Orion Shih

Investigation of terahertz vibration-rotation tunneling spectra for the water octamer.

We report a combined theoretical and experimental study of the water octamer-h16. The calculations used the ring-polymer instanton method to compute tunnelling paths and splittings in full dimensionality. The experiments measured extensive high resolution spectra near 1.4 THz, for which isotope dilution experiments and group theoretical analysis support assignment to the octamer. Transitions appear as singlets, consistent with the instanton paths, which involve the breakage of two hydrogen-bonds and thus give tunneling splittings below experimental resolution.

Cation-cation contact pairing in water: Guanidinium

The formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized by X-ray absorption spectroscopy and first-principles spectral simulations based on molecular dynamics sampling. Observed concentration-induced nitrogen K-edge resonance shifts result from π* state mixing and the release of water molecules from each first solvation sphere as two solvated guanidinium ions associate into a stacked pair configuration. Possible biological implications of this counterintuitive cation-cation pairing are discussed.

Evaporation kinetics of aqueous acetic acid droplets: effects of soluble organic aerosol components on the mechanism of water evaporation

The presence of organic surfactants in atmospheric aerosol may lead to a depression of cloud droplet growth and evaporation rates affecting the radiative properties and lifetime of clouds. Both the magnitude and mechanism of this effect, however, remain poorly constrained. We have used Raman thermometry measurements of freely evaporating micro-droplets to determine evaporation coefficients for several concentrations of acetic acid, which is ubiquitous in atmospheric aerosol and has been shown to adsorb strongly to the air-water interface. We find no suppression of the evaporation kinetics over the concentration range studied (1-5 M). The evaporation coefficient determined for 2 M acetic acid is 0.53 ± 0.12, indistinguishable from that of pure water (0.62 ± 0.09).

Properties of aqueous nitrate and nitrite from x-ray absorption spectroscopy

Nitrate and nitrite ions are of considerable interest, both for their widespread use in commercial and research contexts and because of their central role in the global nitrogen cycle. The chemistry of atmospheric aerosols, wherein nitrate is abundant, has been found to depend on the interfacial behavior of ionic species. The interfacial behavior of ions is determined largely by their hydration properties; consequently, the study of the hydration and interfacial behavior of nitrate and nitrite comprises a significant field of study. In this work, we describe the study of aqueous solutions of sodium nitrate and nitrite via X-ray absorption spectroscopy (XAS), interpreted in light of first-principles density functional theory electronic structure calculations. Experimental and calculated spectra of the nitrogen K-edge XA spectra of bulk solutions exhibit a large 3.7 eV shift between the XA spectra of nitrate and nitrite resulting from greater stabilization of the nitrogen 1s energy level in nitrate. A similar shift is not observed in the oxygen K-edge XA spectra of NO3 (-) and NO2 (-). The hydration properties of nitrate and nitrite are found to be similar, with both anions exhibiting a similar propensity towards ion pairing.

Probing the Acid-Induced Packing Structure Changes of the Molten Globule Domains of a Protein near Equilibrium Unfolding

Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-vis absorption with integrated online size exclusion chromatography, supplemental with molecular dynamics simulations, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I1, I2, and E along the unfolding pathway. The I1-I2 transition, overlooked in all previous studies, involves mainly coordinated reorientations across interconnected molten globule subdomains, and the transition activates a critical pivot domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change of -9.5 kcal mol-1, extracted based on the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein.

Membrane Charging and Swelling upon Calcium Adsorption as Revealed by Phospholipid Nanodiscs

Direct binding of calcium ions (Ca2+) to phospholipid membranes is an unclarified yet critical signaling pathway in diverse Ca2+-regulated cellular phenomena. Here, high-pressure-liquid-chromatography, small-angle X-ray scattering (SAXS), UV-vis absorption, and differential refractive index detections are integrated to probe Ca2+-binding to the zwitterionic lipid membranes in nanodiscs. The responses of the membranes upon Ca2+-binding, in composition and conformation, are quantified through integrated data analysis. The results indicate that Ca2+ binds specifically into the phospholipid headgroup zone, resulting in membrane charging and membrane swelling, with a saturated Ca2+-lipid binding ratio of 1:8. A Ca2+-binding isotherm to the nanodisc is further established and yields an unexpectedly high binding constant K = 4260 M-1 and a leaflet potential of ca. 100 mV based on a modified Gouy-Chapman model. The calcium-lipid binding ratio, however, drops to 40% when the nanodisc undergoes a gel-to-fluid phase transition, leading to an effective charge capacity of a few μF/cm2.

Erratum: “Electrokinetic detection for x-ray spectra of weakly interacting liquids: n-decane and n-nonane” [J. Chem. Phys. 140, 234202 (2014)]

Author(s): Lam, Royce K; Shih, Orion; Smith, Jacob W; Sheardy, Alex T; Rizzuto, Anthony M; Prendergast, David; Saykally, Richard J