TABLE 4.
Proton Affinities and Gas-Phase Basicities for amino acids model compounds with experimental valuea (experimental error in parenthesis) and pKa valuesb in solution as a reference. All quantities are in kcal/mol except pKa.
| Model (Amino Acid) |
CBS-QB3 |
G3B3 |
G3MP2B3 |
PBE0 |
B1B95 |
B3LYP |
QCRNA |
Experiment |
pKa |
|---|---|---|---|---|---|---|---|---|---|
| Proton Affinity | |||||||||
| prolinium (Pro) | 224.3 | 224.8 | 225.0 | 225.4 | 225.3 | 225.0 | 225.0 | 224.0 (2.0) | 1.9 |
| glycinium (Gly) | 211.9 | 212.3 | 212.2 | 213.6 | 213.2 | 212.0 | 212.0 | 211.9 (0.5) | 2.4 |
| acetic acid (Asp) | 347.4 | 348.2 | 348.6 | 347.6 | 347.9 | 345.8 | 346.4 | 347.2 (1.1) | 3.9 |
| propanic acid (Glu) | 346.2 | 347.0 | 347.4 | 347.8 | 347.9 | 346.1 | 345.5 | 347.4 (1.8) | 4.1 |
| 4-methyl-imidazolium (His π) | 229.0 | 229.6 | 229.4 | 231.3 | 231.2 | 230.7 | 230.7 | 227.7 (2.0) | 6.0 |
| 4-methyl-imidazolium (His τ) | 229.6 | 230.3 | 230.1 | 231.8 | 231.7 | 231.3 | 231.3 | − (−) | 6.0 |
| methanethiol (Cys) | 356.8 | 357.4 | 357.7 | 358.2 | 358.7 | 357.3 | 357.3 | 357.3 (1.2) | 8.4 |
| glycine (Gly) | 341.8 | 342.6 | 343.0 | 343.0 | 343.2 | 341.5 | 341.5 | 340.3 (1.1) | 9.8 |
| p-methylphenol (Tyr) | 350.4 | 350.7 | 350.7 | 350.0 | 350.6 | 348.9 | 348.9 | 350.7 (1.3) | 10.5 |
| propanammonium (Lys) | 219.3 | 219.8 | 219.6 | 220.3 | 220.1 | 219.4 | 219.3 | 219.4 (0.5) | 10.5 |
| proline (Pro) | 340.1 | 341.0 | 341.2 | 342.1 | 342.1 | 340.4 | 340.4 | 340.3 (3.1) | 10.6 |
| n-methylguanidine (Arg) | 239.5 | 239.6 | 239.0 | 243.1 | 242.9 | 241.9 | 241.8 | − (−) | 12.5 |
| Gas Phase Basicity | |||||||||
| prolinium (Pro) | 216.8 | 217.3 | 217.4 | 217.8 | 217.6 | 217.4 | 217.4 | 216.0 (2.0) | 1.9 |
| glycinium (Gly) | 204.1 | 204.5 | 204.4 | 205.4 | 205.2 | 204.0 | 204.2 | 203.7 (0.5) | 2.4 |
| acetic acid (Asp) | 339.4 | 340.2 | 340.6 | 341.3 | 341.5 | 339.5 | 338.3 | 341.4 (1.2) | 3.9 |
| propanic acid (Glu) | 340.1 | 341.0 | 341.4 | 339.2 | 339.9 | 337.6 | 339.4 | 340.4 (1.4) | 4.1 |
| 4-methyl-imidazolium (His π) | 221.2 | 221.8 | 221.6 | 223.6 | 223.5 | 223.0 | 223.0 | 220.1 (2.0) | 6.0 |
| 4-methyl-imidazolium (His τ) | 221.9 | 222.6 | 222.4 | 224.2 | 224.0 | 223.7 | 223.6 | − (−) | 6.0 |
| methanethiol (Cys) | 350.4 | 351.0 | 351.3 | 351.8 | 352.3 | 351.0 | 350.9 | 350.6 (2.0) | 8.4 |
| glycine (Gly) | 334.7 | 335.6 | 335.9 | 335.8 | 336.0 | 334.2 | 334.3 | 335.1 (1.4) | 9.8 |
| p-methylphenol (Tyr) | 343.2 | 343.4 | 343.4 | 342.8 | 343.2 | 341.8 | 341.6 | 343.8 (1.2) | 10.5 |
| propanammonium (Lys) | 211.8 | 212.3 | 212.2 | 212.8 | 212.6 | 211.8 | 211.9 | 211.3 (0.5) | 10.5 |
| proline (Pro) | 332.5 | 333.4 | 333.6 | 334.4 | 334.4 | 332.6 | 332.6 | 333.4 (3.0) | 10.6 |
| n-methylguanidine (Arg) | 232.7 | 232.9 | 232.3 | 236.8 | 237.3 | 235.5 | 235.0 | - (−) | 12.5 |