title: Guanidinium Group Remains Protonated in a Strongly Basic Arginine Solution.
abstract: Knowledge of the acid dissociation constant of an amino acid has very important ramifications in the biochemistry of proteins and lipid bilayers in aqueous environments because charge and proton transfer depend on its value. The acid dissociation constant for the guanidinium group in arginine has historically been posited as 12.5, but there is substantial variation in published values over the years. Recent experiments suggest that the dissociation constant for arginine is much higher than 12.5, which explains why the arginine guanidinium group retains its positive charge under all physiological conditions. In this work, we use X-ray photoelectron spectroscopy to study unsupported, aqueous arginine nanoparticles. By varying the pH of the constituent solution, we provide evidence that the guanidinium group is protonated even in a very basic solution. By analyzing the energy shifts in the C and N X-ray photoelectron spectra, we establish a molecular level picture of how charge and proton transport in aqueous solutions of arginine occur.
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
authors.forename: Bo
authors.identifier: http://orcid.org/0000-0002-1120-3423
authors.initials: B
authors.lastname: Xu
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.|Department of Chemistry, University of California, Berkeley, CA, 94720, USA.
authors.forename: Michael I
authors.initials: MI
authors.lastname: Jacobs
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
authors.forename: Oleg
authors.identifier: http://orcid.org/0000-0003-2068-4991
authors.initials: O
authors.lastname: Kostko
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
authors.forename: Musahid
authors.identifier: http://orcid.org/0000-0003-1216-673X
authors.initials: M
authors.lastname: Ahmed
mesh_terms: D001120:Arginine / Q000737:chemistry*; D019791:Guanidine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D056951:Photoelectron Spectroscopy; D011522:Protons; D012996:Solutions
chemical_list: D011522:Protons; D012996:Solutions; D001120:Arginine; D019791:Guanidine
keywords: acid dissociation constant; amino acids; nanoparticles; photoelectron spectroscopy; proton transport