A. R. Burns

Actin restricts Fc|[epsiv]|RI diffusion and facilitates antigen-induced receptor immobilization

The actin cytoskeleton has been implicated in restricting diffusion of plasma membrane components. Here, simultaneous observations of quantum dot-labelled FcɛRI motion and GFP-tagged actin dynamics provide direct evidence that actin filament bundles define micron-sized domains that confine mobile receptors. Dynamic reorganization of actin structures occurs over seconds, making the location and dimensions of actin-defined domains time-dependent. Multiple FcɛRI often maintain extended close proximity without detectable correlated motion, suggesting that they are co-confined within membrane domains. FcɛRI signalling is activated by crosslinking with multivalent antigen. We show that receptors become immobilized within seconds of crosslinking. Disruption of the actin cytoskeleton results in delayed immobilization kinetics and increased diffusion of crosslinked clusters. These results implicate actin in membrane partitioning that not only restricts diffusion of membrane proteins, but also dynamically influences their long-range mobility, sequestration and response to ligand binding.

Polydiacetylene films: a review of recent investigations into chromogenic transitions and nanomechanical properties

Polydiacetylenes (PDAs) form a unique class of polymeric materials that couple highly aligned and conjugated backbones with tailorable pendant sidegroups and terminal functionalities. They can be structured in the form of bulk materials, multilayer and monolayer films, polymerized vesicles, and even incorporated into inorganic host matrices to form nanocomposites. The resulting materials exhibit an array of spectacular properties, beginning most notably with dramatic chromogenic transitions that can be activated optically, thermally, chemically, and mechanically. Recent studies have shown that these transitions can even be controlled and observed at the nanometre scale. These transitions have been harnessed for the purpose of chemical and biomolecular sensors, and on a more fundamental level have led to new insights regarding chromogenic phenomena in polymers. Other recent studies have explored how the strong structural anisotropy that thes em aterials possess leads to anisotropic nanomechanical behaviour. These recen ta dvances suggest that PDAs could be considered as a potential component in nanostructured devices due to the large number of tunable properties. In this paper, we provide a succinct review of the latest insights and applications involving PDA. We then focus in more detail on our work concerning ultrathin films, specifically structural properties, mechanochromism, thermochromism, and in-plane mechanical anisotropy of PDA monolayers. Atomic force microscopy (AFM) and fluorescence microscopy confirm that films 1–3 monolayers thick can be organized into highly ordered domains,with the conjugated backbones parallel to the substrate. The number of stable layers is controlled by the head-group functionality. Local mechanical stress applied by AFM an dn ear-field optical probes induces the chromogenic transition in the film at the nanometre scale. The transition

Multidimensional dynamics in the electron stimulated desorption of ammonia from Pt(111)

We characterize the electron stimulated desorption of neutral ammonia (NH3 and ND3) from Pt(111) with vibrational and rotational quantum resolution by using (2+1) resonance enhanced multiphoton ionization detection. Two significant isotope effects appear: (1) the desorption yield of NH3 is three times that of ND3 and (2) NH3 desorbs with considerably more ‘‘spinning’’ rotational energy than does ND3. We find virtually identical translational energy distributions for each desorbate and roughly equal vibrational energy distributions. Vibrational excitation is found exclusively in the ν2 symmetric deformation or ‘‘umbrella’’ mode, independent of isotope. These effects cannot be explained by desorption induced by vibrational energy transfer. Instead, desorption is the result of excitation of a 3a1 electron principally on the N atom, which causes the pyramidal NH3 adsorbate to rapidly invert. Ab initio calculations of two‐dimensional potential energy surfaces (intramolecular bond angle and surface bond length)...

Large friction anisotropy of a polydiacetylene monolayer

Friction force microscopy measurements of a polydiacetylene monolayer film reveal a 300% friction anisotropy that is correlated with the film structure. The film consists of a monolayer of the red form of N‐(2‐ethanol)‐10,12‐pentacosadiynamide, prepared on a Langmuir trough and deposited on a mica substrate. As confirmed by atomic force microscopy and fluorescence microscopy, the monolayer consists of domains of linearly oriented conjugated backbones with pendant hydrocarbon side chains above and below the backbones. Maximum friction occurs when the sliding direction is perpendicular to the backbones. We propose that this effect is due to anisotropic film stiffness, which is a result of anisotropic side chain packing and/or anisotropic stiffness of the backbone itself. Friction anisotropy is therefore a sensitive, optically‐independent indicator of polymer backbone direction and monolayer structural properties.

Chemisorbed-molecule potential energy surfaces and DIET processes

Abstract We report the use of the local-density approximation, with and without gradient corrections, for the calculation of ground-state potential energy surfaces (PESs) for chemisorbed molecules. We focus on chemisorbed NO and ammonia on Pd(1 1 1) and compare our results with the latest experimental information. We then turn to two aspects of excited-state PESs. First, we compare first-principles calculations of the forces on an ammonia ion as a function of distance from the surface. We find that the image-charge model fails significantly at distances which are the most relevant for dynamics, closer than ∼3 A, and discuss reasons for the failure. We then summarize a purely electronic adiabatic model of the moleuule-surface bond and use empirical parameters to estimate hot carrier-produced excited states of chemisorbed NO. We find multiple PESs and a novel interpretation of the π ∗ resonance, seen in inverse photoemission.

Shear-induced mechanochromism in polydiacetylene monolayers

We use atomic force microscopy to actuate and characterize the nanoscale “mechanochromism” of polydiacetylene monolayers on atomically-flat silicon oxide substrates. We find explicit evidence that the irreversible blue-to-red transformation is caused by shear forces exerted normal to the polydiacetylene polymer backbone. The anisotropic probe-induced transformation is characterized by a significant change in the tilt orientation of the side chains with respect to the surface normal. We discuss preliminary molecular dynamics simulations and electronic structure calculations on twelve-unit polydiacetylene oligomers that allow us to correlate the transformation with bond-angle changes in the conjugated polymer backbone.

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO2 dissociation on Pt(111)

Abstract We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO 2 dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ o ) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO 2 on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the ∼ 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO 2 is N-bonded, ∼ 6° off-normal beams are observed. When the substrate is precovered with θ o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (∼9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.

Electron-stimulated production of NO2(g) from O2 coadsorbed with NO on Pt(111)

Using laser resonance‐enhanced ionization spectroscopy, we have detected O(3PJ=2,1,0) and NO X 2Π3/2,1/2 (ν=5) above a 6–350 eV electron beam‐irradiated Pt(111) surface containing coadsorbed O2 and NO at 90 K. Both product yields have the same chemisorbed NO coverage dependence at saturation O2 precoverage as well as the same ≊10 eV excitation threshold. We conclude that both O(3PJ) and NO(ν=5) are laser‐induced photodissociation fragments of NO2(g). This is established by the observation of identical O(3P2) and NO(ν=5) time‐of‐flight distributions that correspond to NO2 desorption from the surface. The NO2(g) is probably the reaction product of a collision between an O atom (created by electron‐stimulated dissociation of adsorbed O2) and NO(a). We correlate the 10 eV NO2 production threshold with the dissociative ionization of the 3σg molecular bonding orbital of O2(a).

Rotational dynamics and electronic energy partitioning in the electron‐stimulated desorption of NO from Pt(111)

Using laser resonance ionization, we have examined the rotational dynamics of neutral NO molecules desorbed by electron impact from a cold (80 K) Pt(111) surface. Previously [A. R. Burns, E. B. Stechel, and D. R. Jennison, Phys. Rev. Lett. 58, 250 (1987)], we observed ‘‘hot’’ rotational temperatures in the range 481–642 K for the ν=0, 1, 2, and 3 vibrational levels. We now present more detailed data which do not reveal shifts or broadening in the rotationally selected NO velocity distributions as a function of rotational level J in the range J=2.5–29.5. The rotationally independent velocity distributions, peaked at 0.05 eV, as well as the rotational temperature can be understood within the framework of ‘‘electronically stimulated adsorbate rotation’’ (Burns et al., above). The model also predicts complete rotational alignment in a plane normal to the surface. In the laboratory, no alignment is observed, but this is most likely due to the presence of stray magnetic fields. We will also discuss our measurements which show an equal partitioning between the two lowest NO electronic states (2Π1/2 and 2Π3/2), and a preference for the π level which is perpendicular (antisymmetric) to the plane of rotation.Using laser resonance ionization, we have examined the rotational dynamics of neutral NO molecules desorbed by electron impact from a cold (80 K) Pt(111) surface. Previously [A. R. Burns, E. B. Stechel, and D. R. Jennison, Phys. Rev. Lett. 58, 250 (1987)], we observed ‘‘hot’’ rotational temperatures in the range 481–642 K for the ν=0, 1, 2, and 3 vibrational levels. We now present more detailed data which do not reveal shifts or broadening in the rotationally selected NO velocity distributions as a function of rotational level J in the range J=2.5–29.5. The rotationally independent velocity distributions, peaked at 0.05 eV, as well as the rotational temperature can be understood within the framework of ‘‘electronically stimulated adsorbate rotation’’ (Burns et al., above). The model also predicts complete rotational alignment in a plane normal to the surface. In the laboratory, no alignment is observed, but this is most likely due to the presence of stray magnetic fields. We will also discuss our measurem...

Exploring membrane protein dynamics by multicolor single quantum dot imaging using wide field, TIRF, and hyperspectral microscopy

The development of colloidal quantum dots (QDs) for biological imaging has brought a new level of sensitivity to live cell imaging. Single particle tracking (SPT) techniques in particular benefit from the superior photostability, high extinction coefficient and distinct emission spectra of QDs. Here we describe the use of QDs for SPT to study the dynamics of membrane proteins in living cells. We work with the RBL-2H3 mast cell model that signals through the high affinity IgE receptor, Fc&Vegr;RI. Using wide field or Total Internal Reflection Fluorescence (TIRF) microscopy we have achieved simultaneous imaging of two spectrally distinct QDs with frame rates of up to 750 frames/s and localization accuracy of ~10 nm. We also describe the imaging and analysis of QDs using a novel hyperspectral microscope and multivariate curve resolution analysis for multi-color QD tracking. The same QD-tag used for SPT is used to localize proteins at <10 nm resolution by electron microscopy (EM) on fixed membrane sheets.