We present a technique for the directed assembly and self-assembly of micrometer-scale structures based on the control of specific DNA linkages between colloidal particles. The use of DNA links combined with polymer brushes provides an effective way to regulate the range and magnitude of addressable forces between pairs (and further combinations) of different particles. We demonstrate that the autoassembly of alternate microbeads as well as their directed assembly, by using laser tweezers, is reversible. The key to reversibility is preventing the particles from falling into their van der Waals well at close distances. This goal is achieved by the use of adsorbed polymers that limit the number of DNA bridges to one to three between adjacent particles.DNA links ͉ reversible aggregation S tudies of reversible and specific adhesion between colloids are an important step toward understanding various phenomena involving molecular recognition. For instance, they are relevant in the study of cell adhesion (1), cell migration (2), or cell sorting during embryonic development (3). The knowledge and the control of the interplay between nonspecific repulsion and molecular recognition is also fundamental for biotechnological device improvements; e.g., a strategy to improve latex agglutination tests is to reduce aggregation due to nonspecific interactions (4). The present work on DNA links is also a contribution to these more general studies.Controlling and tuning interactions between particles has always been a relevant challenge both experimentally (5, 6) and theoretically (7-11). For example, Tkachenko (11) predicted diverse and unusual crystal morphologies assuming a reversible contact between particles in a binary system of colloids, in which identical particles experience repulsive interactions and differing particles experience attractive ones. Particularly, he predicted a selfassembled diamond lattice structure that would be especially relevant for photonic crystal building. His work was inspired by the work by Mirkin et al. (12), who first used DNA chains as linkers between nanoparticles to build a reversible DNA-mediated assembly of gold nanoparticles. The specificity and magnitude of the attraction is determined by the molecular recognition of complementary DNA strands and the sensitivity of hybridization to solution conditions and temperature. Experimental work involving DNA as a linker between particles has up to now mainly focused on nanosized particles (13-18). To our knowledge, only two studies (19,20) have been reported in the literature with microsized particles, but in both of them the assembly process was not reversible, with the DNA acting as a molecular bridge between the entities of a binary mixture. In this study, we focus on the reversibility of the aggregation process between microsized particles. The specificity and reversibility are proof that the interactions between the colloids are controlled by DNA and thus can be tuned. Materials and MethodsSample Preparation. DNA-functionalized polystyrene mi...

Summary Various rod-shaped bacteria mysteriously glide on surfaces in the absence of appendages such as flagella or pili. In the deltaproteobacterium Myxococcus xanthus, a putative gliding motility machinery (Agl–Glt) localizes to so-called Focal Adhesion sites (FA) that form stationary contact points with the underlying surface. We discovered that the Agl–Glt machinery contains an inner-membrane motor complex that moves intracellularly along a right-handed helical path, and when it becomes stationary at FA sites, it powers a left-handed rotation of the cell around its long axis. At FA sites, force transmission requires cyclic interactions between the molecular motor and adhesion proteins of the outer membrane via a periplasmic interaction platform, which presumably involves a contractile activity of motor components and possible interactions with the peptidoglycan. This work provides the first molecular model for bacterial gliding motility.

We have linked the structural and dynamic properties in aqueous solution of amphiphilic charged diblock copolymers poly(butyl acrylate)-b-poly(acrylic acid), PBA-b-PAA, synthesized by controlled radical polymerization, with the physico-chemical characteristics of the samples. Despite product imperfections, the samples self-assemble in melt and aqueous solutions as predicted by monodisperse microphase separation theory. However, the PBA core are abnormally large; the swelling of PBA cores is not due to AA (the Flory parameter PBA/PAA , determined at 0.25, means strong segregation), but to h-PBA homopolymers (content determined by Liquid Chromatography at the Point of Exclusion and Adsorption Transition LC-PEAT). Beside the dominant population of micelles detected by scattering experiments, capillary electrophoresis CE analysis permitted detection of two other populations, one of h-PAA, and the other of free PBA-b-PAA chains, that have very short PBA blocks and never self-assemble. Despite the presence of these free unimers, the self-assembly in solution was found out of equilibrium: the aggregation state is history dependant and no unimer exchange between micelles occurs over months (time-evolution SANS). The high PBA/water interfacial tension, measured at 20 mN/m, prohibits unimer exchange between micelles. PBA-b-PAA solution systems are neither at thermal equilibrium nor completely frozen systems: internal fractionation of individual aggregates can occur.