The 3D Topography of Mitotic Chromosomes.

A long-standing conundrum is how mitotic chromosomes can compact, as required for clean separation to daughter cells, while maintaining close parallel alignment of sister chromatids. Pursuit of this question, by high resolution 3D fluorescence imaging of living and fixed mammalian cells, has led to three discoveries. First, we show that the structural axes of separated sister chromatids are linked by evenly spaced "mini-axis" bridges. Second, when chromosomes first emerge as discrete units, at prophase, they are organized as co-oriented sister linear loop arrays emanating from a conjoined axis. We show that this same basic organization persists throughout mitosis, without helical coiling. Third, from prophase onward, chromosomes are deformed into sequential arrays of half-helical segments of alternating handedness (perversions), accompanied by correlated kinks. These arrays fluctuate dynamically over <15 s timescales. Together these discoveries redefine the foundation for thinking about the evolution of mitotic chromosomes as they prepare for anaphase segregation.

Sister chromatids separate during anaphase in a three-stage program as directed by interaxis bridges.

During mitosis, from late prophase onward, sister chromatids are connected along their entire lengths by axis-linking chromatin/structure bridges. During prometaphase/metaphase, these bridges ensure that sister chromatids retain a parallel, paranemic relationship, without helical coiling, as they undergo compaction. Bridges must then be removed during anaphase. Motivated by these findings, the present study has further investigated the process of anaphase sister separation. Morphological and functional analyses of mammalian mitoses reveal a three-stage pathway in which interaxis bridges play a prominent role. First, sister chromatid axes globally separate in parallel along their lengths, with concomitant bridge elongation, due to intersister chromatin pushing forces. Sister chromatids then peel apart progressively from a centromere to telomere region(s), step-by-step. During this stage, poleward spindle forces dramatically elongate centromere-proximal bridges, which are then removed by a topoisomerase IIα­dependent step. Finally, in telomere regions, widely separated chromatids remain invisibly linked, presumably by catenation, with final separation during anaphase B. During this stage increased separation of poles and/or chromatin compaction appear to be the driving force(s). Cohesin cleavage licenses these events, likely by allowing bridges to respond to imposed forces. We propose that bridges are not simply removed during anaphase but, in addition, play an active role in ensuring smooth and synchronous microtubule-mediated sister separation. Bridges would thereby be the topological gatekeepers of sister chromatid relationships throughout all stages of mitosis.

One-dimensional spatial patterning along mitotic chromosomes: A mechanical basis for macroscopic morphogenesis.

Spatial patterns are ubiquitous in both physical and biological systems. We have recently discovered that mitotic chromosomes sequentially acquire two interesting morphological patterns along their structural axes [L. Chu et al., Mol. Cell, 10.1016/j.molcel.2020.07.002 (2020)]. First, axes of closely conjoined sister chromosomes acquire regular undulations comprising nearly planar arrays of sequential half-helices of similar size and alternating handedness, accompanied by periodic kinks. This pattern, which persists through all later stages, provides a case of the geometric form known as a "perversion." Next, as sister chromosomes become distinct parallel units, their individual axes become linked by bridges, which are themselves miniature axes. These bridges are dramatically evenly spaced. Together, these effects comprise a unique instance of spatial patterning in a subcellular biological system. We present evidence that axis undulations and bridge arrays arise by a single continuous mechanically promoted progression, driven by stress within the chromosome axes. We further suggest that, after sister individualization, this same stress also promotes chromosome compaction by rendering the axes susceptible to the requisite molecular remodeling. Thus, by this scenario, the continuous presence of mechanical stress within the chromosome axes could potentially underlie the entire morphogenetic chromosomal program. Direct analogies with meiotic chromosomes suggest that the same effects could underlie interactions between homologous chromosomes as required for gametogenesis. Possible mechanical bases for generation of axis stress and resultant deformations are discussed. Together, these findings provide a perspective on the macroscopic changes of organized chromosomes.

Intrinsic Chirality of CdSe/ZnS Quantum Dots and Quantum Rods.

A new class of chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of scientific endeavor. Normally the chirality in semiconductor nanocrystals is induced by the initial presence of chiral ligands/stabilizer molecules. Here we report intrinsic chirality of ZnS coated CdSe quantum dots (QDs) and quantum rods (QRs) stabilized by achiral ligands. As-prepared ensembles of these nanocrystals have been found to be a racemic mixture of d- and l-nanocrystals which also includes a portion of nonchiral nanocrystals and so in total the solution does not show a circular dichroism (CD) signal. We have developed a new enantioselective phase transfer technique to separate chiral nanocrystals using an appropriate chiral ligand and obtain optically active ensembles of CdSe/ZnS QDs and QRs. After enantioselective phase transfer, the nanocrystals isolated in organic phase, still capped with achiral ligands, now display circular dichroism (CD). We propose that the intrinsic chirality of CdSe/ZnS nanocrystals is caused by the presence of naturally occurring chiral defects.

Influence of intermolecular interactions on spectroscopic characteristics of metal nanoparticles and their composites.

In this paper we investigate the possibility to apply the concepts of non-specific intermolecular interactions and dispersive local field effect approach for study of the influence of interactions of metal nanoparticles with matrix molecules on the spectral characteristics of composites. The effect of intermolecular (interparticle) interactions and the influence of the dielectric environment on the peak position of the plasmon resonance band of colloidal solutions and thin films formed from noble metal nanostructures is determined. Simulated and experimental absorption spectra obtained for a colloidal solution of silver and gold nanoparticles, of various shapes and sizes in water and glycerol, are in good agreement.

Anisotropy of optical transitions in ordered ensemble of CdSe quantum rods.

We report on the observation of spectral dependence of absorption anisotropy in a CdSe quantum rod (QR) ensemble, which is aligned in a polymer film with a nanocrystal concentration of 2×10(-5) M. The experimental data on the polarization direction and anisotropy factor were obtained for the lowest excitonic transition and the second group of transitions in the QR. The nonzero constant value of anisotropy was investigated for the high-energy transitions, and is evidence of the one-dimensional confinement in the QR.

Bringing chiral functionality to in vivo applications of nanomaterials.

Chirality is a universal property of an endless number of objects in the universe. Nanotechnology is rapidly expanding to find ways to introduce chirality to artificial nanostructures. In a recent publication in Light: Science and Applications, Das et al. have successfully used capping with chiral ligand molecules to obtain chiral carbon dots. The authors provide a theoretical model to describe the origin of chirality in carbon dots as arising due to exciton coupling in a pair of chiral chromophores. Due to non-toxic chemical composition and sizes as small as 2-5 nm, the chiral carbon dots have the potential to outperform other chiral nanostructures in numerous biomedical applications. However, similarly to chiral drugs, their chiral toxicity must be well understood before the carbon dots are brought to living systems.

Single-Particle Studies Reveal a Nanoscale Mechanism for Elastic, Bright, and Repeatable ZnS:Mn Mechanoluminescence in a Low-Pressure Regime.

Mechanoluminescent materials, which emit light in response to elastic deformation, are demanded for use as in situ stress sensors. ZnS doped with Mn is known to exhibit one of the lowest reported thresholds for appearance of mechanoluminescence, with repeatable light emission under contact pressure <10 MPa. The physical basis for such behavior remains as yet unclear. Here, reliable microscopic detection of mechanoluminescence of single ZnS:Mn microparticles, in combination with nanoscale structural characterization, provides evidence that the mechanoluminescent properties of these particles result from interplay between a non-centrosymmetric crystal lattice and its defects, viz., dislocations and stacking faults. Statistical analysis of the distributions of mechanoluminescence energy release trajectories reveals two distinct mechanisms of excitation: one attributable to a piezo-phototronic effect and the other due to dislocation motion. At pressures below 8.1 MPa, both mechanisms contribute to mechanoluminescent output, with a dominant contribution from the piezo-phototronic mechanism. In contrast, above 8.1 MPa, dislocation motion is the primary excitation source. For the piezo-phototronic mechanism, we propose a specific model that accounts for elastic ZnS:Mn mechanoluminescence under very low pressure. The charged interfaces in stacking faults lead to the presence of filled traps, which otherwise would be empty in the absence of the built-in electric field. Upon application of external stress, local enhancement of the piezoelectric field at the stacking faults' interfaces facilitates release of the trapped carriers and subsequent luminescence. This field enhancement explains how <10 MPa pressure produces thousands of photons.

The chiral nano-world: chiroptically active quantum nanostructures.

Chirality is one of the key factors in molecular recognition, therefore the development of new chiral nanoparticles is of great interest to many fields of scientific endeavour including chemistry, biochemistry, pharmacology and medicine. Knowledge of the fundamental concepts relevant to chirality in nanosystems is also very important for further advancement of nanoscience and nanotechnology in general. Over the past years, the use of stereospecific chiral stabilising molecules has opened a new avenue to the area of nanocrystal research. In this review article we present some recent advances in the development of various chiroptically active quantum nanostructures and discuss the latest progress in various approaches for the preparation of these nanostructures. We also consider the intrinsic chirality in quantum nanostructures due to the presence of chiral defects such as screw dislocations and discuss the structure-property relationship. Furthermore, the corresponding potential applications of these chiral nanomaterials has been analysed for key areas: sensing, cytotoxicity mediation and cell imaging, asymmetric catalysis and enantiomeric separation, circular polarised light emitting devices and spintronics. Finally, we provide an outlook for the future development of chiroptically active quantum nanostructures.

Molecular Recognition of Biomolecules by Chiral CdSe Quantum Dots.

Molecular recognition is one of the most important phenomena in Chemistry and Biology. Here we present a new way of enantiomeric molecular recognition using intrinsically chiral semiconductor nanocrystals as assays. Real-time confocal microscopy studies supported by circular dichroism spectroscopy data and theoretical modelling indicate an ability of left-handed molecules of cysteine and, to a smaller extent, histidine and arginine to discriminate between surfaces of left- and right-handed nanocrystals.

Circular Dichroism of Electric-Field-Oriented CdSe/CdS Quantum Dots-in-Rods.

Here we report anisotropy of intrinsic chiroptical response in CdSe/CdS quantum dot-in-rod systems. These nanostructures being oriented in an external electric field demonstrate dependence of circular dichroism signal on the orientation of the nanocrystals. The type of circular polarization in these nanostructures correlates with preferential direction of linear polarization, and the degree of circular polarization is the maximal for the first circular dichroism band corresponding to the absorption band edge. We also support our experimental data with a theoretical model. Using this model, we show a direct connection between theoretically derived morphological parameters of twisting in nanocrystals lattices and calculated from experimental data parameters of circular dichroism anisotropy.

Preparation of chiral quantum dots.

Chiral quantum dots (QDs) are expected to have a range of potential applications in photocatalysis, as specific antibacterial and cytotoxic drug-delivery agents, in assays, as sensors in asymmetric synthesis and enantioseparation, and as fluorescent chiral nanoprobes in biomedical and analytical technologies. In this protocol, we present procedures for the synthesis of chiral optically active QD nanostructures and their quality control using spectroscopic studies and transmission electron microscopy imaging. We closely examine various synthetic routes for the preparation of chiral CdS, CdSe, CdTe and doped ZnS QDs, as well as of chiral CdS nanotetrapods. Most of these nanomaterials can be produced by a very fast (70 s) microwave-induced heating of the corresponding precursors in the presence of D- or L-chiral stabilizing coating ligands (stabilizers), which are crucial to generating optically active chiral QDs. Alternatively, chiral QDs can also be produced via the conventional hot injection technique, followed by a phase transfer in the presence of an appropriate chiral stabilizer. We demonstrate that the properties, structure and behavior of chiral QD nanostructures, as determined by various spectroscopic techniques, strongly depend on chiral stabilizers and that the chiral effects induced by them can be controlled via synthetic procedures.