EH91 and EH99 refer to the stereochemical target libraries provided in two compilations by Engh and Huber in 1991 (EH91) and 2001 (EH99). The PDB statistics correspond to His residues with unambiguously deduced protonation states from high-resolution crystal structures and to all His residues in 3 resolution structures. The CSD entries were selected using the R-value criterion (R 7.5%). In both cases, the presented statistics are after outlier rejection, as described in the text. The statistics are treated separately depending on the involvement of the N atoms in metal coordination, but the overall statistics (‘all’) are also shown where appropriate.
| Bond () | CGND1 | ND1CE1 | CE1NE2 | NE2CD2 | CD2CG | No. of cases |
|---|---|---|---|---|---|---|
| ND1 protonated | ||||||
| EH91 | 1.378 (11) | 1.345 (20) | 1.319 (13) | 1.382 (30) | 1.354 (11) | |
| EH99 | 1.369 (15) | 1.343 (25) | 1.322 (23) | 1.415 (21)† | 1.353 (17) | 12 |
| PDB, no metals | 1.373 (10) | 1.337 (15) | 1.319 (16) | 1.377 (12) | 1.361(11) | 48 |
| PDB, metals‡ | 1.375 (10) | 1.337 (13) | 1.322 (11) | 1.377 (11) | 1.359 (9) | 142 |
| PDB, all | 1.375 (10) | 1.337 (13) | 1.322 (12) | 1.377 (11) | 1.360 (10) | 190 |
| CSD, no metals | 1.374 (10) | 1.345 (6) | 1.315(10) | 1.376 (9) | 1.358 (15) | 17 |
| CSD, metals‡ | 1.371 (9) | 1.337 (7) | 1.316 (9) | 1.377 (6) | 1.356 (9) | 176 |
| CSD, all | 1.371 (9) | 1.338 (8) | 1.316 (9) | 1.377 (7) | 1.356 (9) | 195 |
| NE2 protonated | ||||||
| EH91 | 1.371 (17) | 1.319 (13) | 1.345 (20) | 1.374 (21) | 1.356 (11) | |
| EH99 | 1.383 (22) | 1.323 (15) | 1.333 (19) | 1.375 (22) | 1.353 (14) | 37 |
| PDB, no metals | 1.383 (9) | 1.322 (8) | 1.335 (12) | 1.368 (10) | 1.360 (10) | 119 |
| PDB, metals‡ | 1.382 (11) | 1.326 (9) | 1.331 (12) | 1.365 (13) | 1.358 (11) | 52 |
| PDB, all | 1.382 (10) | 1.324 (9) | 1.334 (12) | 1.367 (11) | 1.359 (10) | 171 |
| CSD, no metals | 1.381 (9) | 1.319 (8) | 1.336 (13) | 1.367 (9) | 1.359 (9) | 99 |
| CSD, metals‡ | 1.385 (6) | 1.323 (7) | 1.330 (14) | 1.369 (8) | 1.357 (8) | 21 |
| CSD, all | 1.382 (9) | 1.320 (8) | 1.335 (14) | 1.367 (9) | 1.359 (10) | 123 |
| ND1 and NE2 protonated | ||||||
| EH91 | 1.378 (11) | 1.321 (10) | 1.321 (10) | 1.374 (11) | 1.354 (11) | |
| EH99 | 1.380 (10) | 1.326 (10) | 1.317 (11) | 1.373 (11) | 1.354 (9) | 54 |
| PDB, no metals | 1.375 (7) | 1.327 (6) | 1.322 (7) | 1.370 (10) | 1.352 (4) | 32 |
| CSD, no metals | 1.379 (7) | 1.325 (8) | 1.316 (9) | 1.373 (7) | 1.353 (7) | 174 |
| ND1 and NE2 deprotonated | ||||||
| CSD, metals | 1.381 (8) | 1.333 (7) | 1.325 (12) | 1.365 (9) | 1.362 (8) | 7 |
| PDB, resolution 3, all | 1.377 (11) | 1.322 (8) | 1.322 (9) | 1.372 (12) | 1.356 (9) | 39418 |