely that organisms use As in place of P?

TS1 TS1 E, kcal/mol HO- 40 HO HO J5 35 kcal/mol 30 25 20 20 kcal/mol X - As, P 15 10 5 0 ofor X-0-R .| 0-x=0 HOR