A model to estimate stability constants of amino acid chelates with Cu(II) and Ni(II) at different ionic strengths A. Miličević ⁎, N. Raos Institute for Medical Research and Occupational Health, Ksaverska c. 2, P.O. Box 291, HR-10001, Zagreb, Croatia abstract article info Article history: Received 7 October 2011 Accepted 9 November 2011 Available online 19 November 2011 Keywords: Specific ion interaction theory (SIT) Topological indices Regression models The equation for the simultaneous prediction of stability constants at various ionic strengths for copper(II) and nickel(II) mono-complexes with α-amino acids was developed. It is based on valence connectivity index of the 3rd order ( 3 χ v ) and specific ion interaction theory (SIT). The equation was tested on two sets of data. The first set consisted of 31 log K 1 values for copper(II) complexes with four α-amino acids (glycine, alanine, valine and leucine) measured at I =0–2 mol dm −3 . The second set encompassed 24 log K 1 values for nickel(II) complexes with seven α-amino acids (glycine, alanine, valine, leucine, 2-aminobutanoic, 2- aminopentanoic and 2-aminohexanoic acid) measured at I =0–1 mol dm −3 . Both sets yielded fair agreement with the experiment, with the standard error of estimate 0.09 and 0.10, for the first and the second set, re- spectively. Moreover, only two log K 1 values in each set were reproduced with an absolute error >0.2. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The major obstacle in development of a suitable model for predic- tion of stability constants of coordination compounds [1,2] is inhomo- geneity of experimental data, i.e. stability constants were determined at various temperatures and ionic strengths, with different methods and in different background media. Even when the same method was used following the same protocol, experimental data could differ as much as 0.3 log K units [3]. According to IUPAC criteria [4] stability constants are grouped in four categories with respect to standard error in their log K values: recommended (S.E. b 0.05), tentative (0.05 b S.E. b 0.2), doubtful and rejected (S.E. > 0.2). Some researchers skip this problem entirely; they simply use the data from all available sources, irrespectively of experimental condi- tions [5,6]. In contrast to this, when stability constants had been cau- tiously selected (T = 25 °C, I = 0.01 or I →0), log K 1 was predicted with the error of 0.03–0.13, for 12 complexes of four metals with four amino acids (or with the error 0.00–0.29 in log β 2 for 14 com- plexes) [7]. Moreover, when all constants had been determined in the same laboratory, theoretical estimation of stability constants were so successful that it enabled evaluation of two electroanalytical methods, i.e. glass electrode potentiometry (GEP) and square wave voltammetry (SWV) [8]. There are many descriptors and approaches for prediction of stabi- lity constants [1], but we use only one topological descriptor, valence connectivity index of the 3rd order ( 3 χ v ). It was shown that models based on 3 χ v are very successful in predicting the constants of a va- riety of metals (Cu 2+ , Ni 2+ , Co 2+ , Fe 2+ , Mn 2+ , and Cd 2+ ) and li- gands (α-amino acids, their N-alkylated derivatives, diamines, triamines, and peptides) [2,7,9]. Such approach is very simple, both computationally and conceptually, and enables prediction of stability constants from the sole constitutional formulas. However, models have to be calibrated on a considerable number of experimental con- stants, and as they were seldom measured at the same conditions, our approach can hardly be applied to poorly studied systems. The aim of this paper is to solve this problem by developing the model that should deal with the constants measured at different ionic strengths. For this purpose we applied specific ion interaction theory (SIT) [10–12], in its extended (parabolic) form [13–15]. 2. Methods 2.1. Regression functions The stability constant at the given ionic strength, I, is expressed by the equation: logK 1 ¼ logK 1 o þ fI ðÞ: ð1Þ K 1 o is stability constant value at I = 0 mol dm −3 ; it can be expressed as the function of connectivity index 3 χ v [16]: logK 1 o ¼ a 1 3 χ v þ a 2 3 χ v   2 þ b: ð2Þ As the Debye–Hückel equation is accurate only at very low ionic strengths (I b 0.01 mol dm −3 ), its extension is needed for greater I. Journal of Molecular Liquids 165 (2012) 139–142 ⁎ Corresponding author. E-mail address: antem@imi.hr (A. Miličević). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.11.001 Contents lists available at SciVerse ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq