Different effects of redundant feedback loops on a bistable switch

Bistable switches have important roles in cellular decision-making processes. Bistability can be the consequence of positive or double-negative feedback loops. Although necessary, such feedback is not sufficient for bistability, which also requires nonlinearity. Nonlinearity can be provided by synergy of multiple feedback loops or by an ultrasensitive response within a single feedback loop. However, these two possibilities are not mutually exclusive; a combination of them is also possible. Here we analyze a biochemical regulatory network that controls a crucial cell cycle transition in all eukaryotic cells and contains multiple redundant feedback loops and nonlinearity. We show in this realistic biological example that two redundant feedback loops have different effects on the position of one of the saddle-node bifurcations of the system, which determines where the system switches. This illustrates that even though the roles of positive and double-negative feedbacks have been regarded as equivalent, the difference in their architectures can lead to differences in their effects on the system. We speculate that this conclusion could be general for other bistable systems with redundant feedback loops.     

Cation Ordering and Exsolution in Copper-Containing Forms of the Flexible Zeolite Rho (Cu,M-Rho; M=H,Na) and Their Consequences for CO2 Adsorption

The flexibility of the zeolite Rho framework offers great potential for tunable molecular sieving. The fully copper-exchanged form of Rho and mixed Cu,H- and Cu,Na-forms have been prepared. EPR spectroscopy reveals that Cu²⁺ ions are present in the dehydrated forms and Rietveld refinement shows these prefer S6R sites, away from the d8r windows that control diffusion. Fully exchanged Cu-Rho remains in an open form upon dehydration, the d8r windows remain nearly circular and the occupancy of window sites is low, so that it adsorbs CO₂ rapidly at room temperature. Breakthrough tests with 10 % CO₂/40 % CH₄ mixtures show that Cu_4.9-Rho is able to produce pure methane, albeit with a relatively low capacity at this p_CO2 due to the weak interaction of CO₂ with Cu cations. This is in strong contrast to Na-Rho, where cations in narrow elliptical window sites enable CO₂ to be adsorbed with high selectivity and uptake but too slowly to enable the production of pure methane in similar breakthrough experiments. A series of Cu,Na-Rho materials was prepared to improve uptake and selectivity compared to Cu-Rho, and kinetics compared to Na-Rho. Remarkably, Cu,Na-Rho with >2 Cu cations per unit cell exhibited exsolution, due to the preference of Na cations for narrow S8R sites in distorted Rho and of Cu cations for S6R sites in the centric, open form of Rho. The exsolved Cu,Na-Rho showed improved performance in CO₂/CH₄ breakthrough tests, producing pure CH₄ with improved uptake and CO₂/CH₄ selectivity compared to that of Cu_4.9-Rho.     

The proton resonance spectra of heterocycles—VI: The correlation of substituent effects on chemical shifts in bicyclic compounds

The proton chemical shifts are reported for monosubstituted naphthalenes, quinolines and quinoxalines. Together with literature data, these chemical shifts are compared with the parent compounds and substituent effects evaluated statistically. The effect of substituents parallels that in benzenes, but is modified by bond fixation, steric hindrance and other effects which can, at least qualitatively be understood. The treatment enables estimation of likely chemical shifts for ABC spectra in fused aromatic systems which should facilitate the solution of such spectra.     

Continuum description of the cytoskeleton: ring formation in the cell cortex

Motivated by the formation of ringlike filament structures in the cortex of plant and animal cells, we study the dynamics of a two-dimensional layer of cytoskeletal filaments and motor proteins near a surface by a general continuum theory. As a result of active processes, dynamic patterns of filament orientation and density emerge via instabilities. We show that self-organization phenomena can lead to the formation of stationary and oscillating rings. We present state diagrams that reveal a rich scenario of asymptotic behaviors and discuss the role of boundary conditions.     

Synthesis,structural and spectral investigations of new types of dioxovanadium(V) complexes withS-methylthiosemicarbazones derived from salicyl and 2,4-dihydroxybenzaldehydes. X-ray structure of ammonium(salicylaldehydeS-methylthiosemicarbazonato) dioxovanadate(V) monohydrate

Summary The x-ray crystal structure of [VO₂(HL¹)] (where L¹ denotes the dianion of theS-methylthiosemicarbazone of salicylaldehyde) has been determined and refined to R=0.058 (R_w=0.063) for 3377 observed reflections.There are two symmetrically independent molecules in the asymmetric unit, showing no significant differences in their geometries. The vanadium atom is pentacoordinated in a distorted square-pyramidal arrangement.Absorption spectra of [VO₂(HL¹)] and [VO₂(HL²)], in the 10000–45000 cm^–1 range, were calculated from the measured reflectance spectra, applying Kubelka-Munk's theory. The extinction coefficients were determined from the absorption spectra of the solutions of these complexes in methanol. The observed maxima are interpreted on the basis of intraligand transition and charge-transfer spectra on the basis of the presence of approximate groupC _2v for both complexes.     

Quantifying the intrinsic effects of two point mutation models of pro-pro-pro triamino acid diamide. A first-principle computational study

Stabilities and conformational properties of two Pro --> Thr point mutation models were computed at the B3LYP/6-31G(d) level of theory for the parent triamino acid diamide Pro-Pro-Pro (HCO-Pro-Pro-Pro-NH2). Geometrical parameters for the amino acid sequences, used in the molecular orbital computations for Pro-Pro-Thr and Pro-Thr-Pro, were retrieved from the Protein Data Bank. Thermodynamic functions (S, H, G) were computed for the fully optimized geometries. To assess the stabilization energetics of these mutant models, relative to the parent Pro-Pro-Pro reference conformer epsilon(L) epsilon(L) gamma(L), isodesmic reactions were constructed to calculate DeltaS, DeltaH, and DeltaG. The importance of intramolecular hydrogen bonds involving the -OH group of the Thr side chain, which emerged after the point mutations, was also examined to determine the internal stabilization of these peptide models. This study describes an approach to analyzing a point mutation at the center of a peptide chain and compares its stability to that of a point mutation at a terminal end in a small peptide model.     

Transition state infrared spectra for the trans-->cis isomerization of a simple peptide model

Trans-->cis isomerization of N-methylacetylamide (MeCO-NHMe) has been studied at the G3MP2B3 level of theory and the vibration spectrum has been calculated as a function of the torsional mode of motion along the peptide bond. Noticeable spectral differences have been observed for the transition state interconnecting the cis and trans isomers.     

Synthesis,characterisation and antifungal activity of a series of Cobalt(II) and Nickel(II) complexes with ligands derived from reduced <Emphasis Type="Italic">N</Emphasis>, <Emphasis Type="Italic">N</Emphasis>′-<Emphasis Type="Italic">o</Emphasis>-Phenylenebis(Salicylideneimine)

The synthesis and characterisation by elemental analysis, conductivity, FTIR, UV–Visible, ESR and magnetic measurements are described for a series of complexes of nickel(II) and cobalt(II) with three ligands (H₂L^1–3) derived from reduced N, N′-o-Phenylenebis(salicylideneimine). The complexes formed are identified as neutral species, where the ligands are coordinated through N and O donor atoms. The formulae obtained for the complexes are: [CoL(H₂O)₂] with octahedral geometry and [NiL] with tetrahedral geometry. Their antifungal activity is evaluated towards human pathogenic fungi including yeasts of the Candida genus, some opportunistic moulds belonging to the Aspergillus, Scedosporium genus and some dermatophytes. The cobalt complexes show a significant growth inhibition of yeasts tested and also to fungi of the genus Scedosporium which is of interest because these fungi are usually poorly susceptible to current antifungal including Amphotericin B and Itraconazole, chosen as reference in this study. The activity data show that the metal complexes are more potent than the parent ligand.     

Somigliana's method applied to plane problems of elastic half-spaces

In this paper Somigliana's method has been utilized to solve the two-dimensional boundary value problems of elastic half-spaces. We have considered traction and displacement problem, shear problem, contact and crack problems and the solutions, thus derived, have been compared with those derived from dislocation considerations.