A comment on “The Photon Momentum”

In the paper “The Photon Momentum” [1], Dr. Umul makes the erroneous assumption that a photon can be absorbed by a free electron and then discusses the paradoxical consequences of this assumption. In this comment the correct physics is discussed (i.e. Compton scattering [2]).     

High-energy single-transverse-mode Q-switched fiber laser based on a multimode large-mode-area erbium-doped fiber

We demonstrate that appropriately designed doped multimode fibers provide robust single-mode output when used within a fiber laser cavity. Using a novel large-mode-area fiber, we demonstrate what we believe to be record single-mode (M(2) <1.2) pulse energies of >0.5 mJ from a Q -switched fiber laser and even higher pulse energies (as high as 0.85 mJ) with slightly compromised spatial-mode quality (M(2)<2.0) . This approach offers significant scope for extending the range of single-mode output powers and energies that are achievable from fiber-laser-amplifier systems.     

All-optical AND gate based on coupled gap-soliton formation in a fiber Bragg grating

We experimentally demonstrate an all-optical AND gate based on coupled gap-soliton formation in an apodized fiber Bragg grating. A switching contrast of better than 17 dB is obtained with an incident pulse peak power of 2.5 kW.     

The characterisation of a novel, covalently modified, amphiphilic alginate derivative, which retains gelling and non-toxic properties

The characterisation of a novel amphiphilic material, Alg-C4, produced from butanol linked by esterification to alginate is presented. The novel derivative retains the gelling and non-toxic properties of native alginate. FTIR spectra of Alg-C4 contained the characteristic hydroxyl and carboxyl bands, but also featured additional peaks at 1736 and 1134 cm(-1), indicating the presence of ester bonds. NMR studies showed the presence of butyl groups. The endothermic peak and exothermic peak present in the DSC thermogram of native alginate were also apparent in the Alg-C4 thermogram, but had shifted to lower temperatures (from 106 to 87 degrees C and from 254 to 247 degrees C, respectively). In addition, the exothermic peak was significantly reduced for Alg-C4 (5 mW compared to 20 mW in native alginate). Scanning electron microscopy was used to examine surface topography. The native alginate beads appeared smooth while Alg-C4 beads had a different, rougher appearance. Using circular dichroism it was found that the ratio of mannuronic to guluronic residues in the Alg-C4 was markedly increased compared to the native alginate (1.33 to 2.47), suggesting the preferential esterification of butanol to the guluronic residues. Exposure of ovarian granulosa cells in vitro to the Alg-C4 material demonstrated that granulosa cell viability (MTT test) was unchanged when compared to native alginate, which is regarded as non-toxic. The novel material is very stable, giving identical FTIR, DSC and gelling performance after 12 months storage at temperatures ranging from 10 to 20 degrees C. The data support the successful preparation of a stable modified alginate with characteristic hydrophilic properties and, in addition, a novel hydrophobic character.     

Coupled mode equations with free carrier effects: a numerical solution

Abstact We present a numerical method for solving a set of coupled mode equations describing light propagation through a medium with a grating and free carriers. The carriers, which are excited by the light and decay exponentially in time, lower the refractive index, thus rendering the system nonlinear. The method is fourth-order accurate, efficient, stable, easy to implement and well suited for vector and parallel computers.     

Secondary structure analysis of peptides with relevance to iron–sulfur cluster nesting

Peptides coordinated to iron–sulfur clusters, referred to as maquettes, represent a synthetic strategy for constructing biomimetic models of iron–sulfur metalloproteins. These maquettes have been successfully employed as building blocks of engineered heme-containing proteins with electron-transfer functionality; however, they have yet to be explored in reactivity studies. The concept of iron–sulfur nesting in peptides is a leading hypothesis in Origins-of-Life research as a plausible path to bridge the discontinuity between prebiotic chemical transformations and extant enzyme catalysis. Based on past biomimetic and biochemical research, we put forward a mechanism of maquette reconstitution that guides our development of computational tools and methodologies. In this study, we examined a key feature of the first stage of maquette formation, which is the secondary structure of aqueous peptide models using molecular dynamics simulations based on the AMBER99SB empirical force field. We compared and contrasted S…S distances, [2Fe-2S] and [4Fe-4S] nests, and peptide conformations via Ramachandran plots for dissolved Cys and Gly amino acids, the CGGCGGC 7-mer, and the GGCGGGCGGCGGW 16-mer peptide. Analytical tools were developed for following the evolution of secondary structural features related to [Fe-S] cluster nesting along 100 ns trajectories. Simulations demonstrated the omnipresence of peptide nests for preformed [2Fe-2S] clusters; however, [4Fe-4S] cluster nests were observed only for the 16-mer peptide with lifetimes of a few nanoseconds. The origin of the [4Fe-4S] nest and its stability was linked to a “kinked-ribbon” peptide conformation. Our computational approach lays the foundation for transitioning into subsequent stages of maquette reconstitution, those being the formation of iron ion/iron–sulfur coordinated peptides. © 2018 Wiley Periodicals, Inc.     

Investigation of ligand interactions with human RXRα by hydrogen/deuterium exchange and mass spectrometry

Several different agonists of the retinoic X receptor alpha (hRXRalpha) were examined for their effects on the amide H/D exchange kinetics of the homodimeric protein using mass spectrometry. Some agonists, LG 100268, SR11246, and DHA, bind such that slower deuterium exchange-in occurs compared with 9-cis-retinoic acid (9-cis-RA), whereas others, fenretinide and methoprenic acid, result in poorer protection during binding and hence faster exchange-in. Protection against H/D exchange by different agonists and the inhibition of H/D exchange kinetics relative to 9-cis-RA varies markedly in different regions of the protein. Agonists LG 100268, SR11246, and DHA generally inhibit faster exchange processes in the ligand binding regions of hRXRalpha than does the native ligand 9-cis-RA. In at least half of these regions, the level of protection by 9-cis-RA lags behind the agonists even after 60 min. Methoprenic acid did not significantly protect hRXRalpha against amide hydrogen exchange. An efficient method is described for comparing the effects of different agonists on the protein structure of the agonist-RXRalpha complex.