Browsing by Author "Okur, Halil I."

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Open Access
Lysozyme interaction with phospholipid nanodroplets probed by sum frequency scattering vibrational spectroscopy
(American Chemical Society, 2023-04-26) Golbek, Thaddeus W.; Okur, Halil I.; Kulik, Sergey; Dedic, Jan; Roke, Sylvie; Weidner, Tobias
When a nanoparticle (NP) is introduced into a biological environment, its identity and interactions are immediately attributed to the dense layer of proteins that quickly covers the particle. The formation of this layer, dubbed the protein corona, is in general a combination of proteins interacting with the surface of the NP and a contest between other proteins for binding sites either at the surface of the NP or upon the dense layer. Despite the importance for surface engineering and drug development, the molecular mechanisms and structure behind interfacial biomolecule action have largely remained elusive. We use ultrafast sum frequency scattering (SFS) spectroscopy to determine the structure and the mode of action by which these biomolecules interact with and manipulate interfaces. The majority of work in the field of sum frequency generation has been done on flat model interfaces. This limits some important membrane properties such as membrane fluidity and dimensionality─important factors in biomolecule–membrane interactions. To move toward three-dimensional (3D) nanoscopic interfaces, we utilize SFS spectroscopy to interrogate the surface of 3D lipid monolayers, which can be used as a model lipid-based nanocarrier system. In this study, we have utilized SFS spectroscopy to follow the action of lysozyme. SFS spectra in the amide I region suggest that there is lysozyme at the interface and that the lysozyme induces an increased lipid monolayer order. The binding of lysozyme with the NP is demonstrated by an increase in acyl chain order determined by the ratio of the CH3 symmetric and CH2 symmetric peak amplitudes. Furthermore, the lipid headgroup orientation s-PO2– change strongly supports lysozyme insertion into the lipid layer causing lipid disruption and reorientation. Altogether, with SFS, we have made a huge stride toward understanding the binding and structure change of proteins within the protein corona.
Open Access
Molecular mechanism for the ınteractions of hofmeister cations with macromolecules in aqueous solution
(American Chemical Society, 2020) Bruce, E. E.; Okur, Halil I.; Stegmaier, S.; Drexler, C. I.; Rogers, B. A.; van der Vegt, N. F. A.; Roke, S.; Cremer, P. S.
Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.
Open Access
On the stability and necessary electrophoretic mobility of bare oil nanodroplets in water
(American Institute of Physics, 2020) Pullanchery, S.; Kulik, S.; Okur, Halil I.; de Aguiar, H. B.; Roke, S.
Hydrophobic oil droplets, particles, and air bubbles can be dispersed in water as kinetically stabilized dispersions. It has been established since the 19th century that such objects harbor a negative electrostatic potential roughly twice larger than the thermal energy. The source of this charge continues to be one of the core observations in relation to hydrophobicity, and its molecular explanation is still debated. What is clear though is that the stabilizing interaction in these systems is understood in terms of electrostatic repulsion via Derjaguin, Landau, Verwey, and Overbeek theory. Recent work [A. P. Carpenter et al., Proc. Natl. Acad. Sci. U. S. A. 116, 9214 (2019)] has added another element into the discussion, reporting the creation of bare near-zero charged droplets of oil in neat water that are stable for several days. Key to the creation of the droplets is a rigorous glassware cleaning procedure. Here, we investigate these conclusions and show that the cleaning procedure of glassware has no influence on the electrophoretic mobility of the droplets and that oil droplets with near-zero charge are unstable. We provide an alternative possible explanation for the observations involving glass surface chemistry.
Open Access
Transient domains of ordered water induced by divalent ions lead to lipid membrane curvature fluctuations
(Nature Research, 2020) Tarun, O. B.; Okur, Halil I.; Rangaman, P.; Roke, S.
Cell membranes are composed of a hydrated lipid bilayer that is molecularly complex and diverse, and the link between molecular hydration structure and membrane macroscopic properties is not well understood, due to a lack of technology that can probe and relate molecular level hydration information to micro- and macroscopic properties. Here, we demonstrate a direct link between lipid hydration structure and macroscopic dynamic curvature fluctuations. Using high-throughput wide-field second harmonic (SH) microscopy, we observe the formation of transient domains of ordered water at the interface of freestanding lipid membranes. These domains are induced by the binding of divalent ions and their structure is ion specific. Using nonlinear optical theory, we convert the spatiotemporal SH intensity into maps of membrane potential, surface charge density, and binding free energy. Using an electromechanical theory of membrane bending, we show that transient electric field gradients across the membrane induce spatiotemporal membrane curvature fluctuations.