Hard- and Soft-Modeling Approaches for Resolution of Complex Formation of Co 2+ and Ni 2+ with Glycine

Principal component analysis (PCA) was used to obtain information about the number of components in the complex formation equilibria of Co²⁺ and Ni²⁺ with glycine (Gly). In order to obtain a clearer insight into these complex formation systems, multivariate curve resolution–alternating least squares (MCR-ALS) was used. Using MCR-ALS as a soft-modeling method, well-defined concentration and spectral profiles were obtained under unimodality, non-negativity, and closure constraints. Based on the obtained results, an equilibrium model was proposed and subsequently, a hard-modeling method was used to resolve the complex formation equilibria completely. Due to the presence of multiple equilibria, the resolution of such systems is very difficult. The Co–Gly system was best described by a model consisting of M(GlyH), M(Gly), M(Gly)₂, M(Gly)₂H, and M(Gly) ³ (M=Co²⁺) with the overall stability constants determined to be 7.10 ± 0.011, 5.14 ± 0.006, 9.28 ± 0.009, 13.75 ± 0.016, and 11.10 ± 0.010, respectively. On the other hand, the system of Ni–Gly was best fitted by a model containing M(GlyH), M(Gly), M(Gly)₂, M(Gly)₃, and M(Gly)₂(OH) (M = Ni²⁺) with overall stability constants of 10.95 ± 0.04, 6.41 ± 0.03, 11.31 ± 0.03, 15.39 ± 0.06, and 14.32 ± 0.02, respectively.