Anomalous diffusion

Anomalous diffusion is a diffusion process with a non-linear relationship between the mean squared displacement (MSD), \langle r^{2}(\tau )\rangle , and time. This behavior is in stark contrast to Brownian motion, the typical diffusion process described by Einstein and Smoluchowski, where the MSD is linear in time (namely, \langle r^{2}(\tau )\rangle =2dD\tau with d being the number of dimensions and D the diffusion coefficient). Examples of anomalous diffusion in nature have been observed in biology in the cell nucleus, plasma membrane and cytoplasm. Unlike typical diffusion, anomalous diffusion is described by a power law, \langle r^{2}(\tau )\rangle =K_\alpha\tau^\alphawhere K_\alpha is the so-called generalized diffusion coefficient and \tau is the elapsed time. In Brownian motion, α = 1. If α > 1, the process is superdiffusive. Superdiffusion can be the result of active cellular transport processes or due to jum

Single-molecule motions of oligoarginine transporter conjugates on the plasma membrane of Chinese hamster ovary cells

Hui Wang

To explore the real-time dynamic behavior of molecular transporters of the cell-penetrating-peptide (CPP) type on a biological membrane, single fluorescently labeled oligoarginine conjugates were imaged interacting with the plasma membrane of Chinese hamster ovary (CHO) cells. The diffusional motion on the membrane, characterized by single-molecule diffusion coefficient and residence time (tau R), defined as the time from the initial appearance of a single-molecule spot on the membrane (from the solution) to the time the single molecule disappears from the imaging focal plane, was observed for a fluorophore-labeled octaarginine (a model guanidinium-rich CPP) and compared with the corresponding values observed for a tetraarginine conjugate (negative control), a lipid analogue, and a fluorescently labeled protein conjugate (transferrin-Alexa594) known to enter the cell through endocytosis. Imaging of the oligoarginine conjugates was enabled by the use of a new high-contrast fluorophore in the dicyanomethylenedihydrofuran family, which brightens upon interaction with the membrane at normal oxygen concentrations. Taken as a whole, the motions of the octaarginine conjugate single molecules are highly heterogeneous and cannot be described as Brownian motion with a single diffusion coefficient. The observed behavior is also different from that of lipids, known to penetrate cellular membranes through passive diffusion, conventionally involving lateral diffusion followed by membrane bilayer flip-flop. Furthermore, while the octaarginine conjugate behavior shares some common features with transferrin uptake (endocytotic) processes, the two systems also exhibit dissimilar traits when diffusional motions and residence times of single constructs are compared. Additionally, pretreatment of cells with cytochalasin D, a known actin filament disruptor, produces no significant effect, which further rules out unimodal endocytosis as the mechanism of uptake. Also, the involvement of membrane potential in octaarginine-membrane interaction is supported by significant changes in the motion with high [K(+)] treatment. In sum, this first study of single transporter motion on the membrane of a living cell indicates that the mode by which the octaarginine transporter penetrates the cell membrane appears to either be a multimechanism uptake process or a mechanism different from unimodal passive diffusion or endocytosis.

2008

Statistics of the polariton condensate

Antonio Quattropani, Paolo Schwendimann

The influence of polariton-polariton scattering on the statistics of the polariton condensate in a nonresonantly excited semiconductor microcavity is discussed. Taking advantage of the existence of a bottleneck in the exciton-polariton dispersion curve, the polariton states are separated into two domains: reservoir polaritons inside the bottleneck and active polaritons whose energy lies below the bottleneck. In the framework of the master equation formalism, the nonequilibrium stationary reduced density matrix is calculated and the statistics of polaritons in the condensate at q=0 is determined. The anomalous correlations between the polaritons in the condensate and the active ones are responsible for an enhancement of the noise in the condensate. As a consequence, the second order correlation function of the condensate does not show the full coherence that is characteristic of laser emission.

2008

Single-molecule motions of oligoarginine transporter conjugates on the plasma membrane of Chinese hamster ovary cells

Hui Wang

2008