Table 3.
Inductive index values (ii) calculated on the Cα of unmodified and modified amino acids with ionizable side-chains and Trp, measured pKa values of side-chain groups (Asp, Cys, Glu and His) and Mean Fluorescence Lifetime of tryptophan (τ, data for Fig. 4). N-Ac indicates α-amino acetylation. C-NH2 and C-OMe indicate α-carboxyl amidation and methyl esterification, respectively. Doubly modified amino acids are indicated as a combination of modifications. In the case of doubly modified His, the α-carboxyl group was N-methyl amidated (C-NHMe). When an α-group was not modified, it was assumed to be charged at the pH of deprotonation of the side-chain groups, indicated in parentheses. Note that the calculated Inductive Indexes on Cα do not include the side-chain itself, therefore, they are identical for all identical modifications and in the zwitterion form. (−) indicates modification not available for that amino acid.
| Side chain pKa |
τ (ns) of Trp | |||||
|---|---|---|---|---|---|---|
| α-Groups | ii on Cα | Asp | Glu | His | Cys | |
| Zwitterion | 653.0 | 3.81 ± 0.02 | 4.27 ± 0.01 | 6.02 ± 0.02 | 8.27 ± 0.01 | 2.55 |
| N-Ac (α-COO-) | −338.2 | 4.72 ± 0.01 | 4.83 ± 0.01 | 6.98 ± 0.01 | 9.66 ± 0.01 | 4.24 |
| C-NH2 (α-NH3+) | 1305.2 | 2.94 ± 0.02 | 3.88 ± 0.01 | 5.35 ± 0.01 | – | 1.44 |
| C-OMe (α-NH3+) | 1385.7 | – | – | 5.26 ± 0.01 | 6.58 ± 0.01 | – |
| N-Ac/C-NH2 | 314.0 | 4.01 ± 0.01 | 4.34 ± 0.02 | – | – | 2.71 |
| N-Ac/C-OMe | 394.6 | – | – | – | 8.78 ± 0.01 | – |
| N-Ac/C-NHMe | 304.9 | – | – | 6.42 ± 0.01 | – | – |