[show abstract][hide abstract] ABSTRACT: Oligomerization of the HIV-1 protein Rev on the Rev Response Element (RRE) regulates nuclear export of genomic viral RNA and partially spliced viral mRNAs encoding for structural proteins. Single-molecule fluorescence spectroscopy has been used to dissect the multistep assembly pathway of this essential ribonucleoprotein, revealing dynamic intermediates and the mechanism of assembly. Assembly is initiated by binding of Rev to a high-affinity site in stem-loop IIB of the RRE and proceeds rapidly by addition of single Rev monomers, facilitated by cooperative Rev-Rev interactions on the RRE. Dwell-time analysis of fluorescence trajectories recorded during individual Rev-RRE assembly reactions has revealed the microscopic rate constants for several of the Rev monomer binding and dissociation steps. The high-affinity binding of multiple Rev monomers to the RRE is achieved on a much faster timescale than reported in previous bulk kinetic studies of Rev-RRE association, indicating that oligomerization is an early step in complex assembly.
Proceedings of the National Academy of Sciences 02/2009; 106(5):1404-8. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: The one-photon and two-photon absorption properties of cross-shaped chromophores consisting of four donor-substituted styryl branches linked to an aromatic core (benzene or pyrazine) have been investigated and compared with those of linear analogues with only two branches (donor−π−donor distryrylarenes). The areas of the lowest energy two-photon absorption bands of the compounds with four branches were less than twice those of analogues with two branches. The spectral features observed in these chromophores suggest that electronic coupling between the branches is effective but does not lead to significant enhancement of the two-photon cross section when the branches extend in more than one dimension. In a chromophore with two donor-substituted and two acceptor-substituted branches the two-photon cross section is smaller than in the corresponding linear analogues. The main characteristics of both the one-photon and two-photon spectra of multibranched compounds of the type discussed here can be explained qualitatively within the molecular exciton description. In contrast to the case of one-photon absorptivities, the model shows that pure additivity of the two-photon absorption cross section should not be expected when two monomer units are coupled and that the cross section of the dimer depends on the relative orientation of the constituent units and on the strength and sign of the coupling interaction. In particular, the type of coupling effective in the four-branch chromophores presented here should result in a subadditivity of two-photon cross section of the monomers, in agreement with the experimental findings.
Journal of Physical Chemistry C - J PHYS CHEM C. 04/2008; 112(21).
[show abstract][hide abstract] ABSTRACT: Chromophores based on a donor-acceptor-donor structure possessing a large two-photon absorption cross section and one or two mono-aza-15-crown-5 ether moieties, which can bind metal cations, have been synthesized. The influence of Mg(2+) binding on their one- and two-photon spectroscopic properties has been investigated. Upon binding, the two-photon action cross sections at 810 nm decrease by a factor of up to 50 at high Mg(2+) concentrations and this results in a large contrast in the two-photon excited fluorescence signal between the bound and unbound forms, for excitation in the range of 730 to 860 nm. Experimental and computational results indicate that there is a significant reduction of the electron donating strength of the aza-crown nitrogen atom(s) upon metal ion binding and that this leads to a blue shift in the position as well as a reduction in the strength of the lowest-energy two-photon absorption band. The molecules reported here can serve as models for the design of improved two-photon excitable metal-ion sensing fluorophores.
Journal of the American Chemical Society 09/2004; 126(30):9291-306. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: A series of alpha,omega-bis donor substituted oligophenylenevinylene dimers held together by the [2.2]paracyclophane core were synthesized to probe how the number of repeat units and through-space delocalization influence two-photon absorption cross sections. Specifically, the paracyclophane molecules are tetra(4,7,12,15)-(4'-dihexylaminostyryl)[2.2]paracyclophane (3R(D)), tetra(4,7,12,15)-(4' '-(4'-dihexylaminostyryl)styryl)[2.2]paracyclophane (5R(D)), and tetra(4,7,12,15)-(4' "-(4' '-(4'-dihexylaminostyryl)styryl)styryl)[2.2]paracyclophane (7R(D)). The compounds bis(1,4)-(4'-dihexylaminostyryl)benzene (3R) and bis(1,4)-(4' '-(4'-dihexylaminostyryl)styryl)benzene (5R) were also synthesized to reveal the properties of the "monomeric" counterparts. The two-photon absorption cross sections were determined by the two-photon induced fluorescence method using both femtosecond and nanosecond pulsed lasers as excitation sources. While there is a red shift in the linear absorption spectra when going from the "monomer" chromophore to the paracyclophane "dimer" (i.e., 3R --> 3R(D), 5R --> 5R(D)), there is no shift in the two-photon absorption maxima. A theoretical treatment of these trends and the dependence of transition dipole moments on molecular structure rely on calculations that interfaced time-dependent density functional theory (TDDFT) techniques with the collective electronic oscillator (CEO) program. These theoretical and experimental results indicate that intermolecular interactions can strongly affect B(u) states but weakly perturb A(g) states, due to the small dipole-dipole coupling between A(g) states on the chromophores in the dimer.
Journal of the American Chemical Society 09/2004; 126(37):11529-42. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Receptor-based signaling mechanisms are the primary source of cellular regulation. The superfamily of G protein-coupled receptors (GPCR) is the largest and most ubiquitous of the receptor-mediated processes. Desensitization of G-protein-coupled receptors is a fundamental mechanism regulating the cellular response to agonists. We have recently studied the agonist and antagonist of the human melanocortin receptors (hMC1, hMC3, hMC4, and hMC5 receptors), the human delta opioid receptor, and the human gluacagon receptor with the help of synthetic fluorescent labeled ligands and fluorescent protein-labeled beta-arrestin-receptors that shed new insight on cellular signaling and rapid screening of drugs in real time. It was demonstrated that stimulation of these receptors by the cognate agonist triggers the rapid internalization of ligand-receptor complexes, while the interaction of the receptor with antagonists does not follow this pathway. Furthermore, receptor internalization is dependent upon beta-arrestin, which has been shown to be responsible for the rapid desensitization of cAMP-signaling processes.
Journal of the American Chemical Society 07/2004; 126(23):7160-1. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The one- and two-photon spectroscopic properties of four symmetrically substituted donor−acceptor−donor distyrylbenzenes with either di-n-butyl- or diphenylamino donor groups and cyano acceptor groups are reported. It has been found that the position of the substitution of the electron-withdrawing cyano groups on the central phenylene ring as compared to the vinylene bond strongly affects the observed properties. In particular, the molecules with cyano substitution on the α-carbon of the vinylene linkage are characterized by weak fluorescence, short fluorescence lifetimes, and two-photon cross sections (δ) that are comparable to analogous molecules with no acceptor groups. In contrast, the molecules with acceptor substitution on the central phenylene ring are strongly fluorescent and have δ values roughly twice those of the vinyl-substituted molecules. These results are discussed in terms of the larger deviation of the conjugated backbone from planarity and the smaller distance between the donors and acceptors when the cyano groups are substituted on the vinylene carbon rather than the central phenylene ring.
Journal of Physical Chemistry A - J PHYS CHEM A. 11/2002; 106(47).
[show abstract][hide abstract] ABSTRACT: Strong enhancement of the two-photon absorption of organic molecules near silver nanoparticle fractal clusters has been observed and has been exploited to yield composite materials with very strong two-photon absorption and two-photon-excited fluorescence properties. Measurements on cluster films coated with chromophoric polymer or with thiol-bound chromophores give spatially-averaged enhancements of 1000 and 20 000, respectively. Two-photon fluorescence microscopy studies show that the enhancements are spatially inhomogeneous, with peak-enhancement factors of g 10 000 (polymer/cluster) and g 160 000 (thiol chromophore/cluster), and excitation frequency dependent. These results are in accord with theoretical predictions of local-field effects due to strong localization of collective plasmon modes in fractal metal clusters, and demonstrate an approach to ultrasensitive two-photon processes.
Journal of Physical Chemistry B - J PHYS CHEM B. 01/2002; 106(27).