Thermal hysteresis in microtubule assembly/disassembly dynamics: The aging-induced degradation of tubulin dimers.

The assembly/disassembly of biological macromolecules plays an important role in their biological functionalities. Although the dynamics of tubulin polymers and their super-assembly into microtubule structures is critical for many cellular processes, details of their cyclical polymerization/depolymerization are not fully understood. Here, we use a specially designed light scattering technique to continuously examine the effects of temperature cycling on the process of microtubule assembly/disassembly. We observe a thermal hysteresis loop during tubulin assembly/disassembly, consistently with earlier reports on the coexistence of tubulin and microtubules as a phase transition. In a cyclical process, the structural hysteresis has a kinetic component that depends on the rate of temperature change but also an intrinsic thermodynamic component that depends on the protein topology, possibly related to irreversible processes. Analyzing the evolution of such thermal hysteresis loops over successive cycles, we found that the assembly/disassembly ceases after some time, which is indicative of protein aging leading to its inability to self-assemble after a finite number of temperature cycles. The emergence of assembly-incompetent tubulin could have major consequences for human pathologies related to microtubules, including aging, neurodegenerative diseases and cancer.

Phase transitions of liposomes: when light meets heat.

Phase transitions of liposomes are normally studied by differential scanning calorimetry. A suspension of liposomes is subjected to an increase (decrease) of temperature and when heat is absorbed (released), the liposomes transit from a gel (liquid) to a liquid (gel) phase. This endothermic (exothermic) process takes place at a temperature called the melting temperatureTm, which is distinctive of the type of lipids forming the vesicles. The vesicles, though, also modify their size in the transition. Indeed, the thickness of the membranes decreases (increases) because carbon tails misalign (align). Concomitant with the modifications in the membrane thickness, the diameter (D) of the liposomes changes too. Therefore, when they are inspected by light, the scattered signal carries information from such dilatation (contraction) process. We performed careful experiments using dynamic light scattering as a function of temperature to detect the size changes of different liposomes. Gaussian fits of the derivatives of theDvsTcurves coincide within 1% with thermograms, which hints to the possibility of performing thermodynamic studies of lipid systems employing light.

Suspended LRSPP for the development of highly integrated active plasmonic devices.

We present a novel long-range surface plasmon polariton (LRSPP) device consisting of a suspended dielectric matrix in which an electrically active, millimeter-long metallic waveguide is embedded. We show that, by opening an air gap under the lower cladding, the influence of the substrate is suppressed and the symmetry of the thermo-optical distribution around the LRSPP waveguide is preserved over extended ranges of applied electrical current with minimal optical losses. Experimental results show that, compared to a standard nonsuspended structure, our device allows either the induction of a phase change that is three times larger, for a fixed electrical power, or, equivalently, a scaling down of the device to one-tenth of its original length, for a fixed phase change.

Probing complex dynamics with spatiotemporal coherence-gated DLS.

We discuss the specific features of fiber-based implementations of optical sensing techniques based on spatiotemporal coherence-gated dynamic light scattering (DLS). This sensing approach has a number of unique capabilities such as an effective isolation of single scattering, a large sensitivity, and high collection efficiency, and it can also operate over a wide range of optical regimes while providing means for proper ensemble averaging. We review a number of applications in which these specific characteristics permit recovering information beyond the capabilities of traditional light-scattering-based techniques.

Multimode interference dynamic light scattering.

When dealing with dynamic scattering systems, being able to collect strong signals while maintaining a high signal-to-noise ratio (SNR) is critical. It is well known that a spatially coherent measurement provides the largest SNR, while a partially coherent one provides better means for proper spatial averaging. In this Letter, we present a robust implementation of a fiber-based, single-mode, common-path interferometer assisted by multimode interference. We show that light can be efficiently collected from larger coherent regions while keeping a high SNR that is comparable to that of a pure single-mode arrangement. Additionally, our implementation allows having both a stable local oscillator encoding information on the fiber-medium interface and a linear dependence on scattering density. These two attributes, in turn, permit accessing the effective optical properties of the dynamic complex system.

Characterizing Viscoelastic Modulations in Biopolymer Hydrogels by Coherence-Gated Light Scattering.

pH-responsive hydrogels are of great interest for the controlled release of drugs. However, the changes in the structural and mechanical properties of hydrogels during the pH-responsive swelling/contraction process remains largely unknown. In this article, we demonstrate that coherence-gated dynamic light scattering can be used to in situ characterize the structural dynamics of chitosan (CS) hydrogels at different pH values and show that the CS hydrogels undergo viscoelastic modulations during the swelling/contraction/recovery process induced by pH changes. The conditions for the CS hydrogels to undergo these modulations are found by continuously monitoring the nonequilibrium, long-term dynamical process. Our findings are in a close correspondence to the macroscopic observations made at time points where the CS hydrogels are at equilibrium.

Stress homogenization effect in multicore fiber optic bending sensors.

In this work we study the particular case of an optical fiber subjected to compression-bending load, the most common loading configuration for testing fiber optic bending sensors. Our analysis is based on the foundations of column theory and reveals a progressive stress homogenization across the optical fiber with increasing bending. This effect is general to any optical fiber subjected to this load configuration and it is of particular interest for structures with multiple cores since the state of stress experienced by each core can significantly differ even for a condition of constant load. The approach outlined here captures relevant features observed in experiments with multicore fiber optic bending sensors. Also, this approach can be incorporated into coupled-mode theory for assessing the performance of spectrally operated fiber sensors based on multicore coupled structures under realistic conditions commonly encountered in the experiments and without the need of performing computationally expensive simulations. The progressive stress homogenization, as well as the regime of homogeneous stress dominated by the bending contribution, is experimentally demonstrated using a multicore optical fiber with three coupled cores. Our observations are similar to those reported in recent experiments using other multicore fibers with different number of cores.

In situ characterization of structural dynamics in swelling hydrogels.

Characterizing the structural morphology and the local viscoelastic properties of soft complex systems raises significant challenges. Here we introduce a dynamic light scattering method capable of in situ, continuous monitoring of structural changes in evolving systems such as swelling gels. We show that the inherently non-stationary dynamics of embedded probes can be followed using partially coherent radiation, which effectively isolates only single scattering contributions even during the dramatic changes in the scattering regime. Using a simple and robust experimental setup, we demonstrate the ability to continuously monitor the structural dynamics of chitosan hydrogels formed by the Ag(+) ion-triggered gelation during their long-term swelling process. We demonstrate that both the local viscoelastic properties of the suspending medium and an effective cage size experienced by diffusing probe particles loaded into the hydrogel can be recovered and used to describe the structural dynamics of hydrogels with different levels of cross-linking. This characterization capability is critical for defining and controlling the hydrogel performance in different biomedical applications.

In-fiber directional coupler for high-sensitivity curvature measurement.

A curvature fiber optic sensor using a two-core fiber (TCF) is proposed and demonstrated. The TCF is designed to operate as a directional coupler with one core located exactly at the center of the fiber and the other off-axis, but close to the center of the fiber. This design allows straightforward splicing of the TCF to single mode fibers (SMF), and alignment of the off-axis core is not strictly required for optimum operation. The sensor is fabricated by simply splicing a 5 cm long section of TCF between two SMF sections, which provides a sinusoidal spectral response. When the fiber is bent, the coupling parameters are modified due to stress-optic and effective length effects, effectively blue-shifting the sinusoidal spectral response of the sensor and allowing for the measurement of curvature. The sensor exhibits linear response and a sensitivity of -137.87 nm/m(-1) for curvature ranging from 0 to 0.27 m(-1), making it suitable to measure small curvatures with high sensitivity.