Norbert F. Scherer

Single-molecule mechanics of mussel adhesion.

The glue proteins secreted by marine mussels bind strongly to virtually all inorganic and organic surfaces in aqueous environments in which most adhesives function poorly. Studies of these functionally unique proteins have revealed the presence of the unusual amino acid 3,4-dihydroxy-l-phenylalanine (dopa), which is formed by posttranslational modification of tyrosine. However, the detailed binding mechanisms of dopa remain unknown, and the chemical basis for mussels’ ability to adhere to both inorganic and organic surfaces has never been fully explained. Herein, we report a single-molecule study of the substrate and oxidation-dependent adhesive properties of dopa. Atomic force microscopy (AFM) measurements of a single dopa residue contacting a wet metal oxide surface reveal a surprisingly high strength yet fully reversible, noncovalent interaction. The magnitude of the bond dissociation energy as well as the inability to observe this interaction with tyrosine suggests that dopa is critical to adhesion and that the binding mechanism is not hydrogen bond formation. Oxidation of dopa, as occurs during curing of the secreted mussel glue, dramatically reduces the strength of the interaction to metal oxide but results in high strength irreversible covalent bond formation to an organic surface. A new picture of the interfacial adhesive role of dopa emerges from these studies, in which dopa exploits a remarkable combination of high strength and chemical multifunctionality to accomplish adhesion to substrates of widely varying composition from organic to metallic.

Fluorescence‐detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase‐locked pulses

We introduce a novel spectroscopic technique which utilizes a two‐pulse sequence of femtosecond duration phase‐locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in‐phase, in‐quadrature, and out‐of‐phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct de...

Off‐resonant transient birefringence in liquids

Off‐resonant transient birefringence measurements are analyzed using a reduced equation of motion for the ground state density matrix, which is expanded using an effective Hamiltonian. Assuming that the pump field is weak, we express the polarization relevant for the birefringence signal in terms of a convolution of the tensorial polarizability response function with the external fields. The homodyne‐detected birefringence signal is directly compared with the coherent Raman signal. The relationship between off‐resonant birefringence and spontaneous Raman experiments is discussed. By expanding the polarizability in powers of the nuclear coordinates and applying the Brownian oscillator model to the coordinate response function, we separate the birefringence signal into intra‐ and intermolecular coordinate response functions. Off‐resonant transient birefringences of acetonitrile, chloroform, dimethylsulfoxide, and a series of alcohols were measured. The data are transformed to the frequency domain by using a...

Ultrafast solvent dynamics: Connection between time resolved fluorescence and optical Kerr measurements

The vibrational characteristics of liquid dynamics are used to describe the ultrafast relaxations observed in time‐dependent fluorescence Stokes shift [J. Chem. Phys. 95, 4715 (1991)] and heterodyne detected optical Kerr effect measurements on acetonitrile, via a Brownian oscillator model. Introducing a frequency distribution of vibrational modes makes it possible to compare the two experiments. The ultrafast decays observed in the fluorescence Stokes shift and optical Kerr signals are produced by destructive superposition of the high frequency, underdamped modes.

Time resolved dynamics of isolated molecular systems studied with phase‐locked femtosecond pulse pairs

Articles you may be interested in Development of high resolution Michelson interferometer for stable phase-locked ultrashort pulse pair generation Rev. Impulsive effects of phaselocked pulse pairs on nuclear motion in the electronic ground state Fluorescencedetected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phaselocked pulses

Optical trapping and alignment of single gold nanorods by using plasmon resonances.

We demonstrate three-dimensional trapping and orientation of individual Au nanorods by using laser light slightly detuned from their longitudinal plasmon mode. Detuning to the long-wavelength side of the resonance allows stable trapping for several minutes, with an exponential dependence of trapping time on laser power (consistent with a Kramers escape process). Detuning to the short-wavelength side causes repulsion of the rods from the laser focus. Alignment of the long axis of the rods with the trapping laser polarization is observed as a suppression of rotational diffusion about the short axis.

Intracellular transport of insulin granules is a subordinated random walk

We quantitatively analyzed particle tracking data on insulin granules expressing fluorescent fusion proteins in MIN6 cells to better understand the motions contributing to intracellular transport and, more generally, the means for characterizing systems far from equilibrium. Care was taken to ensure that the statistics reflected intrinsic features of the individual granules rather than details of the measurement and overall cell state. We find anomalous diffusion. Interpreting such data conventionally requires assuming that a process is either ergodic with particles working against fluctuating obstacles (fractional Brownian motion) or nonergodic with a broad distribution of dwell times for traps (continuous-time random walk). However, we find that statistical tests based on these two models give conflicting results. We resolve this issue by introducing a subordinated scheme in which particles in cages with random dwell times undergo correlated motions owing to interactions with a fluctuating environment. We relate this picture to the underlying microtubule structure by imaging in the presence of vinblastine. Our results provide a simple physical picture for how diverse pools of insulin granules and, in turn, biphasic secretion could arise.

Femtosecond wave packet and chemical reaction dynamics of iodine in solution: Tunable probe study of motion along the reaction coordinate

One‐ and two‐color time‐domain probing of the resonant dichroic response of iodine in n‐hexane following femtosecond B‐X excitation at 580 nm is described. The detected signals contain both ground and excited state vibrational coherence contributions to the third‐order polarization. The dichroic response can be separated into positive and negative amplitude contributions: B‐X absorption and stimulated emission are positive but absorption from the B‐state can yield either positive or negative signals depending on the direction of the transition moment. Wave packet motion on both the ground and excited states of iodine is studied with a frequency tunable femtosecond probe. It is shown that the positive signals can be interpreted as B‐X dichroic response using the classical Franck principle. The classical Franck principle also provides information about the potential probed in absorption from the B state. From the probe wavelength dependent delay in the signal appearance, it is concluded that the absorptive signal for blue probe wavelengths arises from a repulsive state reached by solvent‐induced predissociation of the B state. Dephasing of B state vibrational coherence results from this solvent‐induced predissociation of iodine. We discuss the evolving reaction in terms of possible dissociative potential energy curves a1g(3Π) and a’0g+(3Σ−). The time evolution of the bluest probe dichroism signals is representative of continuing atom separation; the experiments have not yet probed large enough internuclear separations to evidence a buildup of dissociated product or momentum reversal, i.e., caging.One‐ and two‐color time‐domain probing of the resonant dichroic response of iodine in n‐hexane following femtosecond B‐X excitation at 580 nm is described. The detected signals contain both ground and excited state vibrational coherence contributions to the third‐order polarization. The dichroic response can be separated into positive and negative amplitude contributions: B‐X absorption and stimulated emission are positive but absorption from the B‐state can yield either positive or negative signals depending on the direction of the transition moment. Wave packet motion on both the ground and excited states of iodine is studied with a frequency tunable femtosecond probe. It is shown that the positive signals can be interpreted as B‐X dichroic response using the classical Franck principle. The classical Franck principle also provides information about the potential probed in absorption from the B state. From the probe wavelength dependent delay in the signal appearance, it is concluded that the absorptive ...

Fluorescence‐detected wave packet interferometry. II. Role of rotations and determination of the susceptibility

The recently developed technique of time‐resolved spectroscopy with phase‐locked optical pulse pairs is further explored with additional experimental data and more detailed comparison to theory. This spectroscopic method is sensitive to the overall phase evolution of an optically prepared nuclear wave packet. The phase locking scheme, demonstrated for the B←X transition of gas phase molecular iodine, is extended through the use of in‐quadrature locked pulses and by examination of the dispersed fluorescence signal. The excited state population following the interaction with both pulses is detected as the resultant two‐field‐dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from rovibrational wave packet dynamics of the molecule on the excited state electronic potential energy curve. Quantum interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two‐pulse phase‐locked interferograms are in...

Propagation Lengths and Group Velocities of Plasmons in Chemically Synthesized Gold and Silver Nanowires

Recent advances in chemical synthesis have made it possible to produce gold and silver nanowires that are free of large-scale crystalline defects and surface roughness. Surface plasmons can propagate along the wires, allowing them to serve as optical waveguides with cross sections much smaller than the optical wavelength. Gold nanowires provide improved chemical stability as compared to silver nanowires, but at the cost of higher losses for the propagating plasmons. In order to characterize this trade-off, we measured the propagation length and group velocity of plasmons in both gold and silver nanowires. Propagation lengths are measured by fluorescence imaging of the plasmonic near fields. Group velocities are deduced from the spacing of fringes in the spectrum of coherent light transmitted by the wires. In contrast to previous work, we interpret these fringes as arising from a far-field interference effect. The measured propagation characteristics agree with numerical simulations, indicating that propagation in these wires is dominated by the material properties of the metals, with additional losses due to scattering from roughness or grain boundaries providing at most a minor contribution. The propagation lengths and group velocities can also be described by a simple analytical model that considers only the lowest-order waveguide mode in a solid metal cylinder, showing that this single mode dominates in real nanowires. Comparison between experiments and theory indicates that widely used tabulated values for dielectric functions provide a good description of plasmons in gold nanowires but significantly overestimate plasmon losses in silver nanowires.