Metadynamics study of a β-hairpin stability in mixed solvents

Understanding the molecular mechanisms that allow some organisms to survive in extremely harsh conditions is an important achievement that might disclose a wide range of applications and that is constantly drawing the attention of many research fields. The high adaptability of these living creatures is related to the presence in their tissues of a high concentration of osmoprotectants, small organic, highly soluble molecules. Despite osmoprotectants having been known for a long time, a full disclosure of the machinery behind their activity is still lacking. Here we describe a computational approach that, taking advantage of the recently developed metadynamics technique, allows one to fully describe the free energy surface of a small β-hairpin peptide and how it is affected by an osmoprotectant, glycine betaine (GB) and for comparison by urea, a common denaturant. Simulations led to relevant thermodynamic information, including how the free energy difference of denaturation is affected by the two cosolvents; unlike urea, GB caused a considerable increase of the folded basin stability, which transposes into a higher melting temperature. NMR experiments confirmed the picture derived from the theoretical study. Further molecular dynamics simulations of selected conformations allowed investigation into deeper detail the role of GB in folded state protection. Simulations of the protein in GB solutions clearly showed an excess of osmoprotectant in the solvent bulk, rather than in the protein domain, confirming the exclusion from the protein surface, but also highlighted interesting features on its interactions, opening to new scenarios besides the classic "indirect mechanism" hypothesis.     

Flow injection preconcentration by chemofiltration and spectrophotometric determination of Gd

A new on-line Gd preconcentration and determination system associated to flow injection (FI) method was developed. 2,2′-(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo) bisbenzenearsonic acid (Arsenazo III) was used as a complexing agent at pH 2.5. A reactor containing the polyamide membrane was used for the retention of the Gd complex by chemofiltration. The complex was then removed from the reactor with buffer solution pH 9. The variables affecting the combined on-line preconcentration-absorptiometric method have been evaluated and optimised. The coupling of the on-line preconcentration and spectrophotometric flow through detection led to a detection limit of 15 μg l⁻¹ for a preconcentration time of 5 min at 2 ml/min. The method was successfully applied to biological samples.     

Determination of ethyl formate in magnesium chloride used as Ziegler-Natta catalyst support by Fourier transform infrared attenuated total reflectance spectrometry

FT-IR Spectroscopy with total attenuated reflectance (ATR) was used for the quantitative determination of ethyl formate coordinated in inorganic matrices such as magnesium chloride. The spectra were recorded directly on solutions obtained by dissolving the inorganic Ziegler-Natta supports in water. The method proposed is sensitive and rapid and appears to be very reliable when compared with other methods, such as solvent extraction followed by gaschromatographic or thermogravimetric analysis.     

On the Decompositions of a Quantum State

We classify all the decompositions of a quantum state as a weighted sum of one dimensional projectors. In particular we describe explicitly the set of irreducible decompositions. The physical interest in this problem rests on the possibility of interpreting the decomposition in terms of a classical mixture.     

The density function of the number of moves to complete the Towers of Hanoi puzzle

Any sequence of legal moves leads the Towers of Hanoi puzzle to an arrangement from which the final configuration must be built up. A recursive algorithm which finishes off the puzzle is considered and, assuming a uniform distribution on the possible unfinished situations, the density function of the number of moves it takes is derived.     

Omomorfismi fra grafi e grafi moltiplicativi

Sunto Si studiano vari tipi di omomorfismi fra grafi aventi un arbitrario numero di vertici per ciascun elemento: si studiano poi le relazioni fra gli omomorfismi d'un grafo moltiplicativo in un altro e quelli fra i grafi soggiacenti. Alla memoria di Guido Castelnuovo, nel primo centenario della nascita.     

Sull’approssimazione di una trasformazione fra piani mediante trasformazioni quadratiche

Riassunto Si dimostra che le corrispondenze fra due piani che in una coppia generica ammettono una trasformazione quadratica quasi prossima d’ordine 3 94) sono quelle le cui curve caratteristiche sono rette (rispettivamente le trasformazioni quadratiche).

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Résumé On démontre que les correspondences entre deux plans qui dans un couple générique possèdent une transformation quadratique presque prochaine d’ordre 3 (4) sont celles qu’ont pour courbes caractéristiques des droites (ou respectivement les transformations quadratiques).

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Lavoro eseguito nell’ambito dell’attività dei Gruppi di Ricerca Matematica del Consiglio Nazionale delle Ricerche.