Arginine: Its pK_a value revisited

Abstract

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 6 0.1. This is substantially higher than that of~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.

Ionization Properties of Histidine Residues in the Lipid Bilayer Membrane Environment

“…Arginine also remains cationic when buried in the hydrophobic interior of a folded protein (46). We note furthermore that the experimental value for the aqueous pK a of the Arg guanidinium side chain recently has been revised upward to 13.8 (47).…”

Mechanism of OmpG pH-Dependent Gating from Loop Ensemble and Single Channel Studies

“…This result is not surprising, as the guanidinium group of arginine has been reported to have an unusual ability to maintain the fully protonated state under all physiological conditions, such that its neutral state has not been unambiguously observed. 36 To identify the residues that can sense pH change, we examined residues that have significantly different protonation states when the pH is lowered from 7 to 5. Specifically, we constructed 95% confidence intervals for the difference in proportion of protonated states among low-energy conformations at pH 7 compared to the proportion of protonated states among low energy conformations at pH 5 for each ionizable residue.…”

Energetics of side-chain snorkeling in transmembrane helices probed by nonproteinogenic amino acids

“…The guanidinium group on the Arg side chain has such a high pK a that it remains protonated even when buried in a hydrophobic environment (5). To explore the combined effects of charge and side-chain length [i.e., snorkeling (6, 7)] on membrane insertion, we analyzed a series of Arg analogs with increasingly long side chains of three to eight methyl groups, as well as an analog (l-canavanine), where the δ carbon is replaced by an oxygen atom, thereby lowering the pK a of the guanidinium Significance Membrane proteins are central players in all cells, and their structure and function are under intense study.…”