Resonantly enhanced nonlinear optics in semiconductor quantum wells: An application to sensitive infrared detection

A novel class of coherent nonlinear optical phenomena, involving induced transparency in quantum wells, is considered in the context of a particular application to sensitive long-wavelength infrared detection. It is shown that the strongest decoherence mechanisms can be suppressed or mitigated, resulting in substantial enhancement of nonlinear optical effects in semiconductor quantum wells.Comment: 4 pages, 3 figures, replaced with revised versio

States and transitions in black-hole binaries

With the availability of the large database of black-hole transients from the Rossi X-Ray Timing Explorer, the observed phenomenology has become very complex. The original classification of the properties of these systems in a series of static states sorted by mass accretion rate proved not to be able to encompass the new picture. I outline here a summary of the current situation and show that a coherent picture emerges when simple properties such as X-ray spectral hardness and fractional variability are considered. In particular, fast transition in the properties of the fast time variability appear to be crucial to describe the evolution of black-hole transients. Based on this picture, I present a state-classification which takes into account the observed transitions. I show that, in addition to transients systems, other black-hole binaries and Active Galactic Nuclei can be interpreted within this framework. The association between these states and the physics of the accretion flow around black holes will be possible only through modeling of the full time evolution of galactic transient systems.Comment: 30 pages, 11 figures, To appear in Belloni, T. (ed.): The Jet Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009

An entropic safety catch controls Hepatitis C virus entry and antibody resistance

E1 and E2 (E1E2), the fusion proteins of Hepatitis C Virus (HCV), are unlike that of any other virus yet described, and the detailed molecular mechanisms of HCV entry/fusion remain unknown. Hypervariable region-1 (HVR-1) of E2 is a putative intrinsically disordered protein tail. Here, we demonstrate that HVR-1 has an autoinhibitory function that suppresses the activity of E1E2 on free virions; this is dependent on its conformational entropy. Thus, HVR-1 is akin to a safety catch that prevents premature triggering of E1E2 activity. Crucially, this mechanism is turned off by host receptor interactions at the cell surface to allow entry. Mutations that reduce conformational entropy in HVR-1, or genetic deletion of HVR-1, turn off the safety catch to generate hyper-reactive HCV that exhibits enhanced virus entry but is thermally unstable and acutely sensitive to neutralising antibodies. Therefore, the HVR-1 safety catch controls the efficiency of virus entry and maintains resistance to neutralising antibodies. This discovery provides an explanation for the ability of HCV to persist in the face of continual immune assault and represents a novel regulatory mechanism that is likely to be found in other viral fusion machinery

Evolutionary remodelling of N-terminal domain loops fine-tunes SARS-CoV-2 spike

The emergence of SARS-CoV-2 variants has exacerbated the COVID-19 global health crisis. Thus far, all variants carry mutations in the spike glycoprotein, which is a critical determinant of viral transmission being responsible for attachment, receptor engagement and membrane fusion, and an important target of immunity. Variants frequently bear truncations of flexible loops in the N-terminal domain (NTD) of spike; the functional importance of these modifications has remained poorly characterised. We demonstrate that NTD deletions are important for efficient entry by the Alpha and Omicron variants and that this correlates with spike stability. Phylogenetic analysis reveals extensive NTD loop length polymorphisms across the sarbecoviruses, setting an evolutionary precedent for loop remodelling. Guided by these analyses, we demonstrate that variations in NTD loop length, alone, are sufficient to modulate virus entry. We propose that variations in NTD loop length act to fine-tune spike; this may provide a mechanism for SARS-CoV-2 to navigate a complex selection landscape encompassing optimisation of essential functionality, immune-driven antigenic variation and ongoing adaptation to a new host

Well-defined 2:1 and 2:2 arylcopper-copper bromide aggregates and selective biaryl formation

Reaction of tetranuclear [Li4(C6H4CH2NMe2-2)4] (1) with 4/3 equiv of the copper(I) arenethiolate [Cu3(SC6H4NMe2-2)3] (2) results in the quantitative formation of the arylcopper compound [Cu4(C6H4CH2NMe2-2)4] (3) and the lithium arenethiolate [Li(SC6H4NMe2-2)]n (4). The arylcopper species 3 is a yellow air-sensitive solid, which is tetrameric in the solid state as well as in solution. The reaction of the dinuclear aryllithium reagent [Li2{C6H4(CH2N(Me)CH2CH2NMe2)-2}2] (5) with 3 equiv of CuBr affords the trinuclear mixed 2:1 arylcopper-copper bromide aggregate [Cu3Br{C6H4(CH2N(Me)CH2CH2NMe2)-2}2] (6). The 1:4 reaction of 5 with CuBr affords the tetranuclear mixed 2:2 aggregate [Cu4Br2{C6H4(CH2N(Me)CH2CH2NMe2)-2}2] (7). As solids, aggregates 6 and 7 are less air-sensitive than 3, but their solutions rapidly turn green upon exposure to air. The 1:2 reaction of the naphthyllithium reagent [Li{1-C10H6(CH2N(Me)CH2CH2NMe2)-2}] (9) with CuBr affords the tetranuclear mixed 2:2 aggregate [Cu4Br2{1-C10H6(CH2N(Me)CH2CH2NMe2)-2}2] (10), in which there is a 1:1 arylcopper to copper bromide ratio. Aggregate 10 is obtained as brownish yellow crystals, and solutions of 10 slowly turn bluish green upon exposure to air. The molecular structures of 6 and 10 both contain an R-Cu-R fragment and can be described as comprising a [CuR2]- unit stabilized by a [Cu2Br]+ or a [Cu3Br2]+ cation, respectively. Heating solutions of 6 (55 C; 15 min) and solutions of 10 (110 C; 19 h) results in quantitative biaryl (6) and moderate binaphthyl (10) formation, respectively. The relevance of neutral aggregates such as 6, 7, and 10 as model complexes both for the interpretation of bonding in higher order cuprate species and for understanding of their reactivity is discussed