Stability of "salt bridges" in membrane proteins.

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Abstract
The free energies of transfer of ionized amino acid side chains in water to both their ion-paired and neutral H-bonded states in low-dielectric media were estimated. The difference between the 2 free energies corresponds to the proton transfer free energy in a salt bridge formed between acidic and basic groups (i.e., lysine and glutamine residues). The approach is to use gas phase proton transfer data, pK values, and experimentally determined solvation energies to estimate the standard state free energy changes involved in transferring amino acid side chains, in both ionized and neutral form, from water (dielectric constant .epsilon. = 80) to vacuum .epsilon. = 1). The familiar expressions for the charging energy of a sphere and dipole are used to interpolate between .epsilon. = 1 and .epsilon. = 80. It costs .apprx. 10-16 kcal/mol to transfer a salt bridge from water to a medium of .epsilon. = 2-4, in ionized or neutral form within the resolution of the estimates. The proton transfer energy is thus .apprx. 0. The tendency of salt bridges to form additional H bonds in real proteins suggests that the ion pair will be present in most biological systems.