Author: Loux, Nicholas T.

Source: Chemical Speciation and Bioavailability, Volume 15, Number 2, May 2003 , pp. 47-51(5)

Publisher: Science Reviews 2000 Ltd

Abstract:

Zwitterions are aqueous solvated molecules simultaneously possessing one negatively and one positively charged site. Although electroneutral, the environmental interaction of zwitterions with other ionic species is likely to differ significantly from the behavior of comparable electroneutral species without charged sites. Amino acids, the zwitterionic species that have received the most historical scrutiny, are believed to possess at least four microscopic acidity constants: k_a=[H⁺] [⁺H₃NRCOO⁻]/[⁺H₃NRCOOH], k_b=[H⁺] [H₂NRCOOH]/[⁺H₃NRCOOH], k_c=[H⁺] [H₂NRCOO⁻]/[⁺H₃NRCOO⁻], and k_d=[H⁺] [H₂NRCOO⁻]/[H₂NRCOOH]. Unfortunately, due to their comparable energetics, these microscopic acidity constants cannot be discerned using standard potentiometric titration procedures. In response, experimentally observable macroscopic constants (K₁ and K₂) have historically been related to the microscopic constants with the following relationships: K₁=k_a+k_b and 1/K₂=1/k_c+k_d. It will be demonstrated that these equations are approximations suitable for restricted pH ranges and that more exact expressions can be derived: K₁=k_a+k_b+k_ck_a/[H⁺]+k_dk_b/[H⁺] and 1/K₂=1/k_d+1/k_c+[H⁺]/k_ak_c+[H⁺]/k_bk_d.

Keywords: zwitterions; acidity constants

Document Type: Research article

DOI: http://dx.doi.org/10.3184/095422903782775244

Affiliations: 1: US Environmental Protection Agency, Office of Research and Development, National Exposure Research Laboratory, Ecosystem Research Division, 960 College Station Road, Athens, GA 30605-2700, USA

Publication date: 2003-05-10

More about this publication?

• Chemical Speciation & Bioavailability covers a rapidly expanding area in environmental science.

  Research on the interactions between the chemical forms and behaviour of toxic compounds and their subsequent biological uptake, metabolism and ecological fate involves many scientific fields. These studies are often published in discipline-specific journals, leading to inadequate review and information scatter. This situation hinders both the development of an international community of experienced colleagues and the open flow of information and discussion. Additionally, the importance of speciation and bioavailability research to the development of pollution law and control technologies is being increasingly appreciated by environmental regulatory agencies throughout the world.

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