Guanidinium Group Remains Protonated in a Strongly Basic Arginine Solution

Area of Science:

• Biochemistry
• Physical Chemistry
• Biophysics

Background:

• The acid dissociation constant (pKa) of amino acids is crucial for understanding biochemical processes.
• The pKa of arginine's guanidinium group is historically cited as 12.5, but experimental values vary.
• Recent studies suggest a higher pKa for arginine, explaining its positive charge under physiological conditions.

Purpose of the Study:

• To investigate the acid dissociation constant of the guanidinium group in arginine.
• To elucidate the behavior of arginine in aqueous solutions across different pH levels.
• To provide a molecular-level understanding of charge and proton transport involving arginine.

Main Methods:

• X-ray photoelectron spectroscopy (XPS) was employed to study aqueous arginine nanoparticles.
• The pH of the arginine solutions was systematically varied.
• Energy shifts in C and N XPS spectra were analyzed.

Main Results:

• Evidence was found that the guanidinium group remains protonated even in highly basic solutions.
• The study provides experimental support for an arginine pKa significantly higher than 12.5.
• Molecular-level insights into charge and proton transport mechanisms were established.

Conclusions:

• The guanidinium group of arginine is consistently protonated across a wide pH range, including basic conditions.
• This research refines the understanding of arginine's acid dissociation constant.
• The findings have implications for protein and lipid bilayer behavior in aqueous biological environments.