Proton affinity and pK(a) values of N-formyl-L-histidinamide are found to vary as a function of its backbone and/or side-chain orientation. Proton affinities between the cationic and neutral forms of structurally similar conformers are between -246 and -230 kcal mol(-1), while pK(a) values associated with the same conformers are between 6 and 8. For the neutral-to-anion transition, the following ranges were computed -342 > PA > -350 kcal mol(-1) and 18 < pK(a) < 22. The protonation state of histidines on the surface of a protein depends primarily on the pH. Due to protonation or deprotonation, the side-chain and/or backbone orientation of these histidine residues may undergo considerable changes. Examples are presented and confirmed by ab initio calculations, where proteins were crystallized under various pH conditions, resulting in the same histidine residue to adopt different conformations. Furthermore, a hypothesis is given for a protonation-induced conformational modification of the histidine residue in the catalytic triad of chymotrypsin during catalysis, which lowers the pK(a) value of the catalytic histidine by 1.2 units. Both the experimental and theoretical results support that proton affinity as well as that pK(a) values of histidine residues are strongly conformationally dependent.