Specific Spatial Organization Research Articles

One Photon−Two Free Radical Photoinitiating Systems. Novel Approach to the Preparation of Dissociative, Multicomponent, Electron-Transfer Photoinitiators for Free Radical Polymerization

The photoinitiation abilities of several photoinitiation systems composed of positively charged 3-ethyl-2-(p-(N,N-dimethylamino)styryl)benzothiazolium iodide (dye I) and 3-ethyl-2-(p-pyrrolidinestyryl)benzothiazolium iodide (dye II) acting as light absorbers, n-butyltriphenylborate (Bo-) anion being the electron donor, and alkoxypyridinium (AP+) cation acting as ground-state electron acceptor, are compared. Both (Bo-) and (AP+) in polymerizing formulations were present either as Bo- tetramethylammonium salt (TBAB) and AP+ tetrafluoroborate salt (APTB), respectively, or as an ion pair composed of Bo- anion and AP+ cation (AP+Bo-). The best photoinitiating ability is observed for the system in which borate anion and pyridinium cation form an ion pair. On the basis of experimental results, it is postulated that an enhancement of photoinitiation ability observed for an ion pair is caused by a specific spatial organization of the reactants allowing the secondary reactions, yielding free radicals, in three-comp...

Capabilities of remote sensors to classify coral, algae, and sand as pure and mixed spectra

We investigate the abilities of seven remote sensors to classify coral, algae, and carbonate sand based on 10,632 reflectance spectra measured in situ on reefs around the world. Discriminant and classification analyses demonstrate that full-resolution (1 nm) spectra provide very good spectral separation of the bottom-types. We assess the spectral capabilities of the sensors by applying to the in situ spectra the spectral responses of two airborne hyperspectral sensors (AAHIS and AVIRIS), three satellite broadband multispectral sensors (Ikonos, Landsat-ETM+ and SPOT-HRV), and two hypothetical satellite narrowband multispectral sensors (Proto and CRESPO). Classification analyses of the simulated sensor-specific spectra produce overall classification accuracy rates of 98%, 98%, 93%, 91%, 64%, 58%, and 50% for AAHIS, AVIRIS, Proto, CRESPO, Ikonos, Landsat-ETM+, and SPOT-HRV, respectively. Analyses of linearly mixed sensor-specific spectra reveal that the hyperspectral and narrowband multispectral sensors have the ability to discriminate between coral and algae across many levels of mixing, while the broadband multispectral sensors do not. Applying the results of the general mixing analyses to a specific spatial organization of coral, algae, and sand indicates that the hyperspectral sensors accurately estimate areal cover of the bottom-types regardless of pixel resolution. The narrowband multispectral sensors overestimate coral cover by 11–15%, while the broadband sensors underestimate algae cover by 7–29% and overestimate coral cover by 24–103%. We conclude that currently available satellite sensors are inadequate for assessment of global coral reef status, but that it is both necessary and possible to design a sensor system suited to the task.

Osteogenic growth peptide: From concept to drug design

Recently, the osteogenic growth peptide (OGP) and its C-terminal pentapeptide H-Tyr-Gly-Phe-Gly-Gly-OH [OGP(10-14)] have attracted considerable clinical interest as bone anabolic agents and hematopoietic stimulators. They are present in mammalian serum in micromolar concentrations, increase bone formation and trabecular bone density, and stimulate fracture healing when administered to mice and rats. In cultures of osteoblastic and other bone marrow stromal cells, derived from human and other mammalian species, OGP regulates proliferation, alkaline phosphatase activity and matrix mineralization via an autocrine/paracrine mechanism. In vivo it also regulates the expression of type I collagen and the receptor for basic fibroblast growth factor. In addition, OGP and OGP(10-14) enhance hematopoiesis, including the stimulation of bone marrow transplant engraftment and hematopoietic regeneration after ablative chemotherapy. Apparently, the hematopoietic effects of these peptides are secondary to their effect on the bone marrow stroma. Detailed structure-activity relationship study identified the side chains of Tyr(10) and Phe(12) as the principal pharmacophores for OGP-like activity. Recently, it has been demonstrated that several cyclostereoisomers of OGP(10-14), including the analogue retro-inverso (Gly-Gly-D-Phe-Gly-D-Tyr), share the full spectrum of OGP-like bioactivities. Taken together, OGP represents an interesting case of a "housekeeping" peptide that plays an important role in osteogenesis and hematopoiesis, and interacts with its putative macromolecular target via distinct pharmacophores presented in a specific spatial organization.

Electrophysiological evidence for a chemotopy of biologically relevant odors in the olfactory bulb of the channel catfish.

Extracellular electrophysiological recordings from single olfactory bulb (OB) neurons in the channel catfish, Ictalurus punctatus, indicated that the OB is divided into different functional zones, each processing a specific class of biologically relevant odor. Different OB regions responded preferentially at slightly above threshold to either a mixture of 1) bile salts (10(-7) to 10(-5) M Na(+) salts of taurocholic, lithocholic, and taurolithocholic acids), 2) nucleotides [10(-6) to 10(-4) M adenosine-5'-triphosphate (ATP), inosine-5'-monophosphate (IMP), and inosine-5'-triphosphate (ITP)], or 3) amino acids (10(-6) to 10(-4)M L-alanine, L-methionine, L-arginine, and L-glutamate). Excitatory responses to bile salts were observed primarily in a thin, medial strip in both the dorsal (100-450 microm) and ventral (900-1,200 microm) OB. Excitatory responses to nucleotides were obtained primarily from dorsal, caudolateral OB, whereas excitatory responses to amino acids occurred more rostrally in the dorsolateral OB, but continued more medially in the ventral OB. The chemotopy within the channel catfish OB is more comparable to that previously described by optical imaging studies in zebrafish than by field potential studies in salmonids. The present results are consistent with recent studies, suggesting that the specific spatial organization of output neurons in the OB is necessary for the quality coding/decoding of olfactory information.

Spatial distribution of chaotic transients in unidirectional synchronisation

We consider the transient time to synchronisation in unidirectionally coupled dynamical systems, both discrete maps and continuous flows. We note that while in certain systems the time distribution is typical of chaotic transients, in other cases the distribution possesses a specific spatial organization.

Quadrupolar forces and aggregation of nematic droplets

The electrostatic quadrupolar interaction between spherical nematic droplets in an isotropic (and nonconducting) liquid is calculated. It is found to have an anisotropic form Uq∝1/R5, where R is the distance between droplets, with repulsion for droplets having parallel orientation of the quadrupole moments and attraction at oblique angles around the orthogonal orientation. In an external magnetic field aligning the orientations of the quadrupole moments, a competition of the quadrupolar repulsion and van der Waals attraction (UvdW∝ 1/R6) leads to a specific spatial organization of droplets which is in fact often reported in experimental observations (see the monograph by P. Drzaic, Liquid Crystal Dispersions, World Scientific, Singapore (1995) and references cited therein).

Gene activation by the AraC protein can be inhibited by DNA looping between AraC and a LexA repressor that interacts with AraC: possible applications as a two-hybrid system.

The Escherichia coli activator and repressor proteins AraC and LexA bind DNA as homodimers. Here we show that their heterodimerization through fused cognate dimerization domains results in repression of AraC-dependent gene activation by LexA. Repression also requires a LexA operator half-site located several helical turns downstream of the AraC operator. This requirement for a specific spatial organization of the operators suggests the formation of a DNA loop between operator-bound Ara/LexA heterodimers, and we propose that heterodimerization with the AraC hybrid provides co-operativity for operator binding and repression by the LexA hybrid. Consistent with a mechanism that involves DNA looping, repression increases when the E. coli DNA looping and transcriptional effector protein IHF binds between the AraC and LexA operators. Thus, we have combined the functions of three distinct transcriptional effector proteins to achieve a new mode of gene regulation by DNA looping, in which the activator protein is an essential part of the repressor complex. The flexibility of the DNA loop may facilitate this novel combinatorial arrangement of those proteins on the DNA. The requirement for protein interactions between the AraC and LexA hybrids for gene regulation suggests that this regulatory circuit may prove useful as an E. coli-based two-hybrid system.

Botulinum Neurotoxin Type C Cleaves a Single Lys-Ala Bond within the Carboxyl-terminal Region of Syntaxins

Botulinum neurotoxin serotype C (BoNT/C) is a 150-kDa protein produced by Clostridium botulinum, which causes animal botulism. In contrast to the other botulinum neurotoxins that contain one atom of zinc, highly purified preparations of BoNT/C bind two atoms of zinc per toxin molecule. BoNT/C is a zinc-endopeptidase that cleaves syntaxin 1A at the Lys253-Ala254 and syntaxin 1B at the Lys252-Ala253 peptide bonds, only when they are inserted into a lipid bilayer. The other Lys-Ala bond present within the carboxyl-terminal region is not hydrolyzed. Syntaxin isoforms 2 and 3 are also cleaved by BoNT/C, while syntaxin 4 is resistant. These data suggest that BoNT/C recognizes a specific spatial organization of syntaxin, adopted upon membrane insertion, which brings a selected Lys-Ala peptide bond of its carboxyl-terminal region to the active site of this novel metalloproteinase.

The supramolecular organization of ovomucin. Biophysical and morphological studies

Ovomucin participates in the ovomucin-gel-forming properties because of its shape and its ability to interact in a specific spatial organization. Purified from chicken egg-white by exclusion chromatography with Sephacryl S-300 and Sepharose CL-2B and analysed by light-scattering, it exhibited an Mr of about 40 x 10(6). This large Mr can be explained by the aggregation of polymers that can be degraded into 3 x 10(6)-Mr fragments by reduction with dithiothreitol. The values for hydrodynamic parameters such as Mr, radius of gyration, hydrodynamic radius, mass per unit length and combinations of them suggested that ovomucin is a linear and highly flexible molecule conferring upon it a random-coil-like structure in 0.2 M-NaCl solution. Analysis of the ovomucin molecules by electron microscopy revealed its linear character but also indicated a lower Mr than that obtained in the light-scattering experiments. By temperature-induced non-specific aggregation of an ovomucin solution containing other globular egg proteins, an attempt was made to find out what conditions are required for gel formation and to examine the quality of aggregation that is obtained under these conditions. Results show that the viscosity of the solution did not increase after heat treatment. Apparently, in the ovomucin gel, specific spatial organization of the ovomucin molecules is required for hydrogel formation.

Masking of near-threshold flashes by brief electrical pulses to the optic tract of the cat

A brief electrical pulses delivered bilaterally to the optic tracts evoked a response at the LGN and striate cortex several times larger than that evoked by a brief near-threshold visual stimulus. The electrical pulse could raise the detection threshold about 0.3 log units if it occurred during the first 60 msec of the visually evoked response. If it occurred before or after the early components of the visually evoked response, visual detection was unaffected. This indicates that visual detection in the cat is most dependent on the integrity of the early components of the visually evoked activity. After just a small amount of practice the electrical stimulus ceased to have an effect on the visual threshold. It is concluded that this is due to the protective form in which a visual signal is transmitted and due to the unique characteristics of the activity evoked by the OT stimulus. The ‘signal’ or ‘noise’ within the visual system cannot be quantified by simply counting spike potentials within a certain time frame but must be dependent on some specific spatial or temporal organization of the evoked activity.