Disordered impenetrable two-component fermions in one dimension

We study the one-dimensional Hubbard model for two-component fermions with infinitely strong on-site repulsion (t - 0 model) in the presence of disorder. Our analytical treatment demonstrates that the type of disorder drastically changes the nature of the emerging phases. The case of spin-independent disorder can be treated as a single-particle problem with Anderson localization. On the contrary, recent numerical findings show that spin-dependent disorder, which can be realized as a random magnetic field, leads to the many-body localization-delocalization transition. We find an explicit analytic expression for the matrix elements of the random magnetic field between the eigenstates of the t - 0 model with potential disorder on a finite lattice. Analysis of the matrix elements supports the existence of the many-body localization-delocalization transition in this system and provides an extended physical picture of the random magnetic field.</p

Preparation of facilities for fundamental research with ultracold neutrons at PNPI

The WWR-M reactor of PNPI offers a unique opportunity to prepare a source for ultracold neutrons (UCN) in an environment of high neutron flux (about 3*10^12 n/cm^2/s) at still acceptable radiation heat release (about 4*10^-3 W/g). It can be realized within the reactor thermal column situated close to the reactor core. With its large diameter of 1 m, this channel allows to install a 15 cm thick bismuth shielding, a graphite premoderator (300 dm^3 at 20 K), and a superfluid helium converter (35 dm^3). At a temperature of 1.2 K it is possible to remove the heat release power of about 20 W. Using the 4pi flux of cold neutrons within the reactor column can bring more than a factor 100 of cold neutron flux incident on the superfluid helium with respect to the present cold neutron beam conditions at the ILL reactor. The storage lifetime for UCN in superfluid He at 1.2 K is about 30 s, which is sufficient when feeding experiments requiring a similar filling time. The calculated density of UCN with energy between 50 neV and 250 neV in an experimental volume of 40 liters is about 10^4 n/cm^3. Technical solutions for realization of the project are discussed.Comment: 10 pages, more detail

Quantitative Treatment of Decoherence

We outline different approaches to define and quantify decoherence. We argue that a measure based on a properly defined norm of deviation of the density matrix is appropriate for quantifying decoherence in quantum registers. For a semiconductor double quantum dot qubit, evaluation of this measure is reviewed. For a general class of decoherence processes, including those occurring in semiconductor qubits, we argue that this measure is additive: It scales linearly with the number of qubits.Comment: Revised version, 26 pages, in LaTeX, 3 EPS figure

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Characterisation of bacterially expressed structural protein E2 of hepatitis C virus

The E2 glycoprotein is a structural component of the hepatitis C virus (HCV) virion. It interacts with putative cellular receptors, elicits production of neutralising antibodies against the virus, and is involved in viral morphogenesis. The protein is considered as a major candidate for anti-HCV vaccine. Despite this, relatively little is known about this protein. Previous studies have focused on the antigenic and functional analysis of the glycosylated forms. This report describes expression of the ectodomain of E2 (recE2) in Escherichia coli cells, its purification, and initial characterisation of its structural and functional properties. It is demonstrated that the purified protein forms small soluble aggregates, which retain functional characteristics of its native counterpart, i.e., it interacts with a putative cellular receptor, CD81, and is recognised by both conformation-dependent and -independent anti-E2 monoclonal antibodies

Dimerization of the antimicrobial peptide polyphemusin I into one polypeptide chain: Theoretical and practical consequences

A strategy of sequential dimerization of monomers of antimicrobial peptides (AMPs) into one polypeptide chain has been implemented on the example of a beta-structural AMP polyphemusin I which is one of the most effective candidate for use as an antibiotic. The possible polyphemusin I monomer and dimer structures in lipid membrane were studied in this work via molecular modeling. To this end, these molecules were chemically synthesized so that the dimer represented two monomers connected in series into one polypeptide chain with a flexible linker. The antimicrobial effects of monomer and dimer were then tested on various bacterial cultures, and their similarity was shown. Therefore, we can conclude that the pore formation is not a putative mechanism of the polyphemusin I action. © 2019 State Research Institute for Genetics and Selection of Industrial Microorganisms

Получение и характеристика безметионинового варианта термостабильного шаперона GroEL из Thermus thermophilus

Chaperone GroEL is the subject of extensive studies concerning its organization and functioning at folding cell proteins as well as the possibility of using it when developing new expression systems to obtain recombinant proteins in soluble forms. In this work, a new variant of Thermus thermophilus chaperone GroEL is developed, in which all methionine residues are substituted for leucine residues. Obtained variant of the chaperone was purified to homogeneity. Modified GroEL retained tetruary structure characteristic for initial chaperone consisting of double heptamer, and also retained the initial thermostability. Obtained variant of the chaperone is interesting not only from the point of view of GroEL structural organization, but it can be effectively used as a leader to obtain recombinant target proteins. The substitution of methionine residues in a leader allows to significantly simplify the following purification of target polypeptide.Шаперон GroEL является объектом интенсивных исследований как с точки зрения его устройства и функционирования при сворачивании белков в клетке, так и возможностей использования в создаваемых системах экспрессии для получения рекомбинантных белков в растворимых формах. Создан вариант шаперона GroEL из Thermus thermophilus, в котором все остатки метионина заменены на остатки лейцина. Полученный вариант шаперона был очищен до гомогенного состояния. Измененный GroEL полностью сохранял четвертичную структуру, присущую исходному шаперону, в виде частицы из двойного гептамера, а также сохранял исходную термостабильность. Полученный вариант шаперона интересен не только с точки зрения структурной организации GroEL, но и может быть эффективно использован в качестве белка-носителя при получении целевых рекомбинантных белков. Замена остатков метионина в белке-носителе позволяет существенно упростить последующие процедуры очистки целевых полипептидов