From calls to communities: a model for time-varying social networks

Social interactions vary in time and appear to be driven by intrinsic mechanisms thatshape the emergent structure of social networks. Large-scale empirical observations ofsocial interaction structure have become possible only recently, and modelling theirdynamics is an actual challenge. Here we propose a temporal network model which builds onthe framework of activity-driven time-varying networks with memory. Themodel integrates key mechanisms that drive the formation of social ties – socialreinforcement, focal closure and cyclicclosure, which have been shown to give rise to community structure andsmall-world connectedness in social networks. We compare the proposed model with areal-world time-varying network of mobile phone communication, and show that they shareseveral characteristics from heterogeneous degrees and weights to rich communitystructure. Further, the strong and weak ties that emerge from the model follow similarweight-topology correlations as real-world social networks, including the role of weakties.     

Synthesis and structural characterization of cis- and trans-fused 4a,5,6,7,8,8a-hexahydro-2H,4H-1,3-benzodithiines and their 2-methyl and 2,2-dimethyl derivatives

Both cis- and trans-fused 4a,5,6,7,8,8a-hexahydro-2H,4H-1,3-benzodithiine together with their 2-methyl and 2,2-dimethyl derivatives were prepared as racemates from the appropriate dithiols obtained via multistep syntheses. The products were characterized by (1)H and (13)C NMR, mass spectrometry, and for two of the cis-fused compounds by X-ray diffraction. (1)H,(1)H vicinal coupling constants indicated that all compounds attain chair-chair conformations as their predominant conformations. All three trans-fused isomers exist in totally biased chair-chair conformations and are essentially conformationally locked, whereas the cis-fused compounds are conformationally mobile and can potentially attain either the S-in or the S-out conformation. The interconversion of the conformers is fast on the NMR time-scale at ambient temperatures, but at 213 K 4ar,5,6,7,8,8ac-hexahydro-1,3-benzodithiine freezes out into a 83:17 mixture of the S-in and S-out forms, respectively. Both 2c-methyl-4ar,5,6,7,8,8ac-hexahydro-1,3-dithiine and the dimethyl derivative adopt almost exclusively the S-in conformer at ambient temperature whereas 2t-methyl-4ar,5,6,7,8,8ac-hexahydro-1,3-dithiine is a 5:1 mixture of the S-out and S-in conformers.     

Synthesis and X‐ray Structures of the Polycyclic Dimers {[PhB(μ‐NtBu)2AsN(tBu)H]LiI}2 and [PhB(μ‐NtBu)2AsN(tBu)Li]2

The metathesis of [PhB(μ‐NtBu)₂]AsCl and tBuN(H)Li in 1:1 molar ratio in diethyl ether produced the amido derivative [PhB(μ‐NtBu)₂AsN(tBu)H] ( 1 ) in good yield. The lithiation of 1 with one equivalent of nBuLi afforded the lithium salt [PhB(μ‐NtBu)₂AsN(tBu)Li] ( 2a ). Both 1 and 2a were characterized by multinuclear NMR spectroscopy. The crystal structure of 2a is comprised of a U‐shaped, centrosymmetric dimer in which the monomeric [PhB(μ‐NtBu)₂AsN(tBu)]^?Li⁺ units are linked by Li‐N interactions to give a six‐rung ladder. Oxidation of 2a with one‐half equivalent of I₂ in diethyl ether resulted in hydrogen abstraction from the solvent to give the dimeric lithium iodide adduct {[PhB(μ‐NtBu)₂AsN(tBu)H]LiI}₂ ( 1 ·LiI) with a central Li₂I₂ ring.     

Selective N3- and 5′-O-Alkylation of 2′,3′-O-isopropylideneuridine with methyl iodide

The 2′,3′-O-isopropylideneuridine ( 1 ) reacts with MeI in the presence of an excess of NaH in THF giving 2′,3′-O-isopropylidene-5′-O-methyluridine ( 2 ). Prolonged reaction time gives rise to 2′,3′-O-isopropylidene-3,5′-O-dimethyluridine ( 4 ). The use of an equimolar amount of base and alkylating agent results predominantly in methylation at N(3) (→ 3).     

Interferometric studies of interfaces at milliKelvin temperatures

A brief review of our recent work on interfaces in quantum systems at millikelvin temperatures is presented. In this paper, we concentrate on high-resolution interferometry on superfluid/solid interface in⁴He. Our results show a novel surface transition at small inclination angles off the c-axis, slow facet growth which cannot be assigned to the regular screw-dislocation-mediated mechanism, and fast spiral growth moderated by step localization.     

Matrix Gla protein inhibits ectopic calcification by a direct interaction with hydroxyapatite crystals

Mice lacking the gene encoding matrix gla protein (MGP) exhibit massive mineral deposition in blood vessels and die soon after birth. We hypothesize that MGP prevents arterial calcification by adsorbing to growing hydroxyapatite (HA) crystals. To test this, we have used a combined experimental-computational approach. We synthesized peptides covering the entire sequence of human MGP, which contains three sites of serine phosphorylation and five sites of γ-carboxylation, and studied their effects on HA crystal growth using a constant-composition autotitration assay. In parallel studies, the interactions of these sequences with the {100} and {001} faces of HA were analyzed using atomistic molecular dynamics (MD) simulations. YGlapS (amino acids 1-14 of human MGP) and SK-Gla (MGP43-56) adsorbed rapidly to the {100} and {001} faces and strongly inhibited HA growth (IC(50) = 2.96 μg/mL and 4.96 μg/mL, respectively). QR-Gla (MGP29-42) adsorbed more slowly and was a moderate growth inhibitor, while the remaining three (nonpost-translationally modified) peptides had little or no effect in either analysis. Substitution of gla with glutamic acid reduced the adsorption and inhibition activities of SK-Gla and (to a lesser extent) QR-Gla but not YGlapS; substitution of phosphoserine with serine reduced the inhibitory potency of YGlapS. These studies suggest that MGP prevents arterial calcification by a direct interaction with HA crystals that involves both phosphate groups and gla residues of the protein. The strong correlation between simulated adsorption and measured growth inhibition indicates that MD provides a powerful tool to predict the effects of proteins and peptides on crystal formation.     

Synthesis, 1 H, 13 C, 15 N, and 113 Cd NMR, ESI-TOF MS, Semiempirical MO (PM3), ab initio/HF and Cation/Anion Binding Studies of N-deoxycholyl-l-tryptophan

The synthesis, structural characterization, and cation/anion binding properties ofa new bile acid-amino acid conjugate, N-deoxycholyl-l-tryptophan, aredescribed. The structures of the ligand and its cadmium adduct at different pHconditions and various cadmium perchlorate concentrations were determined bymodern multinuclear magnetic resonance spectroscopic as well as ESI-TOF MStechniques. Also semiempirical PM3 and ab initio/HF molecular modellingstudies were performed. Based on ¹H,¹H NOESY measurementsN-deoxycholyl-l-tryptophan in alkaline conditions was found to appearin a bent conformation which was clearly different from the conformations in neutraland acidic solutions. According to molecular modelling in its minimum energy structurethe tryptophan backbone of the ligand was folded close to the deoxycholic acid skeletonand the structure was stabilized by an intramolecular hydrogen bond. The multinuclearmagnetic resonance experiments indicated that Cd²⁺-cation was bound with theligand in neutral and alkaline conditions while in acidic conditions protons block thebinding site. ESI-TOF MS revealed clearly a competition between sodium and cadmiumions, the ligand having a stronger affinity for sodium. Cadmium binding occurred onlywhen excess of cadmium was used. Further, ESI-TOF MS spectra showed that variouschlorine oxyanions originated from perchlorate anion formed together with cationsdifferent adducts with the ligand.     

Colour interference and confinement effects in W-pair production

In the reactione ⁺ e ^– W ⁺ W ^– ( ) ( ) we expect that normally the colour singlet systems ( ) and ( ) hadronize independently into two hadron chains or strings. However also the pairs ( ) and ( ) form colour singlets with probability 1/N _c ² =1/9. This probability could be further enhanced by gluon exchange. We therefore expect that the hadronization can give recoupled colour strings between these quark-antiquark pairs with some small probability, and a study of this effect will give interesting information about the vacuum structure. In this paper we discuss a possible experimental signal for recoupled events and show that such events can be identified also when the effect of gluon emission before the recoupling is taken into account.