Table 4.

Computed Proton Transfer Energies (kcal/mol) Using HF/6–311+G(d,p) and Signed Errors from PHO-QM/MM Calculations for the All-Trans Amino Acids^a

$\begin{array}{c}{}^{-}{\mathrm{O}}_2{\mathrm{C}\mathrm{C}\mathrm{H}}_2{\mathrm{C}}_{B1}\left[{\mathrm{H}}_2{\left({\mathrm{C}\mathrm{H}}_2\right)}_n{\mathrm{H}}_2\right]\\ {\mathrm{C}}_{B2}{\mathrm{C}\mathrm{H}}_2{\mathrm{N}\mathrm{H}}_3^{+}\end{array}$	HF (full QM)	PHO (QM/MM)
n = 1	−81.0	1.9
n = 2	−87.2	−1.3
n = 3	−91.7	−1.0
n = 4	−95.6	−0.8

^aHydrogen atoms and methylene groups in square parentheses are modeled by the CHARMM force field. The energy change for the proton transfer process is defined by $\mathrm{\Delta }{E}_{\mathrm{P}\mathrm{T}\mathrm{E}}=E\left(\mathrm{n}\mathrm{e}\mathrm{u}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{l}\right)-E\left(\mathrm{z}\mathrm{w}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{o}\mathrm{n}\right)$.