Thermodynamic study on polyaspartic acid biopolymer in solution and prediction of its chemical speciation and bioavailability in natural fluids

Abstract

The interaction of polyaspartic acid 2 kDa (PASP) biopolymer with the major cationic constituent of natural fluids (H+, Na+, K+, Mg2+, Ca2+) were studied by ISE-H+ titrations. The acid-base properties of PASP were determined in different ionic media [NaNO3, NaCl, KCl, (CH3)4NCl and (C2H5)4NI] at different temperatures and ionic strengths. The data analysis was performed according to the diprotic like model, assuming that a monomeric unit consists of two α and two β aspartyl residues, finding four protonation constants in the range 2.0 < pH < 8.0. Corresponding thermodynamic data (ΔG0, ΔH0 and TΔS0) revealed a slightly endothermic process for the 1st and 2nd steps and a slightly exothermic process for the 3rd and 4th steps, suggesting that amino groups are not involved in the proton binding. The CaL species is more stable than the MgL one at low ionic strength and T = 298.15 K, the contrary is observed at I = 1.0 mol kg−1 and T = 318.15 K, indicating that the latter process is endothermic, while the former is exothermic. Finally, speciation diagrams in the conditions of sea water and human blood plasma are drawn. In sea water, for example, PASP is present as MgL (48%), CaL (41%), Na2L (8%) and NaL species (3%).