Arginine: Its pKa value revisited

Using complementary approaches of potentiometry and NMR spectroscopy, we have determined that the equilibrium acid dissociation constant (pK(a) value) of the arginine guanidinium group is 13.8 +/- 0.1. This is substantially higher than that of approximate to 12 often used in structure-based electrostatics calculations and cited in biochemistry textbooks. The revised intrinsic pK(a) value helps explains why arginine side chains in proteins are always predominantly charged, even at pH values as great as 10. The high pK(a) value also reinforces the observation that arginine side chains are invariably protonated under physiological conditions of near neutral pH. This occurs even when the guanidinium moiety is buried in a hydrophobic micro-environment, such as that inside a protein or a lipid membrane, thought to be incompatible with the presence of a charged group.