Reply to the 'Comment on "Theoretical study of the NO radical reaction with CHClBr, CHICl, CHBrI, CHClBr, and CHClBr"' by C. J. Nielsen and Y. Tang, , 2022, , DOI: 10.1039/D2CP03013F.

In this Reply, we answer the main argument raised in the Comment about the energy of the NO radical and its influence in the reaction profiles of the reaction of the NO radical with CHClBr, CHICl, CHBrI, CHClBr, and CHClBr by C. J. Nielsen and Y.

The Chemistry of Mercury in the Stratosphere.

Mercury, a global contaminant, enters the stratosphere through convective uplift, but its chemical cycling in the stratosphere is unknown. We report the first model of stratospheric mercury chemistry based on a novel photosensitized oxidation mechanism. We find two very distinct Hg chemical regimes in the stratosphere: in the upper stratosphere, above the ozone maximum concentration, Hg oxidation is initiated by photosensitized reactions, followed by second-step chlorine chemistry.

Theoretical study of the NO radical reaction with CHClBr, CHICl, CHBrI, CHClBr, and CHClBr.

The potential reaction of the nitrate radical (NO), the main nighttime atmospheric oxidant, with five alkyl halides, halons (CHClBr, CHICl, CHBrI, CHClBr, and CHClBr) has been studied theoretically. The most favorable reaction corresponds to a hydrogen atom transfer. The stationary points on the potential energy surfaces of these reactions have been characterized.

Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere.

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg to the atmosphere where it is oxidized to reactive Hg compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized Hg and Hg species postulate their photodissociation back to Hg as a crucial step in the atmospheric Hg redox cycle.

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury.

Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg and its oxidized forms Hg and Hg . The efficient gas-phase photolysis of Hg and Hg has recently been reported. However, whether the photolysis of Hg leads to other stable Hg species, to Hg , or to Hg and its competition with thermal reactivity remain unknown.

Gas-Phase Photolysis of Hg(I) Radical Species: A New Atmospheric Mercury Reduction Process.

The efficient gas-phase photoreduction of Hg(II) has recently been shown to change mercury cycling significantly in the atmosphere and its deposition to the Earth's surface. However, the photolysis of key Hg(I) species within that cycle is currently not considered. Here we present ultraviolet-visible absorption spectra and cross-sections of HgCl, HgBr, HgI, and HgOH radicals, computed by high-level quantum-chemical methods, and show for the first time that gas-phase Hg(I) photoreduction can occur at time scales that eventually would influence the mercury chemistry in the atmosphere.

Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition.

Anthropogenic mercury (Hg(0)) emissions oxidize to gaseous Hg(II) compounds, before deposition to Earth surface ecosystems. Atmospheric reduction of Hg(II) competes with deposition, thereby modifying the magnitude and pattern of Hg deposition. Global Hg models have postulated that Hg(II) reduction in the atmosphere occurs through aqueous-phase photoreduction that may take place in clouds.

Definition and determination of the triplet-triplet energy transfer reaction coordinate.

A definition of the triplet-triplet energy transfer reaction coordinate within the very weak electronic coupling limit is proposed, and a novel theoretical formalism is developed for its quantitative determination in terms of internal coordinates The present formalism permits (i) the separation of donor and acceptor contributions to the reaction coordinate, (ii) the identification of the intrinsic role of donor and acceptor in the triplet energy transfer process, and (iii) the quantification of the effect of every internal coordinate on the transfer process. This formalism is general and can be applied to classical as well as to nonvertical triplet energy transfer processes. The utility of the novel formalism is demonstrated here by its application to the paradigm of nonvertical triplet-triplet energy transfer involving cis-stilbene as acceptor molecule.

Miltefosine and BODIPY-labeled alkylphosphocholine with leishmanicidal activity: Aggregation properties and interaction with model membranes.

Miltefosine (hexadecylphosphocholine, MT) afforded successful oral treatment against human visceral and cutaneous leishmaniasis. Knowledge of MT aggregation in aqueous solutions and of its interaction with lipid membranes is important to understand pharmacokinetics, bioavailability and antiparasitic effects. Methods based on surface tension and fluorescence spectroscopy gave the value of 50μM for critical micelle concentration (CMC) in buffered water solution, and the value is influenced by salt content.

A BODIPY-embedding miltefosine analog linked to cell-penetrating Tat(48-60) peptide favors intracellular delivery and visualization of the antiparasitic drug.

Therapeutic application of many drugs is often hampered by poor or denied access to intracellular targets. A case in point is miltefosine (MT), an orally active antiparasitic drug, which becomes ineffective when parasites develop dysfunctional uptake systems. We report here the synthesis of a fluorescent BODIPY-embedding MT analogue with appropriate thiol functionalization allowing linkage to the cell-penetrating Tat(48-60) peptide through disulfide or thioether linkages.

Synthesis and photophysics of novel biocompatible fluorescent oxocines and azocines in aqueous solution.

The spectroscopic properties in water solution of the different prototropic forms of the strongly fluorescent hemiacetal 4,9-dihydroxy-1,2-dihydro-4,11a-methanooxocino[4,5-b]benzofuran-5(4H)-one (1a, monardine), the aza analogue 4,9-dihydroxy-3,4-dihydro-1H-4,11a-methanobenzofuro[2,3-d]azocin-5(2H)-one (2a, azamonardine) and the respective 2-carboxyl derivatives (1b, 2b) have been studied by experimental and quantum-chemical methods. Monardine and carboxymonardine are the major products of new fluorogenic, room-temperature reactions of hydroxytyrosol or salvianic acid in aqueous solution, respectively, and present unique photophysical properties. Near neutral pH (pKa = 7.

Edelfosine and miltefosine effects on lipid raft properties: membrane biophysics in cell death by antitumor lipids.

Edelfosine (1-O-octadecyl-2-O-methyl-sn-glycero-phosphocholine) and miltefosine (hexadecylphosphocholine) are synthetic alkylphospholipids (ALPs) that are reported to selectively accumulate in tumor cell membranes, inducing Fas clustering and activation on lipid rafts, triggering apoptosis. However, the exact mechanism by which these lipids elicit these events is still not fully understood. Recent studies propose that their mode of action might be related with alterations of lipid rafts biophysical properties caused by these lipid drugs.

Drug uptake, lipid rafts, and vesicle trafficking modulate resistance to an anticancer lysophosphatidylcholine analogue in yeast.

The ether-phospholipid edelfosine, a prototype antitumor lipid (ATL), kills yeast cells and selectively kills several cancer cell types. To gain insight into its mechanism of action, we performed chemogenomic screens in the Saccharomyces cerevisiae gene-deletion strain collection, identifying edelfosine-resistant mutants. LEM3, AGP2, and DOC1 genes were required for drug uptake.

Fluorescent labeling of Acanthamoeba assessed in situ from corneal sectioned microscopy.

Acanthamoeba keratitis is a serious pathogenic corneal disease, with challenging diagnosis. Standard diagnostic methods include corneal biopsy (involving cell culture) and in vivo reflection corneal microscopy (in which the visualization of the pathogen is challenged by the presence of multiple reflectance corneal structures). We present a new imaging method based on fluorescence sectioned microscopy for visualization of Acanthamoeba.

Interaction of miltefosine with intercellular membranes of stratum corneum and biomimetic lipid vesicles.

Miltefosine (MT) is an alkylphospholipid approved for breast cancer metastasis and visceral leishmaniasis treatments, although the respective action mechanisms at the molecular level remain poorly understood. In this work, the interaction of miltefosine with the lipid component of stratum corneum (SC), the uppermost skin layer, was studied by electron paramagnetic resonance (EPR) spectroscopy of several fatty acid spin-labels. In addition, the effect of miltefosine on (i) spherical lipid vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and (ii) lipids extracted from SC was also investigated, by EPR and time-resolved polarized fluorescence methods.

Defeating Leishmania resistance to miltefosine (hexadecylphosphocholine) by peptide-mediated drug smuggling: a proof of mechanism for trypanosomatid chemotherapy.

Miltefosine (hexadecylphosphocholine, HePC), the first orally active drug successful against leishmaniasis, is especially active on the visceral form of the disease. Resistance mechanisms are almost exclusively associated to dysfunction in HePC uptake systems. In order to evade the requirements of its cognate receptor/translocator, HePC-resistant Leishmania donovani parasites (R40 strain) were challenged with constructs consisting of an ω-thiol-functionalized HePC analogue conjugated to the cell-penetrating peptide (CPP) Tat(48-60), either through a disulfide or a thioether bond.

Structure and formation of the fluorescent compound of Lignum nephriticum.

The intense blue fluorescence of the infusion of Lignum nephriticum (Eysenhardtia polystachya), first observed in the sixteenth century, is due to a novel four-ring tetrahydromethanobenzofuro[2,3-d]oxacine which is not present in the plant but is the end product of an unusual, very efficient iterative spontaneous oxidation of at least one of the tree's flavonoids.

Synthesis of BODIPY-labeled alkylphosphocholines with leishmanicidal activity, as fluorescent analogues of miltefosine.

Two general synthetic methods are described, by which the highly fluorescent and photostable BODIPY group can be inserted in and aligned with the alkyl backbone of linear lipids. These methods have been used to prepare strongly emitting analogues of the leishmanicidal drug miltefosine, in which the antiparasite activity in vitro of the original drug is preserved.

Synthesis and biological evaluation of fluorescent leishmanicidal analogues of hexadecylphosphocholine (miltefosine) as probes of antiparasite mechanisms.

The leishmanicidal mechanism of miltefosine (hexadecylphosphocholine, MT) is not clearly understood. Valuable insights into its mode of action could be obtained by fluorescence techniques, given suitably emitting analogues. In this regard, the synthesis and biological characterization of two fully competent MT fluorescent analogues is reported here: all-(E)-13-phenyltrideca-6,8,10,12-tetraenylphosphocholine (PTE-MT) and all-(E)-13-phenyltrideca-8,10,12-trien-6-ynylphosphocholine (PTRI-MT).

The phenyltetraene lysophospholipid analog PTE-ET-18-OMe as a fluorescent anisotropy probe of liquid ordered membrane domains (lipid rafts) and ceramide-rich membrane domains.

The conjugated phenyltetraene PTE-ET-18-OMe (all-(E)-1-O-(15'-phenylpentadeca-8',10',12',14'-tetraenyl)-2-O-methyl-rac-glycero-3-phosphocholine) is a recently developed fluorescent lysophospholipid analog of edelfosine, (Quesada et al. (2004) J. Med.

Synthesis of 16-mercaptohexadecylphosphocholine, a miltefosine analog with leishmanicidal activity.

The alkylphosphocholine miltefosine (n-hexadecylphosphocholine, MT) has been introduced recently as a very effective drug for the oral treatment of human leishmaniasis. However, the parasiticidal mechanism of MT at a molecular level is far from being understood. Here we report the synthesis and biological characterization of 16-mercaptohexadecylphosphocholine, a thiol analog of MT which was designed to facilitate the search of MT interacting targets within the parasite by a variety of analytical methods.

Dynamics of bolaamphiphilic fluorescent polyenes in lipid bilayers from polarization emission spectroscopy.

The rotational motions of the biamphiphilic polyenes (bolapolyenes) dimethyl all-(E)-octacosa-10,12,14,16,18-pentaenedioate (DE28:5) and dimethyl all-(E)-tetratriaconta-13,15,17,19,21-pentaenedioate (DE34:5), with head-to-head distances of 34 and 42A, respectively, have been examined by fluorescence anisotropy methods. The membrane-spanning bolapolyenes, which contain a central emitting pentaene group tethered to two methoxycarbonyl opposite polar heads by symmetric C(8) (DE28:5) and C(11) (DE34:5) polymethylene chains, were dispersed in lipid bilayers of DPPC or DMPC, and the stationary and picosecond-resolved emission was recorded as a function of temperature. In fluid-phase DMPC bilayers, three relaxation times could be determined, assigned to fast (0.

Fluorescent phenylpolyene analogues of the ether phospholipid edelfosine for the selective labeling of cancer cells.

Edelfosine (ET-18-OCH3), a synthetic antitumor ether lipid, is taken up by malignant but not by normal cells, triggering apoptosis in a large variety of human tumor cells. The synthesis of the first fluorescent edelfosine analogue (6), with apoptotic activity comparable to that of the parent drug, is described. Fluorescence microscopy experiments show that 6 selectively labels human cancer cells, accumulating into specific domains of the plasma membrane.

Protein self-association in crowded protein solutions: a time-resolved fluorescence polarization study.

The self-association equilibrium of a tracer protein, apomyoglobin (apoMb), in highly concentrated crowded solutions of ribonuclease A (RNase A) and human serum albumin (HSA), has been studied as a model system of protein interactions that occur in crowded macromolecular environments. The rotational diffusion of the tracer protein labeled with two different fluorescent dyes, 8-anilinonaphthalene-1-sulfonate and fluorescein isothiocyanate, was successfully recorded as a function of the two crowder concentrations in the 50-200 mg/mL range, using picosecond-resolved fluorescence anisotropy methods. It was found that apoMb molecules self-associate at high RNase A concentration to yield a flexible dimer.