Adrian C. Murza

Adsorption of acridine drugs on silver: surface-enhanced resonance Raman evidence of the existence of different adsorption sites

Abstract The adsorption of the acridine derivative drugs Quinacrine (QA) and 9-aminoacridine (9AA) on silver surfaces was comparatively studied by surface-enhanced Raman spectroscopy (SERS). The effects of the excitation wavelength, pH and chloride concentration on the SERS spectra were investigated. The analysis of the Raman spectra reveals that each drug is able to interact with the metal surface through different mechanisms of adsorption. This is related to the distinct chemical structure of both drugs. The results shown in this work demonstrate the existence of different adsorption sites on the metal surface.

Synchronization and entrainment of coupled circadian oscillators

Circadian rhythms in mammals are controlled by the neurons located in the suprachiasmatic nucleus of the hypothalamus. In physiological conditions, the system of neurons is very efficiently entrained by the 24 h light–dark cycle. Most of the studies carried out so far emphasize the crucial role of the periodicity imposed by the light–dark cycle in neuronal synchronization. Nevertheless, heterogeneity as a natural and permanent ingredient of these cellular interactions seemingly plays a major role in these biochemical processes. In this paper, we use a model that considers the neurons of the suprachiasmatic nucleus as chemically coupled modified Goodwin oscillators, and introduce non-negligible heterogeneity in the periods of all neurons in the form of quenched noise. The system response to the light–dark cycle periodicity is studied as a function of the interneuronal coupling strength, external forcing amplitude and neuronal heterogeneity. Our results indicate that the right amount of heterogeneity helps the extended system to respond globally in a more coherent way to the external forcing. Our proposed mechanism for neuronal synchronization under external periodic forcing is based on heterogeneity-induced oscillator death, damped oscillators being more entrainable by the external forcing than the self-oscillating neurons with different periods.

Interaction of Antimalarial Drug Quinacrine with Nucleic Acids of Variable Sequence Studied by Spectroscopic Methods

Abstract The interaction of antimalarial drug quinacrine (QA) with polynucleotides is studied by UV- visible absorption, fluorescence and surface-enhanced Raman spectroscopy(SERS). The polynucleotides employed for such a study were calf thymus DNA, poly(A).poly(T), poly(A).poly(U), poly(C).poly(G) and poly(dG-dC).poly(dG-dC). Absorption and fluorescence spectra of QA complexes indicate that an interaction with the biomolecule is taking place, although different interaction mechanisms are probable depending on the sequence. The SERS spectra also reflect spectral changes which depend on the polymer sequence and that can be correlated to those observed by fluorescence, with the advantage of the detailed structural information provided by this vibrational technique. QA interacts with polynucleotides through its diprotonated form and by ring stacking. The strength of such interaction is extremely sequence dependent, thus suggesting different interaction mechanisms in each case. The SERS technique allows the simultaneous study of those polynucleotide moieties that are directly involved in the interaction thanks to the short-range character of the SERS spectroscopy. The interaction of QA with the above nucleic acids lead to a different change in the chain stability and flexibility which is further related to the different denaturation tendency of the polymer in the presence of the metal surface.

Affinity chromatography of plasma proteins (guanidinobenzoatase): use of mimetic matrices and mimetic soluble ligands to prevent the binding of albumin on target affinity matrices

Serum albumin is the most abundant protein in plasma and it has a high capacity to bind many small compounds and macromolecules. In this way, albumin may promote important interferences during affinity chromatography of plasma proteins. Guanidinobenzoatase (GB) is a very relevant plasma protease that seems to be related to tumoral processes. This enzyme may be adsorbed on tailor-made agmatine-amide-agarose (CH-A) supports (e.g., the ones having 2 micromol of guanidino groups per ml of agarose attached to the support, through a 6 C aliphatic chain). Such tailor-made supports containing a very low concentration of ionized groups are hardly able to adsorb any protein by anion-exchange. However, they are able to strongly adsorb albumin. In order to solve this problem new mimetic affinity matrices have been designed: (i) by using the same ligand immobilized through a different chemical linkage [guanidino groups attached via secondary amino bonds, (AEA)] or (ii) by using slightly different ligands (e.g., 1,8-octanediamine containing a primary amino group instead of a guanidino one) also attached to the support via amido bonds (CH-DAO). Albumin adsorbs on the target and on the two mimetic matrices while GB is mainly adsorbed on the target one. Moreover, the adsorption of albumin on the affinity matrix (CH-A) is very strongly inhibited by the presence of low concentrations of soluble ligands (e.g., 1,8-octanediamine containing two ionized primary amino groups). On the contrary, the adsorption of GB on CH-A is hardly inhibited by the presence of such mimetic soluble ligand. In this way, the former offering of crude GB samples to AEA plus the use of mimetic inhibitors during adsorption of the extract on CH-A completely prevent the undesirable adsorption of albumin. In a such way, an extremely selective adsorption of GB can be performed. Such an improved chromatography procedure allows a very easy affinity purification and detection of GB.

Oscillation patterns in tori of modified FHN neurons

Abstract We analyze the dynamics of a network of electrically coupled, modified FitzHugh–Nagumo (FHN) oscillators. The network building-block architecture is bidimensional squared array shaped as torus, with unidirectional nearest neighbor coupling in both directions. Linear approximation about the origin of a single torus reveals that the array is able to oscillate via a Hopf bifurcation, controlled by the interneuronal coupling constants. Group theoretic analysis of the dynamics of one torus leads to discrete rotating waves moving diagonally in the squared array under the influence of the direct product group Z N × Z N × Z 2 × S 1 . Then, we studied the existence multifrequency patterns of oscillations, in networks formed by two coupled tori. We showed that when acting on the traveling waves, this group leaves them unchanged, while when it acts on the in-phase oscillations, they are shifted in time by ϕ. We therefore proved the possibility of a pattern of oscillations in which one torus produces traveling waves of constant phase shift, while the second torus shows synchronous in-phase oscillations, at N-times the frequency shown by the traveling waves.

Shear flow dynamics in the Beris-Edwards model of nematic liquid crystals

We consider the Beris-Edwards model describing nematic liquid crystal dynamics and restrict it to a shear flow and spatially homogeneous situation. We analyse the dynamics focusing on the effect of the flow. We show that in the co-rotational case one has gradient dynamics, up to a periodic eigenframe rotation, while in the non-co-rotational case we identify the short- and long-time regimes of the dynamics. We express these in terms of the physical variables and compare with the predictions of other models of liquid crystal dynamics.

Global dynamics of a family of 3-D Lotka–Volterra systems

In this article, we analyse the flow of a family of three-dimensional Lotka–Volterra systems restricted to an invariant and bounded region. The behaviour of the flow in the interior of this region is simple: either every orbit is a periodic orbit or orbits move from one boundary to another. Nevertheless, the complete study of the limit sets in the boundary allows one to understand the bifurcations which take place in the region as a global bifurcation that we denote by focus-centre-focus bifurcation.