Several kinds of small molecules and nanoparticles (NP) are known for effective inhibition of amyloid fibrillation, which is known as a precursor for many neurodegenerative diseases. We address the role of the surface charge of NP in this process from a systematic study using charged NP and surfactants. The fibrillation kinetics is investigated using time-resolved Thioflavin T fluorescence and transmission electron microscopy. It is found that if the protein residues corresponding to the β-sheet (key secondary structure for the formation of fibrils) are charged, addition of oppositely charged NP will inhibit the fibrillation process, irrespective of the material composition of the NP. Molecular dynamics simulations show that electrostatic interaction between the β-sheet forming residues and the NP is responsible for this effect. The finding is further validated by using oppositely charged surfactants: like-charged surfactants and NP do not affect the fibrillation kinetics. Further, even the preformed fibrils are dissociated into protein aggregates upon adding oppositely charged NP/surfactants. These results suggest that NP can be used as therapeutic agents even after the fibrils are formed and that they can be dissolved into soluble aggregates. The inhibitory effect is essentially decided by the charge of the inhibitor, irrespective of the size, shape, and material composition. The study provides new insight for the rational design of nanoparticle-based therapeutics, with appropriate surface charge, to inhibit the onset of amyloid β fibrillation and to dissociate the preformed fibrils.

Kinesin motors are essential for the transport of cellular cargo along microtubules. How the motors step, detach, and cooperate with each other is still unclear. To dissect the molecular motion of kinesin-1, we developed germanium nanospheres as ultraresolution optical trapping probes. We found that single motors took 4-nanometer center-of-mass steps. Furthermore, kinesin-1 never detached from microtubules under hindering load conditions. Instead, it slipped on microtubules in microsecond-long, 8-nanometer steps and remained in this slip state before detaching or reengaging in directed motion. Unexpectedly, reengagement and thus rescue of directed motion was more frequent. Our observations broaden our knowledge on the mechanochemical cycle and slip state of kinesin. This state and rescue need to be accounted for to understand long-range transport by teams of motors.

Biocompatible and colloidally stable gold nanorods (GNRs) with well-defined plasmonic properties are essential for biomedical and theranostic applications. The as-synthesized GNRs using the seed-mediated method are stabilized by the surfactant, cetyltrimethylammonium bromide (CTAB), which is known for its cytotoxicity in many cell lines. Biocompatible GNRs synthesized using known protocols exhibit some extent of cytotoxicity and colloidal instability because of the incomplete removal of CTAB. We report a facile method for the efficient removal of CTAB molecules with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) phospholipid molecules, which are naturally present in cell membranes. The kinetics of the ligand exchange process is studied using surface-enhanced Raman scattering (SERS) and corroborated with matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. From colloidal stability studies using dynamic light scattering (DLS) and UV-Vis spectroscopy, the optimal lipid concentration and duration required for the successful ligand exchange of CTAB by DMPC are reported. Using thermogravimetric analysis, the surface concentration of DMPC on colloidally stable GNRs is found to be approximately 9 molecules per nm. The 3-(4,5-dimethylthiozol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and lactate dehydrogenase (LDH) assays show that the surface-modified DMPC-GNRs have significantly better biocompatibility than those of CTAB-GNRs. Studies on the ligand exchange, colloidal stability and biocompatibility of DMPC-GNRs with aspect ratios ranging from 2.2 to 4.2 demonstrate the robustness of the proposed method. The results provide insights into the important factors to be considered while designing biocompatible GNRs suitable for applications in nanomedicine.

Extracellular plaques of amyloid beta (Aβ) fibrils and neurofibrillary tangles are known to be associated with neurological diseases such as Alzheimer's disease. Studies have shown that spherical nanoparticles inhibit the formation of Aβ fibrils by intercepting the nucleation and growth pathways of fibrillation. In this report, gold nanorods (AuNRs) are used to inhibit the formation of Aβ fibrils and the shape-dependent plasmonic properties of AuNRs are exploited to faciliate faster dissolution of mature Aβ fibrils. Negatively charged, lipid (DMPC) stabilized AuNRs inhibit the formation of fibrils due to selective binding to the positevly charged amyloidogenic sequence of Aβ protein. The kinetics of inhibition is characterized by thioflavin T (ThT) fluorescence, transmission electronic microscopy (TEM), atomic force microscopy (AFM), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). An increase in the aspect ratio of DMPC-AuNR in the range of 2.2-4.2 decreased the fibrils content proportionally. Further, the fibrils content is decreased by increasing the concentration of AuNR for all aspect ratios. As AuNR absorb near-infrared (NIR) light and creates a localized hotspot, NIR laser (800 nm) is applied for 2 min to facilitate the thermal dissolution of mature Aβ fibrils. Majority of Aβ fibrils are disintegrated into smaller fragments after exposure to NIR in the presence of AuNR. Thus, the DMPC-AuNRs exhibit a dual effect: inhibition of fibrillation and NIR laser facilitated dissolution of mature amyloid fibrils. This study essentially provides guidelines to design efficient nanoparticle-based therapeutics for neurodegenerative diseases.

Force spectroscopy on single molecular machines generating piconewton forces is often performed using optical tweezers.1–3 Since optical forces scale with the trapped particle volume, piconewton force measurements require micron-sized probes practically limiting the spatiotemporal resolution.1,2,4,5 Here, we have overcome this limit by developing high-refractive index germanium nanospheres as ultraresolution trapping probes. Using these probes, we have dissected the molecular motion of the cytoskeletal motor kinesin-1 that transports vesicles along microtubule filaments. With a superior spatiotemporal resolution, we have resolved a controversy unifying its stepping and detachment behavior. We found that single motors took 4-nm-center-of-mass steps with alternating force dependence of their dwell times. At maximum force, motors did not detach but switched to a weakly bound state. In this state, motors slid on the microtubule with 8-nm steps on a microsecond time scale. Kinesins remained in this intermediate slip state before either truly detaching or reengaging in directed motion. Surprisingly, reengagement and, thus, rescue of directed motion occurred in about 80 percent of events. Such rescue events suggest that macroscopically observed run lengths of individual motors are concatenations and rescues need to be accounted for to understand long-range transport. Furthermore, teams of motors involved in transport may be synchronized through the weakly bound slip state. Apart from ultraresolution optical trapping, germanium nanospheres are promising candidates for applications ranging from nanophotonics to energy storage.