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Table 2 Effect of protonation on the barriers of the sequential reaction [a]

From: Effects of protonation on the hydrolysis of triphosphate in vacuum and the implications for catalysis by nucleotide hydrolyzing enzymes

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
  1. [a] Energy barriers (AM1/d, in kcal mol−1) along the sequential reaction for different protonation states. [b] nP 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 Pγ-Oβγ 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 Wa attack onto the metaphosphate, relative to the reactant. [f] ΔE 2 ǂ = (E 2 - E meta) is the activation barrier for water Wa attack onto the metaphosphate. [g] Various searches for a sequential reaction failed (hydrolysis always proceeded through a concurrent mechanism)