How Salt bridges effect protein properties

In proteins, salt bridges occur between amino acid side-chains with opposite positive or negative full-electron charges, namely, Glu- or Asp- vs. Arg+ His+ Lys+. They may also occur between ionized organic ligands, such as acetylcholine+ (or example at right: 1cbr), or inorganic ions, such as K+ or SO4=, and amino acid side-chains.
A salt bridge is generally considered to exist when the centers of charge are 4 Å or less apart. The center of charge of the arginine sidechain is the zeta carbon. The energetic significance of such complementary charge pairs is a complex function of the local environment.
Proteins from thermophiles have more salt bridges than do proteins from mesophiles. These additional salt bridges contribute to stability, resisting denaturation by high temperature.
Which amino acid has an R-group with a pKa closest to physiological pH?
A) Lysine
B) Histidine
C) Glutamate
D) Arginine
E) Aspartate

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Histidine

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