Browsing by Subject "Thermoresponsive polymers"

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    Molecular interactions and binding mechanisms of hydrophobic hofmeister cations to macromolecules
    (2023-08) Ertekin, Umay Eren
    It’s been known for over a century that ions specifically affect the bulk properties of solutions, behavior of macromolecules and a myriad of interfacial phenomena occurring in solution. Yet, the molecular mechanisms underlying these so-called Hofmeister effects are only recently being realized within the last few decades. In the resurgence in specific ion effects studies, the role attributed to cations has been relatively understated in comparison to the effects of anions. Whereas various molecular mechanisms have been elucidated for a diverse spectrum of anions, cationic effects have largely remained limited to common metal ions. Within the cationic Hofmeister series for cations, strongly-hydrated cations exhibit very weak binding to polar, electronegative groups, but weakly-hydrated cations in particular are classified as non-interacting. This thesis brings a much-needed expansion to specific cation effects by investigating the interactions between the weakly-hydrated tetraalkylammonium cations and model neutral thermoresponsive polymers, principally poly(N-isopropylacrylamide) (PNIPAM). The hydrophobicity of the cations is incrementally tuned by increasing the length of their alkyl chains, thus forming the series of salts investigated herein: NR4Cl where R = H, Me, Et, n-Pr, n-Bu, and the anion is kept constant as Cl-. By using a multi-instrumental approach, it is demonstrated that the largest of these cations exhibit a significant binding to the polymers and that the resulting salting-in effects are comparable in magnitude to those observed for sodium salts of weakly-hydrated anions. Thermodynamic phase transition measurements of the polymers are complemented by ATR-FTIR and quantitative 1H-NMR spectroscopic studies to systematically investigate the nature and molecular-level mechanism of the interaction. In stark contrast to the known behavior of the strongly-hydrated cations, through the temperature-controlled ATR-FTIR investigations it is found that carbonyl moieties are not the primary sites of interaction. Instead, it is found that these weakly-hydrated, ‘greasy’ cations preferentially interact with the most hydrophobic groups on the polymer: the isopropyl group on the PNIPAM side-chain, as revealed by a quantitative externally-referenced 1H-NMR methodology developed to elucidate ion-macromolecule interactions. The binding generally follows a Langmuir-type saturation behavior and exhibits site-specific dissociation constants as low as KD ≈ 0.2 M. This unprecedented, hydrophobically-mediated interaction between weakly-hydrated tetraalkylammonium cations and neutral macromolecules is then demonstrated to be a general mechanism and is shown to extend to polymers of vastly different molecular architectures. The results presented, thus, signify a new, more expansive view of cationic Hofmeister effects, where the far weakly-hydrated region of the series interacts with a novel mechanism entirely unlike that of other cations.
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    The mechanisms of ATP - biomacromolecule interactions
    (2023-12) Ayvaz, Cansın
    Adenosine triphosphate (ATP), one of the most important biomolecules of life, plays a vital role as the primary energy source within cells for essential biological functions. It has recently been discovered that ATP can also serve as a biological hydrotrope to destabilize protein aggregates and fibers. This thesis aims to investigate the recently discovered hydrotropic behavior of ATP and its interaction mechanisms with biomacromolecules, particularly poly(N-isopropylacrylamide) (PNIPAM), using a multi-experimental approach combined with molecular dynamics (MD) simulations. Adapting the bottom-up approach, the phase behavior of macromolecules is examined through phase transition and ATR-FTIR measurements. Additionally, site-specific interactions are identified with quantitative 1H-NMR spectroscopic studies, and the hydration shell structure and cluster morphologies of ATP molecules are explored through Multivariate Curve Resolution (MCR) Raman experiments. It is demonstrated that adenine and adenosine subgroups show negligible effect on the solubility of macromolecules, whereas ATP, AMP, and triphosphate exhibited purely salting-out behavior, and induced the aggregation of macromolecules. In stark contrast to the recently discovered hydrotropic behavior of ATP, no specific interactions between the macromolecule and ATP were observed in spectroscopic ATR-FTIR and 1H-NMR measurements, as well as MD simulations. Surprisingly, at elevated concentrations, self-association of ATP was observed leading to partial destabilization of larger PNIPAM aggregates to smaller ones. In the absence of ATP binding sites, interactions with random-coil-like structured macromolecules do not lead to effective hydrotropic action of ATP. Instead, they function more as stabilizers rather than solubilizing the macromolecules.