Table 4.

QCFF/PI-EVB energetics of the ion pair states^a

System	$\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}(\text{I})$b	pKa (SBH+) b	$\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}(\text{II})$c	Δ GPTd
water	–	7.0	–	6.5
bR	13.0	16.4	2.2	7.7
K	11.0	15.0	3.0	8.2
L	9.0	13.5	1.8	6.3
M1	11.0	15.0	2.0	5.8
M2	7.0	12.1	− 14.0	− 4.0
M2′	4.0	9.9	− 17.0	− 6.0
M22	7.0	12.1	− 18.0	− 7.0

^aMicroscopic free energies in kcal/mol, where R82 and D212 are ionized, Asp85 is designated by A in the equations below.

^b $\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}(\text{I})=\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}({\text{SBH}}^{+},{\text{A}}^{-}\to {\text{SBH}}_{\text{np}},{\text{A}}^{-})$ is the change in the solvation of the Schiff base upon moving from solution to the protein site. The corresponding absolute pK_a (when D85 is ionized) is given in pK_a units.

^c $\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}(\text{II})=\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}({\text{SBH}}^{+},{\text{A}}^{-}\to {\text{SBH}}_{\text{np}},{\text{A}}_{\text{np}})$, where the ion pair is held at the same distance in water and in the protein.

^d $\mathrm{\Delta }{G}_{PT}^{\prime }$ is evaluated by, $\mathrm{\Delta }{G}_{PT}^p=\mathrm{\Delta }{G}_{PT}^w(\infty )-\mathrm{\Delta }{V}_{QQ}^{ip}/20+\mathrm{\Delta }{V}_{QQ}^{\mathit{polar}}/4+\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}(\text{II})+\mathrm{\Delta }\mathrm{\Delta }{\text{G}}_{\text{sol}}^{\text{w}\to \text{p}}(\text{III})$, where $\mathrm{\Delta }\mathrm{\Delta }{\text{V}}_{\text{QQ}}^{\text{ip}}$ is the gas-phase interaction within the ion pair and $\mathrm{\Delta }\mathrm{\Delta }{\text{G}}_{\mathit{sol}}^{\text{w}\to \text{p}}(\text{III})=\mathrm{\Delta }\mathrm{\Delta }{G}_{\mathit{sol}}^{w\to p}({SB}_{np},{AH}_{np}\to {SB}_{po},{AH}_{po}).$