Energetics and structural parameters for the lowest-energy conformers of Trp-cage protomers.

In each row the following information is reported: protonation pattern (first column); energy difference with respect to the most stable protomer ( in kJ/mol); (intrinsic) internal energy variation (), proton affinity (PA) and gas-phase basicity (GPB) relative to the most favourable protomer (see text for a definition of these quantities; all values are in kJ/mol); ionized residues (IR); salt-bridges (SB); hydrogen bonds between salt-bridged residues (sHB); ionized hydrogen bonds where either the donor or the acceptor is ionized, A or D H-bonds (iHB); neutral hydrogen bonds (HB). Hydrogen bonds are identified according to the donor-acceptor () distance and the donor-acceptor H-bond angle (). The following geometric criterion was adopted: Å and . A salt-bridge is formed if the distance between any oxygen atom of the acidic residue and any protonable nitrogen atom of the basic residue is less than 4.0 Å. Structural data for the most stable protonation state in aqueous solution () are also reported (data in Italics).