Modifying
the properties of bio-based materials has garnered increasing
interest in recent years. In related applications, the ability of
alginates to complex with metal ions has been shown to be effective
in liquid-to-gel transitions, useful in the development of foodstuff
and pharma products as well as biomaterials, among others. However,
despite its ubiquitous use, alginate behavior as far as interactions
with cations is not fully understood. Hence, this study presents a
detailed comparison of alginate’s complexation with Na+ and Ca2+ and the involved intramolecular hydrogen
bonding and biomolecular chain geometry. Using all-atom molecular
dynamics simulations, we find that in contrast to accepted models,
calcium cations strongly bind to alginate chains by disruption of
hydrogen bonds between neighboring residues, stabilizing a left-hand,
3-fold helical chain structure that enhances chain stiffness. Hence,
while present, the traditionally accepted egg-box binding mode was
a minor subset of possible conformations. For a single chain, most
of the cation binding occurred as single-cation interaction with a
carboxyl group, without the coordination of other alginate oxygens.
The monovalent Na+ ions were found to be mostly nonlocalized
around alginate and therefore do not compete with intramolecular hydrogen
bonding. The different binding modes observed for Na+ and
Ca2+ contribute toward explaining the different solubility
of sodium and calcium alginate.
Alginic
acid, a naturally occurring anionic polyelectrolyte, forms
strong physically cross-linked hydrogels in the presence of metal
cations. The latter engage in electrostatic interactions that compete
with intra- and intermolecular hydrogen bonds, determining the gel
structure and properties of the system in aqueous media. In this study,
we use all-atom molecular dynamics simulations to systematically analyze
the interactions between alginic acid chains and Na+ and
Ca2+ counterions. The formed alginates originate from the
competition of intramolecular hydrogen bonding and water coordination
around the polyelectrolyte. In contrast to the established interpretation,
we show that calcium cations strongly bind to alginate by disrupting
hydrogen bonds within (1 → 4)-linked β-d-mannuronate
(M) residues. On the other hand, Na+ cations enhance intramolecular
hydrogen bonds that stabilize a left-hand, fourfold helical chain
structure in poly-M alginate, resulting in stiffer chains. Hence,
the traditionally accepted flexible flat-chain model for poly-M sequence
is not valid in the presence of Na+. The two cations have
a distinct effect on water coordination around alginate and therefore
on its solubility. While Ca+ disrupts water coordination
directly around the alginate chains, mobile Na+ cations
significantly disrupt the second hydration layer.
With an increasing need to develop carbon capture technologies, research regarding the use of cage-based porous materials has garnered great interest. Typically, the study of gas adsorption in porous organic...
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