Table 2.

Effect of protonation on the barriers of the sequential reaction ^[a]

nP [b]		ΔE 1 ǂ [c]	E meta [d]	E 2 [e]	ΔE 2 ǂ [f]
0		14.2	11.2	45.9	34.7
1	α:	1.82	−7.07	30.4	37.5
1	β:	1.99	−36.0	−4.6	31.4
1	γ: [g]	---	---	---	---
2	αβ:	0.34	−38.2	−9.0	29.2
2	αγ:	14.5	13.9	40.9	27.0
2	βγ:	8.02	3.8	29.1	25.3
3	αβγ:	4.9	−3.27	18.6	21.9

^[a] Energy barriers (AM1/d, in kcal mol⁻¹) along the sequential reaction for different protonation states. ^[b] n_P is the number of protons on the triphosphate (α, β and γ indicate which phosphate is protonated). ^[c] ΔE ₁ ^ǂ is the barrier of the metaphosphate formation, i.e., the energy difference between the saddle point for P_γ-O_βγ bond cleavage and the reactant. Note that ΔE ₁ ^ǂ is the same as E ₁ defined in the main text. ^[d] E _meta is the energy of the geometry-optimised metaphosphate intermediate, relative to the initial reactant. ^[e] E ₂ is the energy of the saddle point for W_a attack onto the metaphosphate, relative to the reactant. ^[f] ΔE ₂ ^ǂ = (E ₂ - E _meta) is the activation barrier for water W_a attack onto the metaphosphate. ^[g] Various searches for a sequential reaction failed (hydrolysis always proceeded through a concurrent mechanism)