Alexander Kisliuk

Protein and solvent dynamics: How strongly are they coupled?

Analysis of Raman and neutron scattering spectra of lysozyme demonstrates that the protein dynamics follow the dynamics of the solvents glycerol and trehalose over the entire temperature range measured 100-350 K. The proteins fast conformational fluctuations and low-frequency vibrations and their temperature variations are very sensitive to behavior of the solvents. Our results give insight into previous counterintuitive observations that protein relaxation is stronger in solid trehalose than in liquid glycerol. They also provide insight into the effectiveness of glycerol as a biological cryopreservant.

Dynamics of Biological Macromolecules: Not a Simple Slaving by Hydration Water

We studied the dynamics of hydrated tRNA using neutron and dielectric spectroscopy techniques. A comparison of our results with earlier data reveals that the dynamics of hydrated tRNA is slower and varies more strongly with temperature than the dynamics of hydrated proteins. At the same time, tRNA appears to have faster dynamics than DNA. We demonstrate that a similar difference appears in the dynamics of hydration water for these biomolecules. The results and analysis contradict the traditional view of slaved dynamics, which assumes that the dynamics of biological macromolecules just follows the dynamics of hydration water. Our results demonstrate that the dynamics of biological macromolecules and their hydration water depends strongly on the chemical and three-dimensional structures of the biomolecules. We conclude that the whole concept of slaving dynamics should be reconsidered, and that the mutual influence of biomolecules and their hydration water must be taken into account.

Optical properties and enhancement factors of the tips for apertureless near-field optics

Resonant excitation of surface plasmons of a metal or metal-coated tip is crucial for achieving high enhancement of an optical signal with apertureless near-field optics. However, it remains a challenge to measure the optical spectrum of a tip with sub-wavelength dimensions. We present a technique based on total internal reflection microscopy to measure the optical properties of tips. A dependence of the optical resonance on the metal deposited is shown for silver-coated and gold-coated tips. These tips were also used to measure the tip-enhanced Raman spectra of silicon and a polymer blend of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (PEDOT/PSS) at 514.5 and 647 nm incident wavelengths. Qualitative agreement was observed between the tip-enhanced Raman spectra and the optical resonance of the tip measured with the technique developed.

Crossover in dynamics of polymeric liquids: Back toTll?

Light scattering spectra of two polymers, polyisobutylene (PIB) and polystyrene (PS), were analyzed in the broad frequency range at temperatures above the glass transition (Tg ). At high temperatures, the spectra followed the qualitative scenario suggested by mode-coupling theory (MCT) of the glass transition. The crossover temperature (Tc ) was defined to be approximately 1.35 Tg in PIB and approximately 1.15 Tg in PS. At lower temperatures (T < Tc ), the light scattering spectra deviated strongly from the idealized MCT scenario. Different signs of the dynamic transition around Tc are discussed. The difference between the suggested interpretation and an old idea of the liquid–liquid transition in polymeric liquids is stressed: we describe the transition as purely dynamic in nature.

Protein dynamics in viscous solvents

The mechanism of protein stabilization by glassy solvents is not entirely clear, and the stabilizer effective for a given protein is often discovered empirically. We use low frequency Raman spectroscopy as an effective tool to directly evaluate the ability of different solvents to suppress the conformational fluctuations that can lead to both protein activity and denaturation. We demonstrate that while trehalose provides superior suppression at high temperatures, glycerol is more effective at suppressing protein dynamics at low temperatures. These results suggest that viscosity of the solvent is not the only parameter important for biopreservation. It is also shown that glycerol and water enhance the high temperature conformational fluctuations relative to dry lysozyme, which explains the lower melting temperatures Tm in the hydrated protein and protein formulated in glycerol.

Decoupling charge transport from the structural dynamics in room temperature ionic liquids

Light scattering and dielectric spectroscopy measurements were performed on the room temperature ionic liquid (RTIL) [C4mim][NTf2] in a broad temperature and frequency range. Ionic conductivity was used to estimate self-diffusion of ions, while light scattering was used to study structural relaxation. We demonstrate that the ionic diffusion decouples from the structural relaxation process as the temperature of the sample decreases toward T(g). The strength of the decoupling appears to be significantly lower than that expected for a supercooled liquid of similar fragility. The structural relaxation process in the RTIL follows well the high-temperature mode coupling theory (MCT) scenario. Using the MCT analysis we estimated the dynamic crossover temperature in [C4mim][NTf2] to be T(c) ~ 225 ± 5 K. However, our analysis reveals no sign of the dynamic crossover in the ionic diffusion process.

Controlling Interfacial Dynamics: Covalent Bonding versus Physical Adsorption in Polymer Nanocomposites

It is generally believed that the strength of the polymer-nanoparticle interaction controls the modification of near-interface segmental mobility in polymer nanocomposites (PNCs). However, little is known about the effect of covalent bonding on the segmental dynamics and glass transition of matrix-free polymer-grafted nanoparticles (PGNs), especially when compared to PNCs. In this article, we directly compare the static and dynamic properties of poly(2-vinylpyridine)/silica-based nanocomposites with polymer chains either physically adsorbed (PNCs) or covalently bonded (PGNs) to identical silica nanoparticles (RNP = 12.5 nm) for three different molecular weight (MW) systems. Interestingly, when the MW of the matrix is as low as 6 kg/mol (RNP/Rg = 5.4) or as high as 140 kg/mol (RNP/Rg= 1.13), both small-angle X-ray scattering and broadband dielectric spectroscopy show similar static and dynamic properties for PNCs and PGNs. However, for the intermediate MW of 18 kg/mol (RNP/Rg = 3.16), the difference between physical adsorption and covalent bonding can be clearly identified in the static and dynamic properties of the interfacial layer. We ascribe the differences in the interfacial properties of PNCs and PGNs to changes in chain stretching, as quantified by self-consistent field theory calculations. These results demonstrate that the dynamic suppression at the interface is affected by the chain stretching; that is, it depends on the anisotropy of the segmental conformations, more so than the strength of the interaction, which suggests that the interfacial dynamics can be effectively tuned by the degree of stretching-a parameter accessible from the MW or grafting density.

Slow relaxation process in DNA.

A dynamic transition at temperatures ∼200–230K is observed in manyhydrated bio-polymers. It shows up as a sharp increase of the mean-squaredatomic displacements above this temperature range. We present neutronscattering data of DNA at different levels of hydration. The analysis showsthat the dynamic transition in DNA is related to a slow relaxation processin the MHz-GHz frequency range. This slow relaxation process iscompletely suppressed in the dry DNA sample where no dynamic transitionwas observed. The nature of the slow process is discussed. We ascribe it toa global relaxation of DNA molecule that involves cooperative motion ofmany base-pairs and backbone.

Influence of solvent on dynamics and stability of a protein

Abstract Proteins are often dissolved in viscous glass-forming solvents to provide thermal stability and preserve biochemical activity. However, the mechanisms by which this preservation is achieved are unclear. This issue of biopreservation is undoubtedly affected by both thermodynamic and dynamic parameters. The latter parameters will control the rate of conformational transitions of the protein that accompany biological activity. In the present communication we observe variations of local conformational motions of lysozyme in different solvents by using low-frequency Raman spectroscopy. We demonstrate that at low temperatures liquid glycerol provides a stronger suppression of the fast conformational motions of the protein than glassy trehalose. This demonstrates that solvent viscosity is not the only parameter that controls protein dynamics, and details of the protein–solvent interactions might be important in the biopreservation process.

Relaxation spectra in poly(methylmethacrylate): Comparison of acoustic attenuation and light scattering data

Relaxation processes in polymethylmethacrylate were investigated using light scattering and mechanical relaxation techniques. The acoustic attenuation data cover the frequency range from 1 Hz to 17 GHz and the light scattering from 0.5 to 10 000 GHz. Analysis of the data in a broad temperature range (80–350 K) shows that the model of the asymmetric double-well potentials, traditionally used for description of the acoustic attenuation in glasses, cannot describe all the data consistently. It is shown that assuming an additional relaxation in GHz region with a constant loss spectrum provides a good agreement of the model with experimental data.