Restricted Rotational Motion Research Articles

Size-Dependent Magnetic Responsiveness of a Photonic Crystal of Graphene Oxide Nanosheets.

A magnetically responsive photonic crystal of colloidal nanosheets can exhibit a controllable structural color, offering diverse potential applications. In this study, we systematically investigated how the lateral sizes of graphene oxide (GO) nanosheets affect their magnetic responsiveness in a photonic system. Contrary to the prediction that larger lateral sizes of nanosheets would be more responsive to an applied magnetic field based on the magnetic energy of anisotropic materials, we discovered that GO nanosheets with larger lateral sizes in the photonic system scarcely responded to a 12 T magnetic field. The lack of magnetic response may be due to the strongly restricted rotational motion of GO nanosheets by mutual electrostatic forces. In contrast, GO nanosheets with medium lateral sizes readily responded to the 12 T magnetic field, forming a uniaxially oriented structure that resulted in a vivid structural color. However, smaller GO nanosheets displayed a less vivid structural color, possibly because of less structural ordering of GO nanosheets. Finally, we found that the photonic crystal of GO nanosheets with optimized lateral sizes responded effectively to the 12 T magnetic field across various GO concentrations, resulting in a vivid and tunable structural color.

Effects of stiffness-altered sport compression garments on lower-limb biomechanics in cutting maneuvers

Athletes commonly use compression garments (CGs) for perceived effectiveness in preventing injury occurrence. However, limited evidence is available on whether lower-limb CGs reduce the risk of injury. This study aimed at (1) evaluating the effects of CGs on mitigating the risk factors of cutting-related knee injuries; (2) identifying undesirable side-effects of CGs on other joints and cutting performance; and (3) identifying possible interactions between sex and condition. 62 healthy adults performed pre-planned 90˚ cutting tasks under four conditions: control, knee sleeves, placebo leggings and stiffness-altered leggings. Joint angle at initial contact, range of motion, moments, and ground reaction force were measured. A mixed two-way (sex*condition) ANOVA was performed, followed by post-hoc comparisons and subset analyses for sexes. Results showed that the leggings restricted hip sagittal (45.4 ± 1.3 vs. control 50.0 ± 1.3˚, p = 0.001) and rotational (16.8 ± 0.8 vs. control 22.5 ± 1.1˚, p < 0.001) motion. At initial contact, the stiffness-altered leggings reduced knee valgus (0.4 ± 0.8 vs. control −2.1 ± 0.8˚, p = 0.031). However, the altered alignment of lower-limb joints did not reduce multiplanar knee joint moments (p > 0.05). CGs were not effective protective equipment yet. There was no significant difference between knee sleeves and control, nor between leggings conditions (p > 0.05). Force plate measurements, such as increased rate of force development (stiffness-altered 42.6 ± 1.1 & placebo 42.9 ± 1.1 vs. control 39.9 ± 1.0 BW/s, p < 0.028), implied the possibility of performance enhancement through CGs. While further investigations on the optimal compression and stiffness alterations are warranted, athletes are recommended to be aware of the discrepancies between the claimed and actual biomechanical effects of CGs.

Dendrimeric calcium-sensitive MRI probes: the first low-field relaxometric study.

Different classes of small- or nano-sized calcium-sensitive probes for magnetic resonance imaging (MRI) have been proposed in the last two decades. These compounds have been developed mainly for functional MRI purposes and tested in vivo in different animal models. Most of them are paramagnetic systems that change their relaxivity in the presence of the divalent ion calcium, resulting in increased T1 or T2 contrast. In this work, we report the investigation of their relaxometric behavior at low magnetic fields, specifically the comparison of the monomeric Ca-sensitive probe and the corresponding dendrimer conjugates of generations 0, 1 and 2 (G0, G1 and G2, respectively). As a result, a relaxivity hump between 10 and 100 MHz of the Larmor frequency progressively appeared with an increase in the size of the investigated contrast agent, indicative of a restricted rotational motion of the complexes as long as the size of the molecule increases. The same trend with a more pronounced effect was detectable in the presence of calcium. The relaxivity enhancement for the Ca2+ adducts, primarily caused by an increase of the hydration state of Gd3+, went from ca. 130% for the monomeric probe to ca. 310% for the G2 dendrimer conjugate at 0.5 T and 25 °C. T1 weighted magnetic resonance images acquired at 1 T displayed the strong ability of these systems to change their contrast according to the presence of calcium at this field, thus laying the basis for promising future in vivo applications.

Matrix Isolation Spectroscopy: Aqueous p-Toluenesulfonic Acid Solvation.

Interaction between p-toluenesulfonic acid (pTSA) and water is studied at -20 °C in a CCl4 matrix. In CCl4 water exists as monomers with restricted rotational motion about its symmetry axis. Additionally, CCl4 is transparent in the hydrogen-bonded region; CCl4 thus constitutes an excellent ambient thermal energy matrix isolation medium for diagnosing interactions with water. Introducing pTSA-nH2O gives rise to two narrow resonances at 3642 cm-1 and at 2835 cm-1 plus a broad 3000-3550 cm-1 absorption. In addition, negative monomer symmetric and asymmetric stretch features relative to nominally dry CCl4 indicate that fewer water monomers exist in the cooled (-20 °C) acid solution than in room-temperature anhydrous CCl4. The negative peaks along with the broad absorption band indicate that water monomers are incorporated into clusters. The 3642 cm-1 resonance is assigned to the OH-π interaction with a cluster containing many water molecules per acid molecule. The 2835 cm-1 resonance is assigned to the (S-)O-H stretch of pTSA-dihydrate. The coexistence of these two species provides insights into interactions in this acid-water CCl4 system.

Reduced stress‐optical coefficient of polycarbonate by antiplasticization

ABSTRACTWe have investigated the effect of antiplasticization on the stress‐optical behavior of polycarbonate (PC) containing terphenyls (tPh) and di(2‐ethylhexyl)adipate (DEHA). Addition of the three tPhs (p‐, o‐, and m‐tPh) and DEHA at contents of 5–10 wt % increases the tensile storage modulus (E') of PC owing to the antiplasticization effect. In particular, p‐tPh increases E' more than the other additives, suggesting that the rod‐like shape matches the free volume of PC in the glassy state. The three tPh isomers improve the glassy birefringence of PC while DEHA does not change the glassy birefringence, which corresponds to the polarizability anisotropy. The stress‐optical coefficient, a ratio of stress and birefringence, of PC decreases with increasing additive content in order of p‐tPh ≫ o‐tPh &gt; m‐tPh = DEHA. This result is agreement with a restricted rotational motion of additive molecule in PC, which is observed in dynamic mechanical and birefringence data. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1837–1842

Molecular Dynamics of a Hydrated Collagen Peptide: Insights into Rotational Motion and Residence Times of Single-Water Bridges in Collagen.

Magnetic resonance transverse spin relaxation time constants (T2) of water protons in ordered collagenous tissues are dependent on the orientation of the tissue relative to the static magnetic field. This dependence is commonly referred to as the magic angle (MA) effect and has been attributed to the restricted rotational motion of icelike water bridges in the hydrated triple-helix collagen molecule. Understanding of the molecular mechanism of the MA effect is important for clinical and research applications of magnetic resonance spectroscopy and imaging to tissues, such as articular cartilage, tendons, and ligaments. In this work, we have used molecular dynamics simulations to investigate the subnanosecond time scale dynamics of single-water bridges in a model collagen peptide. We ascertain the residence times and the patterns of restricted rotational motion of water molecules. The key findings are strongly anisotropic rotation patterns of water molecules at bridge sites and a dynamic, rather than icelike, nature of the single-water bridges within the individual triple-helix collagen molecule.

Spin relaxation of radicals in cryptochrome and its role in avian magnetoreception.

Long-lived spin coherence and rotationally ordered radical pairs have previously been identified as key requirements for the radical pair mechanism of the avian magnetic compass sense. Both criteria are hard to meet in a biological environment, where thermal motion of the radicals creates dynamic disorder and drives efficient spin relaxation. This has long been cited as a major stumbling block of the radical pair hypothesis. Here we combine Redfield relaxation theory with analytical solutions to a rotational diffusion equation to assess the impact of restricted rotational motion of the radicals on the operation of the compass. The effects of such motions are first investigated generally in small, model systems and are then critically examined in the magnetically sensitive flavin-tryptophan radical pair that is formed photochemically in the proposed magnetoreceptor protein, cryptochrome. We conclude that relaxation is slowest when rotational motion of the radicals within the protein is fast and highly constrained; that in a regime of slow relaxation, the motional averaging of hyperfine interactions has the potential to improve the sensitivity of the compass; and that consideration of motional effects can significantly alter the design criteria for an optimal compass. In addition, we demonstrate that motion of the flavin radical is likely to be compatible with its role as a component of a functioning radical-pair compass, whereas the motion of the tryptophan radical is less ideal, unless it is particularly fast.

(Invited) How Spatial Heterogeneities Affect Dynamics of Ionic Liquids

Despite that the heterogeneous structure of ionic liquids (ILs) has been established, the consequences of the nanoscopic segregation on the dynamics of ILs has remained elusive. In particular, the spatial heterogeneity in ILs may result in inherently different local solute environments when using ILs as solvents. Such different local interactions will affect the ability to re-structure and dissipate excess energy during a chemical reaction, which determines the performance as a solvent. We study these solvent dynamics using femtosecond infrared spectroscopy for a series of alkylammonium ionic liquids [1, 2]. In these experiments vibrational excitation of the ND or NH stretching vibrations results in an ultrafast, local temperature jump and we monitor the subsequent equilibration as a response to the dissipated energy. Owing to the intrinsic molecular specificity of infrared vibrations we can thus probe specifically the different moieties of the ionic liquid’s ions, providing access to the intrinsic solvation dynamics in the absence of any probe molecules. Our results show that ionic liquids are not only structurally heterogeneous, but exhibit a substantial dynamic heterogeneity, with the equilibration within the hydrophobic domains being two-fold faster than in the ionic domains. The excitation anisotropy, which provides information on the local rotational mobility of the ionic fragments of the cations, is rather insensitive to the size of the cations: Our results even suggest that increasing the hydrophobic domain size goes along with an acceleration of the restricted rotational motion of the ammonium group of the cationic fragments of the IL’s ions. Such accelerated dynamics are indicative of reduced directional interionic interactions, which in turn signify a more fragile three-dimensional structure as the size of the hydrophobic fragments increases. Our study shows that local dynamics in these ionic liquids are decoupled from macroscopic properties, like e.g. viscosity. Thus, our findings underline that the structural heterogeneity of ILs makes local dynamics heterogeneous, with substantially different solvation properties within the hydrophobic and the ionic sub-domains. [1] J. Hunger, T. Sonnleitner, L Liu, R. Buchner, M. Bonn and H.J. Bakker, „Hydrogen-Bond Dynamics in a Protic Ionic Liquid: Evidence of Large-Angle Jumps“ J. Phys. Chem. Lett. 3, 3034 (2012). [2] Z.P. Zheng, W.H. Fan, S. Roy, K. Mazur, A. Nazet, R. Buchner, M. Bonn, and J. Hunger “Ionic Liquids: Not only Structurally but also Dynamically Heterogeneous”, Angew. Chem. Int. Ed. 54 , 687 (2015).

Molecular dynamics study of rhodamine 6G diffusion atn-decane–water interfaces

Two-dimensional diffusion of a rhodamine 6G fluorescent tracer molecule at the n-decane-water interface was studied with all-atom molecular dynamics simulations. In agreement with experimental data, we find increased mobility of the tracer at the n-decane-water interfaces in comparison to its mobility in bulk water. Orientational ordering of water and n-decane molecules near the interface is observed, and may change the interfacial viscosity as suggested to explain the experimental data. However, the restricted rotational motion of the rhodamine molecule at the interface suggests that the Saffman-Delbrück model may be a more appropriate approximation of rhodamine diffusion at n-decane-water interfaces, and, without any decrease in interfacial viscosity, suggests faster diffusion consistent with both experimental and simulation values.

Stimuli-sensitive breathing of Cucurbit[7]uril cavity: monitoring through the environment responsive fluorescence of 1'-hydroxy-2'-acetonaphthone (HAN).

In this work, we have focused on the supramolecular interactions of a water-soluble host Cucurbit[7]uril (CB[7]) with an excited state intramolecular proton transfer (ESIPT) probe 1'-hydroxy-2'-acetonaphthone (HAN) through steady-state and time-resolved fluorescence measurements. In water HAN is almost nonfluorescent in nature with a very low fluorescence quantum yield (Φ = 0.009). With gradual addition of CB[7] absorption maximum of HAN is red-shifted (292 cm(-1)) and hence confirming the formation of an inclusion complex in the ground state between HAN and CB[7]. Due to this complexation CB[7] offers a hydrophobic microenvironment to the HAN which is completely different from that of homogeneous water. Upon encapsulation into the nanocavity of CB[7], HAN exhibits a 20-fold increase in fluorescence intensity along with a 36 nm (1618 cm(-1)) hypsochromic shift in emission maxima. This hypsochromic shift is an indication about the modulation of excited state photophysical behavior of HAN due to the formation of HAN-CB[7] inclusion complex. Moreover, huge partition coefficient of HAN from water to CB[7] along with a∼ 12-fold increase in fluorescence lifetime confirm the favorable interaction between HAN and CB[7]. We have also observed the stimuli-sensitive (temperature and cationic stimuli) breathing of CB[7] cavity i.e., in the presence of different additives the portals of CB[7] open up to release HAN in water and take up the additives. Time-resolved anisotropy measurements further indicate about the probable location of HAN inside the CB[7]. The observation of a 1.7 ns component in the presence of CB[7] signifies the highly restricted rotational motion of HAN inside the cavity of CB[7] corroborates our finding.

Molecular mobility of scaffolds' biopolymers influences cell growth.

Understanding biocompatibility of materials and scaffolds is one of the main challenges in the field of tissue engineering and regeneration. The complex nature of cell-biomaterial interaction requires extensive preclinical functionality testing by studying specific cell responses to different biomaterial properties, from morphology and mechanics to surface characteristics at the molecular level. Despite constant improvements, a more general picture of biocompatibility is still lacking and tailormade scaffolds are not yet available. The scope of our study was thus the investigation of the correlation of fibroblast cell growth on different gelatin scaffolds with their morphological, mechanical as well as surface molecular properties. The latter were thoroughly investigated via polymer molecular mobility studied by site-directed spin labeling and electron paramagnetic resonance spectroscopy (EPR) for the first time. Anisotropy of the rotational motion of the gelatin side chain mobility was identified as the most correlated quantity with cell growth in the first days after adhesion, while weaker correlations were found with scaffold viscoelasticity and no correlations with scaffold morphology. Namely, the scaffolds with highly mobile or unrestricted polymers identified with the cell growth being five times less efficient (N(cells) = 60 ± 25 mm(-2)) as compared to cell growth on the scaffolds with considerable part of polymers with the restricted rotational motion (N(cells) = 290 ± 25 mm(-2)). This suggests that molecular mobility of scaffold components could play an important role in cell response to medical devices, reflecting a new aspect of the biocompatibility concept.

Rotational ratchets with dipolar interactions

We report results from a computer simulation study on the rotational ratchet effect in systems of magnetic particles interacting via dipolar interactions. The ratchet effect consists of directed rotations of the particles in an oscillating magnetic field, which lacks a net rotating component. Our investigations are based on Brownian dynamics simulations of such many-particle systems. We investigate the influence of both the random and deterministic contributions to the equations of motion on the ratchet effect. As a main result, we show that dipolar interactions can have an enhancing as well as a dampening effect on the ratchet behavior depending on the dipolar coupling strength of the system under consideration. The enhancement is shown to be caused by an increase in the effective field on a particle generated by neighboring magnetic particles, while the dampening is due to restricted rotational motion in the effective field. Moreover, we find a nontrivial influence of the short-range, repulsive interaction between the particles.

Cation and anion dynamics in supercooled and glassy states of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate: Results from13C,31P, and19F NMR spectroscopy

The rotational dynamics of cations and anions in the room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C${}_{4}$mim]PF${}_{6}$) have been investigated in the supercooled liquid and glassy states using ${}^{13}$C, ${}^{31}$P, and ${}^{19}$F NMR spectroscopy. The \ensuremath{\alpha}-relaxation process of the supercooled liquid corresponds well with that of the isotropic rotational reorientation of the [C${}_{4}$mim]${}^{+}$ cations. The timescale of the reorientational motion of the butyl chain in the cations, which is reminiscent of the conformational isomerization, is found to be slower than that of the imidazolium ring. This counterintuitive result can be attributed to the presence of local structures in the form of polar and nonpolar nanodomains in the liquid and significant steric and coulombic interactions between the rings or chains in the cations and such domains. On the other hand, the dynamics of the constituent PF${}_{6}$${}^{\ensuremath{-}}$ anions is dominated by free rotational diffusion at temperatures above \ensuremath{\sim}230 K, while a restricted rotational or librational motion dominates at lower temperatures. This transition temperature can be identified with the mode coupling critical temperature ${T}_{\mathrm{c}}$ where the anion rotational timescale decouples from that of the [C${}_{4}$mim]${}^{+}$ cations. The librational motion of the anions has a characteristic timescale on the order of 10${}^{\ensuremath{-}10}$ s with an activation energy of \ensuremath{\sim}0.16 eV typical of a \ensuremath{\beta}-relaxation process. This dynamical process continues below ${T}_{\mathrm{g}}$, well into the glassy state of this ionic liquid.

Fractional protein dynamics seen by nuclear magnetic resonance spectroscopy: Relating molecular dynamics simulation and experiment

We propose a fractional Brownian dynamics model for time correlation functions characterizing the internal dynamics of proteins probed by NMR relaxation spectroscopy. The time correlation functions are represented by a broad distribution of exponential functions which are characterized by two parameters. We show that the model describes well the restricted rotational motion of N-H vectors in the amide groups of lysozyme obtained from molecular dynamics simulation and that reliable predictions of experimental relaxation rates can be obtained on that basis.

Ions and Hydrogen Bonding in a Hydrophobic Environment: CCl4

It is generally expected that ions in an aqueous ionic solution in contact with a hydrophobic phase enter the hydrophobic phase accompanied by a hydration shell. This expectation suggests that the ion mole fraction in the hydrophobic phase is less than, or at most, equal to that of water. Both gravimetric and spectroscopic evidence shows that for a model hydrophobic phase, carbon tetrachloride, this is not the case: In contact with a 1 M simple salt solution (sodium or potassium halide), the salt concentration in carbon tetrachloride ranges from 1.4 to nearly 3 times that of water. Infrared spectra of the OH stretch region support a model in which water associates with the cation, primarily as water monomers. Salts containing larger, more polarizable anions can form outer-sphere ion pairs that support water dimers, giving rise to a spectral signature at 3440 cm(-1). In CCl(4), the infrared spectral signature of the normally strongly ionized acid HCl clearly shows the presence of molecular HCl. Additionally, the presence of a Q branch for HCl indicates restricted rotational motion. The spectral and gravimetric data provide compelling evidence for ion clusters in the hydrophobic phase, which is a result that may have implications for hydrophobic matter in both biological and environmental systems.

Single Molecule Spectroscopy Reveals Heterogeneous Transport Mechanisms for Molecular Ions in a Polyelectrolyte Polymer Brush

Single molecule polarization and fluorescence correlation spectroscopy were used to evaluate heterogeneous transport mechanisms of molecular ions within supported polyelectrolyte brushes. Modes of diffusive transport include periods of significantly restricted rotational motion, often maintained over tens of milliseconds; periods of fast molecular rotation; and occasional adsorption of fluorescent probe molecules in the brush. The studies reveal rapid switching between orientational states during each observed mode of motion. Through quantitative analysis of state occupation times, the rate constants for transitions from weakly associated to strongly associated states were extracted. Additionally, the pH dependence of the ion transport rates in the brush exhibits an abrupt, rather than continuous, trend. These single molecule studies demonstrate the presence of dynamic anisotropic interactions between the charged molecular probe and the polymer brush and provide experimental evidence of stimuli responsive switchable transport functionality in the polyelectrolyte brush.

Reply to comment on ‘aggregation-induced phosphorescent emission (AIPE) of iridium(iii) complexes’: origin of the enhanced phosphorescence

An obvious enhanced phosphorescence emission in the solid state (EPESS) was observed for two iridium(III) complexes with restricted (even fixed) intramolecular rotational motion, which is contrary to the opinion ("restricted intramolecular rotational motion as the origin of EPESS") of Park et al., and our observation confirmed that the pi-pi stacking of adjacent pyridyl rings of ppy ligands played a key role in EPESS.

Ultrafast resonance energy transfer from a site-specifically attached fluorescent chromophore reveals the folding of the N-terminal domain of CP29

The photosynthetic minor antenna complex CP29 of higher plants was singly mutated, overexpressed in Escherichia coli, selectively labeled with the fluorescent dye TAMRA at three positions in the N-terminal domain, and reconstituted with its natural pigments. Picosecond fluorescence experiments revealed rapid excitation energy transfer (∼20 ps) from TAMRA covalently attached to a cysteine at either position 4 or 97 (near the beginning and end of the N-terminal domain) to the chlorophylls in the hydrophobic part of the protein. This indicates that the N-terminus is folded back on the hydrophobic core. In 20% of the complexes, efficient transfer was lacking, indicating that the N-terminus can adopt different conformations. Time-resolved polarized fluorescence measurements demonstrate that the non-transferring conformations only allow restricted rotational motion of the dye molecule. When TAMRA was attached to a cysteine at position 40, the overall transfer efficiency was far lower, reflecting a larger distance to the hydrophobic region.

Comparison of the Fluorescence Characteristics of Dye Molecules in Liquid Crystal and Conventional Solvents

We investigated the uorescence characteristics of hemicyanine dye molecules in conventional solvents and in the isotropic phase of liquid-crystal 4-pentyl-4'-cyanobiphenyl. Fluorescence decay lifetime of hemicyanine in liquid crystal decreased monotonically as the temperature was increased. As compared to glycerol, the uorescence decay lifetime of hemicyanine in the isotropic phase of liquid crystal was appreciably longer. This longer decay lifetime of hemicyanine in liquid crystal can be explained by a restricted internal rotational motion of the hemicyanine molecule due to strong interaction between the dye molecule and the liquid crystal molecules as compared to other solvents.

Segmental dynamics in nanophase separated comb-like polymers with long side chains

Poly(α-n-alkyl β-l-aspartate)s, PnAALAs, are comb-like polymers with rod-like helical conformations which present crystallinity for long alkyl side chains (n>10). Dielectric spectroscopy experiments for PnAALAs (10⩽n⩽18) show the existence at low temperatures of two secondary modes corresponding to localized motions of the carbonyl group. At higher temperatures two cooperative mobilities coexist. The first corresponds to the segmental mobility of the amorphous side chains and has the characteristic of a polyethylene-like dynamic glass transition common to various series of nanophase segregated side chain polymers with different morphologies. A second segmental mode is attributed to the restricted rotational motion of the rigid rods which is hindered when the spacing between rods decreases. An extension to the results found for other polymer series with shorter lateral chains is presented, showing the tendency to approximate amorphous polyethylene dynamics in these confined nanodomains of increasing sizes.