On the initial binding of alginate by calcium ions. The tilted egg-box hypothesis.

The initial binding of calcium ions by alginate chains was investigated in dilute solution. The use of viscometry, light scattering, circular dichroism, and fluorescence quenching performed on both Ca(2+) and Mg(2+) alginate systems allowed collecting new experimental data in addition to those already reported in the literature. This led us to propose an ion multicomplex binding modality and to disprove the Ca(2+)-alginate monocomplex formation. The first mode of bonding was proposed to be formed by four G residues from facing chains in a conformation which, albeit ordered, is different from the well-known "egg box" one. This was indicated as a locally tilted conformation (tilted egg-box). The addition of further bonding ions caused the well-known cooperative egg-box conformation (strong bonding) with notable variation in the physical-chemical properties of the dilute solution.