Biomedical subjects

Youli Li

Publications and source records attributed to Youli Li.

5 recordsLinked to original sources

PubMed2006

Jumbo squid beaks: inspiration for design of robust organic composites.

The hard tissues found in some invertebrate marine organisms represent intriguing paradigms for robust, lightweight materials. The present study focuses on one such tissue: that comprising the beak of the jumbo squid (Dosidicus gigas). Its main constituents are chitin fibers (15-20wt.%) and histidine- and glycine-rich proteins (40-45%). Notably absent are mineral phases, metals and halogens. Despite being fully organic, beak hardness and stiffness are at least twice those of the most competitive synthetic organic materials (notably engineering polymers) and comparable to those of Glycera and Nereis jaws. Furthermore, the combination of hardness and stiffness makes the beaks more resistant to plastic deformation when in contact with blunt abrasives than virtually all metals and polymers. The 3,4-dihydroxy-l-phenylalanine and abundant histidine content in the beak proteins as well as the pigmented hydrolysis-resistant residue are suggestive of aromatic cross-linking. A high cross-linking density between the proteins and chitin may be the single most important determinant of hardness and stiffness in the beak. Beak microstructure is characterized by a lamellar arrangement of the constituents, with a weak interface that promotes crack deflection and endows the structure with high fracture toughness. The susceptibility of this microstructure to cracking along these interfaces from contact stresses at the external surface is mitigated by the presence of a protective coating.

Animals↗

PubMed2006

Microtubule protofilament number is modulated in a stepwise fashion by the charge density of an enveloping layer.

Microtubules are able to adjust their protofilament (PF) number and, as a consequence, their dynamics and function, to the assembly conditions and presence of cofactors. However, the principle behind such variations is poorly understood. Using synchrotron x-ray scattering and transmission electron microscopy, we studied how charged membranes, which under certain conditions can envelop pre-assembled MTs, regulate the PF number of those MTs. We show that the mean PF number, , is modulated primarily by the charge density of the membranes. decreases in a stepwise fashion with increasing membrane charge density. does not depend on the membrane-protein stoichiometry or the solution ionic strength. We studied the effect of taxol and found that increases logarithmically with taxol/tubulin stoichiometry. We present a theoretical model, which by balancing the electrostatic and elastic interactions in the system accounts for the trends in our findings and reveals an effective MT bending stiffness of order 10-100 k(B)T/nm, associated with the observed changes in PF number.

Animals↗

PubMed2005

Cationic liposome-microtubule complexes: pathways to the formation of two-state lipid-protein nanotubes with open or closed ends.

Intermolecular interactions between charged membranes and biological polyelectrolytes, tuned by physical parameters, which include the membrane charge density and bending rigidity, the membrane spontaneous curvature, the biopolymer curvature, and the overall charge of the complex, lead to distinct structures and morphologies. The self-assembly of cationic liposome-microtubule (MT) complexes was studied, using synchrotron x-ray scattering and electron microscopy. Vesicles were found to either adsorb onto MTs, forming a "beads on a rod" structure, or undergo a wetting transition and coating the MT. Tubulin oligomers then coat the external lipid layer, forming a tunable lipid-protein nanotube. The beads on a rod structure is a kinetically trapped state. The energy barrier between the states depends on the membrane bending rigidity and charge density. By controlling the cationic lipid/tubulin stoichiometry it is possible to switch between two states of nanotubes with either open ends or closed ends with lipid caps, a process that forms the basis for controlled chemical and drug encapsulation and release.

Cytoplasmic Vesicles↗

PubMed2004

Ordered patterns of liquid crystal toroidal defects by microchannel confinement.

In this article we present experimental results demonstrating an approach to controlling the size and spatial patterning of defect domains in a smectic liquid crystal (LC) by geometric confinement in surface-modified microchannels. By confining the LC 4'-octyl-4-cyanobiphenyl in mum-sized rectangular channels with controlled surface polarity, we were able to generate defect domains that are not only nearly uniform in size but also arranged in quasi-2D ordered patterns. Atomic force microscopy measurements revealed that the defects have a toroidal topology, which we argue is dictated by the boundary conditions imposed by the walls of the microchannel. We show that the defects can be considered to be colloidal objects, which interact with each other to form ordered patterns. This method opens the possibility for exploiting the unique optical and rheological properties associated with LC defects to making new materials. For example, the control of the shape, size, and spatial arrangement of the defects at the mesoscale suggests applications in patterning, templating, and when extended to lyotropic LCs, a process leading to uniform-sized spherical particles for chemical encapsulation and delivery.

Biphenyl Compounds↗

PubMed2003

Lamellar phase of stacked two-dimensional rafts of actin filaments.

We examined liquid crystalline phases of the cytoskeletal polyelectrolyte filamentous (F-)actin in the presence of multivalent counterions. As a function of increasing ion concentration, the F-actin rods in either an isotropic or a nematic phase will transform into a new and unexpected lamellar phase of cross-linked rafts (L(XR) phase), before condensing into a bundled phase of parallel, close-packed rods. This behavior is generic for alkali earth divalent ions Mg2+, Ca2+, Sr2+, and Ba2+, and the structural transitions are achieved without any architecture-specific actin-binding linker proteins.

Actins↗