Molecular Structure And Diffusion Study Of Monolayers At The Air Water Interface

Author : Saranya Pullanchery Sankara Narayanan
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (11 download)

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Download or read book Probing Interfacial Water Structure Next to Lipid Membranes Using Vibrational Sum Frequency Spectroscopy written by Saranya Pullanchery Sankara Narayanan and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Phospholipids are one of the major structural components of biological membranes. The hydrophilic headgroups of lipid molecules interact with the adjacent interfacial water molecules. The hydrogen bonding structure of interfacial water is determined by the chemical structure of the lipid headgroups themselves. Moreover, the first few molecular layers of interfacial water mediate all the chemical interactions at the membrane/water interface. Herein, the changes in interfacial water structure associated with lipid-lipid and ion-lipid interactions next to various zwitterionic and charged lipid membranes are investigated. In the first part of the dissertation, the interfacial water structure and phosphate group hydration of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) monolayers at air/water interfaces are described. Both vibrational sum frequency spectroscopy (VSFS) and Langmuir monolayer compression measurements were made. The PC lipids oriented water molecules predominantly through their phosphate-choline (P-N) dipoles and carbonyl moieties. Upon the introduction of low concentrations of 1,2 dioleoyl-3-trimethylammonium propane (DOTAP), a positively charged double chain surfactant, the TAP headgroups were attracted to the phosphate moieties on adjacent PC lipids. This attraction caused the monolayers to contract, expelling water molecules that were hydrogen bonded to the phosphate groups. Moreover, amplitude of the OH stretch signal decreased. At higher DOTAP concentrations, the positive charge on the monolayer caused an increase in the area per headgroup and water molecules in the near-surface bulk region became increasingly aligned. Under these latter conditions, the OH stretch amplitude was linearly proportional to the surface potential. By contrast, introducing 1,2-dioleoyl-sn-glycero-3-phosphatidylglycerol (DOPG), a negatively charged lipid, did not change the area per lipid or the phosphate-water hydrogen bonding network. As the interfacial potential grew more negative, the OH stretch amplitude increased continuously. Significantly, changes in the interfacial water spectrum were independent of the chemistry employed to create the positive or negative interfacial potential. For example, Ca2+ and tetracaine (both positively charged) disrupted the water structure similarly to low DOTAP concentrations, while SCN- and ibuprofen (both negatively charged) enhanced the water structure. The changes in interfacial water peaks as a function of increasing surface potential were deconvoluted into changes in directly bound water molecules and water molecules ordered by the electric field. At high surface potential values, the water molecules ordered by the electric field were found to dominate the VSFS signal changes. In a second set of experiments, the effect of inverting the P-N dipole on the interfacial water ordering was studied using VSFS. The inverted headgroup dipole was found to order less water molecules compared to PC headgroups. The second part of this dissertation describes various ion-lipid interactions and the associated changes in interfacial water structure. Using a combination of fluorescence imaging, surface pressure-area isotherms and VSFS, the differences between the interaction of Cu2+, Ca2+, Mg2+ and Zn2+ ions with the PS headgroup were characterized. Cu2+ ions bound to two PS lipids without attenuating the surface charge, whereas Ca2+, Mg2+ and Zn2+ ions neutralized the negative charge on the headgroups. Zn2+ and Ca2+ ions induced three-dimensional bleb formation in PS lipid bilayers because of their ability to form contact ion pairs with the PS headgroup moieties and to reduce the headgroup area dramatically. The molecular details of phase changes within a PS lipid monolayer induced by Zn2+ were further characterized in detail using VSFS. Zn2+ ions were found to dehydrate the PS lipid headgroups and rigidify the lipid alkyl tails. The changes in interfacial water structure next to PS lipid monolayers were utilized to determine the dissociation constant for Zn2+-PS interactions. The approach developed in this dissertation to interpret interfacial water signal changes during ion-lipid and lipid-lipid interactions can potentially be useful for future nonlinear optical experiments.