Fluctuation-Optical Method for Determining Gelation Point of Biopolymer Systems

Method for determining temperature of the transition of biopolymer systems to gellike state was developed using the earlier discovered effect of decaying amplitude of spontaneous fluctuations of system optical density, which significantly exceed the level of intrinsic noise of measuring devices of Specord M40 and Specord M400 spectrophotometers. These fluctuations are caused by the thermal convection resulting in the emergence of randomly distributed regions with different optical properties in the analyzed systems. The inhomogeneity can by amplified by the fraction of supermolecular particles (SMP). The effect of convection markedly decreases as a result of the loss of fluidity during the system gelation, thus underlying the instrumental registration of the sol–gel transition whose efficiency was confirmed in this work using aqueous preparations of plant polysaccharides (pectin, agar, furcellaran, and -carrageenan). It was shown that, in the cases of initially optically homogeneous systems, the role of the SMPs can be played (if necessary) by the relatively small amounts of the particles of chemically inert compounds (for example, cellulose) at slight mixing of a system using magnetic stirrer. This method was applied for constructing the fragment of the phase diagram of the furcellaran–water system during its cooling. The interpretation of its semi-logarithmic transformation in terms of the Eldridge–Ferry theory resulted in the estimation of the amount of heat evolved during the formation of gel network junctions (presumably, due to the pair associations of double helixes), which appeared to be equal to –63 kJ mol^–1.     

Structural Anisotropy of Fractal Aggregates and Its Exhibition in Electrooptical Effects

The first experimental examination of orientational turbidimetric effect [differential orientational spectrum of optical density D() in suspensions of fractal clusters was presented. Experiments were performed for aqueous suspensions of powdered Aerosil OX-50 aggregates ordered by the alternating electric field oriented along or across the unpolarized light beam. Experimental differential spectra D() are essentially different from the known spectra for bacterial and yeast suspensions. In particular, the points of zero effect lie in the red (but not in the blue) region of spectrum, and the |D()| dependence is a decreasing function in the range 300–800 nm. To explain these results, a theory was developed based on the three-dimensional lattice model of cluster-cluster aggregation and the optical model of interacting dipoles. This theory confirms main conclusions obtained previously in the Rayleigh–Debye–Hans approximation (Khlebtsov, N.G. and Mel'nikov, A.G., Kolloidn. Zh., 1998, vol. 60, no. 6, p. 843) and agrees with experimental D() spectra. It was shown experimentally and theoretically that the structural anisotropy of aggregates exhibited scale invariance, and the differential spectra for the aggregates with various numbers of particles are described by a single universal curve D(kR _g), where k= 2/ and R _gis the mean radius of gyration of aggregates.     

A Multilayer Model for Gold Nanoparticle Bioconjugates: Application to Study of Gelatin and Human IgG Adsorption Using Extinction and Light Scattering Spectra and the Dynamic Light Scattering Method

A new model of colloidal gold (CG) bioconjugates is proposed. The model consists of a gold core and a primary polymer shell formed during conjugate synthesis. Additionally, the conjugate includes a secondary shell formed during its interaction with target molecules. Each of the inhomogeneous shells is modeled by the arbitrary number of discrete layers. Using Mie theory for multilayered spheres, we calculated the extinction and static light scattering (SLS, at 90°) spectra, as well as differential spectra A(), I() describing the effects of primary and secondary shells. Our calculations are performed for the conjugates with gold particle diameters d = 10–160 nm and two 5-nm shells. The primary shell consists of two 2.5-nm layers with the refractive indices of 1.50 and 1.45; the secondary shell, of two 2- and 3-nm layers with the refractive indices of 1.45 and 1.40. The differential spectra are related to the adsorption of target molecules and possess a characteristic resonance that is shifted to the red region of spectra compared to the usual localized plasmon resonances of gold particles. The maximal values of differential resonances A _max and I _max are observed for gold particles with diameters about 40–60 nm (extinction spectra) or 70–90 nm (the SLS spectra). The adsorption of human gamma-globulin (hIgG) and gelatin onto 15- and 34-nm gold particles was studied using the SLS and extinction spectra in combination with the dynamic light scattering measurements. It is shown that the thickness of adsorbed layer is equal to 5–6 nm for hIgG and to 15–18 nm for gelatin. The experimental extinction and SLS spectra for CG + hIgG conjugates are well explained by a simple model with the gold core and homogeneous polymer coating. For the CG + gelatin conjugates, we used the new model with inhomogeneous polymer coating, which is modeled by 10 discrete layers with the total thickness of 16–18 nm and exponential spatial profile of shell refractive index.     

Random replicators with high-order interactions

We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations.     

Optical assessment of <Emphasis Type="Italic">Dunaliella salina</Emphasis> microalgae viability upon toxicological testing

A method has been developed for monitoring the dynamics of the number and viability of Dunaliella salina microalgae taking into account the spectral contribution from aggregates of metal particles tested as a toxic agent in a microplate assay system. The method is based on the in vivo determination of the chlorophyll content from the intensity of an absorption peak at 680 nm corrected for the nonselective extinction as calculated using the values of nearest local minima at 640 and 740 nm.