A General Strategy Toward pH-Resistant Phenolic Fluorophores for High-Fidelity Sensing and Bioimaging Applications

Aryl alcohol-type or phenolic fluorophores offer diverse opportunities for developing bioimaging agents and fluorescence probes. Due to the inherently acidic hydroxyl functionality, phenolic fluorophores provide pH-dependent emission signals. Therefore, except for developing pH probes, the pH-dependent nature of phenolic fluorophores should be considered in bioimaging applications but has been neglected. Here we show that a simple structural remedy converts conventional phenolic fluorophores into pH-resistant derivatives, which also offer “medium-resistant” emission properties. The structural modification involves a single-step introduction of a hydrogen-bonding acceptor such as morpholine nearby the phenolic hydroxyl group, which also leads to emission bathochromic shift, increased Stokes shift, enhanced photo-stability and stronger emission for several dyes. The strategy greatly expands the current fluorophores’ repertoire for reliable bioimaging applications, as demonstrated here with ratiometric imaging of cells and tissues.     

Solar Light Driven H2O2 Production and Selective Oxidations Using a Covalent Organic Framework Photocatalyst Prepared by a Multicomponent Reaction

A chemically stable 2D microporous COF ( PMCR-1 ) was synthesized via the multicomponent Povarov reaction. PMCR-1 exhibits a remarkable and long-term stable photocatalytic H₂O₂ production rate (60 h) from pure and sea water under visible light. The H₂O₂ production is markedly enhanced when benzyl alcohol (BA) is added as reductant, which is also due to a strong π–π interaction of BA with dangling phenyl moieties in the COF pores introduced by the multicomponent Povarov reaction. Motivated by the concomitant BA oxidation to benzaldehyde during H₂O₂ formation, the photocatalytic oxidation of various organic substrates such as benzyl amine and methyl sulfide derivatives was investigated. It is shown that the well-defined micropores of PMCR-1 enable size-selective photocatalytic oxidation.     

Effect of gamma radiation on growth and survival of common seed-borne fungi in India

The present work describes radiation-induced effects of major seeds like Oryza sativa Cv-2233, Oryza sativa Cv-Shankar, Cicer arietinum Cv-local and seed-borne fungi like Alternaria sp., Aspergillus sp., Trichoderma sp. and Curvularia sp. ⁶⁰Co gamma source at 25 °C emitting gamma ray at 1173 and 1332 keV energy was used for irradiation. Dose of gamma irradiation up to 3 kGy (0.12 kGy/h) was applied for exposing the seed and fungal spores. Significant depletion of the fungal population was noted with irradiation at 1–2 kGy, whereas germinating potential of the treated grain did not alter significantly. However, significant differential radiation response in delayed seed germination, colony formation of the fungal spores and their depletion of growth were noticed in a dose-dependent manner. The depletion of the fungal viability (germination) was noted within the irradiation dose range of 1–2 kGy for Alternaria sp. and Aspergillus sp., while 0.5–1 kGy for Trichoderma sp. and Curvularia sp. However, complete inhibition of all the selected fungi was observed above 2.5 kGy.     

From large 12-membered macrometallacycles to ionic (NHC)2M+Cl- type complexes of gold and silver by modulation of the N-substituent of amido-functionalized N-heterocyclic carbene (NHC) ligands

A series of structurally diverse gold and silver complexes extending from ionic (NHC) 2M(+)Cl(-) (M=Au, Ag) type complexes to large 12-membered macrometallacycles have been prepared by the appropriate modification of the N-substituent of amido-functionalized N-heterocyclic carbenes. Specifically, the ionic, [1-(R)-3-{ N-(t-butylacetamido)imidazol-2-ylidene}]2M(+)Cl(-), (R=t-Bu, i-Pr; M=Au, Ag; 1b, 1c, 2b, 2c) complexes, were obtained in case of the N- t-butyl substituent of the amido-functionalized sidearm while 12-membered macrometallacycles, [1-(R)-3-{N-(2,6-di i-propylphenylacetamido)imidazol-2-ylidene}]2M2, (R=t-Bu, i-Pr; M=Au, Ag; 3b, 3c, 4b, 4c) were obtained in case of the 2,6-di i-propylphenyl N-substituent. These structurally diverse complexes of gold and silver were, however, prepared employing a common synthetic pathway involving the reactions of the imidazolium chloride salts (1a, 2a, 3a, 4a) with Ag2O to give the silver complexes (1b, 2b, 3b, 4b) and which, when treated with (SMe2)AuCl, gave the gold complexes (1c, 2c, 3c, 4c). Detailed density functional theory studies of 1b, 1c, 2b, 2c, 3b, 3c, 4b, and 4c were carried out to gain insight about the structure, bonding, and the electronic properties of these complexes. The NHC-metal interaction in the ionic 1b, 1c, 2b, and 2c complexes is primarily composed of the interaction of the carbene lone pair with the empty p orbital of the metal (5p for Ag and 6p for Au) while the same in the macrometallacyclic 3b, 3c, 4b, and 4c complexes consisted of the interaction of the carbene lone pair with the empty s orbital of the metal (5s for Ag and 6s for Au). The observation of a low energy emission in about the 580-650 nm region has been tentatively assigned to originate from the presence of weak metallophilic interaction in these macrometallacyclic 3b, 3c, 4b, and 4c complexes.     

Size dependence of structural, electronic, elastic, and optical properties of selenium nanowires: a first-principles study

We have studied the structural, elastic, and optical properties of selenium nanowires, as well as bulk selenium, by performing first-principles density functional theory calculations. The nanowires are structurally similar to bulk trigonal Se, in that they consist of hexagonal arrays of helices, though there is a slight structural rearrangement in response to the finite size of the nanowires. These small structural changes result in Young's modulus decreasing slightly for progressively thinner nanowires. However, there is a significant effect on electronic structure and optical properties. The thinner the nanowire, the greater the band gap, and the greater the anisotropy in optical conductivity. The latter is due to the effects of finite size being much more marked for the case where the electric field is polarized perpendicular to the helical axis, than in the case where the polarization is parallel to c. For the case of bulk Se, we obtain good agreement with experimental data on the structure, elastic constants, and dielectric function.     

Shorter argentophilic interaction than aurophilic interaction in a pair of dimeric {(NHC)MCl}(2) (M = Ag, Au) complexes supported over a N/O-functionalized N-heterocyclic carbene (NHC) ligand

Synthesis, structure, bonding, and photoluminescence studies of a pair of neutral dimeric silver and gold complexes of a N/O-functionalized N-heterocyclic carbene ligand exhibiting closed-shell d10...d10 argentophilic and aurophilic interactions, are reported. In particular, dimeric complexes of the type {[1-(benzyl)-3-(N-tert-butylacetamido)imidazol-2-ylidene]MCl}2 [M = Ag (2); Au (3)] displayed attractive metallophilic interaction in the form of a close ligand-unsupported metal...metal contact [3.1970(12) A in 2; 3.2042(2) A in 3] as observed from X-ray diffraction study and also was further verified by low temperature photoluminescence study at 77 K that showed the characteristic emission [527 nm for 2; 529 nm for 3] owing to the metal...metal interaction. The nature of the metallophilic interaction in these complexes was further probed using computational studies that estimated the metal...metal interaction energy to be 12.8 (2) and 8.6 kcal/mol (3). Notably, the argentophilic interaction was found to be stronger than the aurophilic interaction in this series of neutral dimeric complexes. The complexes 2 and 3 were synthesized sequentially, with the silver 2 complex prepared by the reaction of the 1-(benzyl)-3-(N-tert-butylacetamido)imidazolium chloride with Ag2O in 66% yield, while the gold 3 complex was obtained by the transmetallation reaction of the silver 2 complex with (SMe2)AuCl in 86% yield.     

Interplay between bonding and magnetism in the binding of NO to Rh clusters

We have studied the binding of NO to small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111), suggesting that Rh clusters may be good catalysts for NO reduction. However, binding to NO also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the NO binding energy, and also washes out the clear size-dependent trend observed in the nonmagnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.     

Organophosphites: An Addition to the Arsenal of Organocatalysts

Organophosphites are nucleophilic in nature and can act as a good leaving group owing to the stability of the phosphite anion. This dual reactivity makes them good candidates for nucleophilic organocatalysis. However, phosphites were introduced only in 2004 as the umpolung catalyst for acylsilane substrates utilizing sequential Brook rearrangements. Very recently, phosphites have been reported to catalyze aza-rearrangements and radical reactions. In this review, we discuss the reactivity parameters to understand its lack of use, as well as the potential for catalysis.     

Efficient Palladium‐Assisted One‐Pot Deprotection of (Acetamidomethyl)Cysteine Following Native Chemical Ligation and/or Desulfurization To Expedite Chemical Protein Synthesis

The acetamidomethyl (Acm) moiety is a widely used cysteine protecting group for the chemical synthesis and semisynthesis of peptide and proteins. However, its removal is not straightforward and requires harsh reaction conditions and additional purification steps before and after the removal step, which extends the synthetic process and reduces the overall yield. To overcome these shortcomings, a method for rapid and efficient Acm removal using Pd^II complexes in aqueous medium is reported. We show, for the first time, the assembly of three peptide fragments in a one‐pot fashion by native chemical ligation where the Acm moiety was used to protect the N‐terminal Cys of the middle fragment. Importantly, an efficient synthesis of the ubiquitin‐like protein UBL‐5, which contains two native Cys residues, was accomplished through the one‐pot operation of three key steps, namely ligation, desulfurization, and Acm deprotection, highlighting the great utility of the new approach in protein synthesis.     

Magnetism in Simple Metal and 4<Emphasis Type="Italic">d</Emphasis> Transition Metal Clusters

In this review article I discuss two aspects of magnetism in small metal clusters. The first question discussed is whether simple metal clusters, that obey electronic shell models and mimic properties of elemental atoms, also obey Hund’s rule of maximum spin multiplicity. The second question is whether small clusters of 4d transition metal atoms, that are non-magnetic in the bulk, have magnetic ground states. The question arises because calculations showed that small V clusters are magnetic although the bulk metal is not. We discuss known results on Rh clusters in detail to show that small clusters are generally magnetic, but it is difficult to unequivocally identify the ground state due to the presence of many isomers and spin states that are very close in energy.