Long-lived Complex Research Articles

A mechanism for reversible mesoscopic aggregation in liquid solutions

Solutions of proteins and other molecules exhibit puzzling, mesoscopically sized inclusions of a solute-rich liquid, well outside the region of stability of the solute-rich phase. This mesoscopic size is in conflict with existing views on heterophase fluctuations. Here we systematically work out a microscopic mechanism by which a metastable solute-rich phase can readily nucleate in a liquid solution. A requisite component of the mechanism is that the solute form long-lived complexes with itself or other molecules. After nucleated in this non-classical fashion, individual droplets grow until becoming mechanically unstable because of a concomitant drop in the internal pressure, the drop caused by the metastability of the solute-rich phase. The ensemble of the droplets is steady-state. In a freshly prepared solution, the ensemble is predicted to evolve in a way similar to the conventional Ostwald ripening, during which larger droplets grow at the expense of smaller droplets.

Effect of an Optical Breakdown on the Stimulated Raman Scattering in Water in the Field of Picosecond Laser Pulses

Experiments on nonstationary stimulated Raman scattering (SRS) of laser pulses and an optical breakdown are performed to study water samples with gas nanobubbles (bubstons) and without the bubston phase. The SRS excitation threshold in water with the bubston phase turns out to be higher than that in bubston-free water, and the optical breakdown threshold in bubston-containing water is lower than that in bubston-free water. The spectral composition of the SRS signal from bubston-free water samples is found to differ from that of bubston-containing water samples. This effect is caused by the formation of long-lived high-molecular H2O complexes near the bubston surface, which are stabilized by the dispersion forces acting from this surface. Since these complexes are located inside 1–2 monolayers adjacent to the surface, the volume density of these complexes is negligibly small as compared to the density of “free” (not bound to bubstons) molecules. To interpret the experimental results, we advance a hypothesis that the detected effect is related to the giant enhancement caused by the generation of a resonance plasmon inside a bubston during an optical breakdown. As follows from our estimates, the bubston-related plasma frequency falls in the range 2600–3200 cm–1; it is this range that contains the SRS spectral lines of water containing the bubston phase.

Low temperature gas phase reaction rate coefficient measurements: Toward modeling of stellar winds and the interstellar medium

AbstractStellar winds of Asymptotic Giant Branch (AGB) stars are responsible for the production of ∼85% of the gas molecules in the interstellar medium (ISM), and yet very few of the gas phase rate coefficients under the relevant conditions (10 – 3000 K) needed to model the rate of production and loss of these molecules in stellar winds have been experimentally measured. If measured at all, the value of the rate coefficient has often only been obtained at room temperature, with extrapolation to lower and higher temperatures using the Arrhenius equation. However, non-Arrhenius behavior has been observed often in the few measured rate coefficients at low temperatures. In previous reactions studied, theoretical simulations of the formation of long-lived pre-reaction complexes and quantum mechanical tunneling through the barrier to reaction have been utilized to fit these non-Arrhenius behaviours of rate coefficients.Reaction rate coefficients that were predicted to produce the largest change in the production/loss of Complex Organic Molecules (COMs) in stellar winds at low temperatures were selected from a sensitivity analysis. Here we present measurements of rate coefficients using a pulsed Laval nozzle apparatus with the Pump Laser Photolysis - Laser Induced Fluorescence (PLP-LIF) technique. Gas flow temperatures between 30 – 134 K have been produced by the University of Leeds apparatus through the controlled expansion of N2 or Ar gas through Laval nozzles of a range of Mach numbers between 2.49 and 4.25.Reactions of interest include those of OH, CN, and CH with volatile organic species, in particular formaldehyde, a molecule which has been detected in the ISM. Kinetics measurements of these reactions at low temperatures will be presented using the decay of the radical reagent. Since formaldehyde and the formal radical (HCO) are potential building blocks of COMs in the interstellar medium, low temperature reaction rate coefficients for their production and loss can help to predict the formation pathways of COMs observed in the interstellar medium.

SPR-Measured Dissociation Kinetics of PROTAC Ternary Complexes Influence Target Degradation Rate.

Bifunctional degrader molecules, known as proteolysis-targeting chimeras (PROTACs), function by recruiting a target to an E3 ligase, forming a target/PROTAC/ligase ternary complex. Despite the importance of this key intermediate species, no detailed validation of a method to directly determine binding parameters for ternary complex kinetics has been reported, and it remains to be addressed whether tuning the kinetics of PROTAC ternary complexes may be an effective strategy to improve the efficiency of targeted protein degradation. Here, we develop an SPR-based assay to quantify the stability of PROTAC-induced ternary complexes by measuring for the first time the kinetics of their formation and dissociation in vitro using purified proteins. We benchmark our assay using four PROTACs that target the bromodomains (BDs) of bromodomain and extraterminal domain proteins Brd2, Brd3, and Brd4 to the von Hippel–Lindau E3 ligase (VHL). We reveal marked differences in ternary complex off-rates for different PROTACs that exhibit either positive or negative cooperativity for ternary complex formation relative to binary binding. The positively cooperative degrader MZ1 forms comparatively stable and long-lived ternary complexes with either Brd4BD2 or Brd2BD2 and VHL. Equivalent complexes with Brd3BD2 are destabilized due to a single amino acid difference (Glu/Gly swap) present in the bromodomain. We observe that this difference in ternary complex dissociative half-life correlates to a greater initial rate of intracellular degradation of Brd2 and Brd4 relative to Brd3. These findings establish a novel assay to measure the kinetics of PROTAC ternary complexes and elucidate the important kinetic parameters that drive effective target degradation.

Intersystem crossing in the exit channel.

Intersystem crossing plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin-orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet-triplet intersection seam. Here we present evidence for a different intersystem crossing mechanism in the bimolecular reaction of O(3P) with alkylamines. In crossed-beam experiments, product velocity-flux maps are measured for aminoalkyl radicals produced from H abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential energy surface that could support such a complex. Multi-reference ab initio calculations suggest, instead, that intersystem crossing occurs in the exit-channel region due to the long-range dipole-dipole interaction between the nascent radical product pair coupled with the vanishing singlet-triplet splitting at long range. Intersystem crossing then leads to a deep hydroxylamine well before OH elimination.

Single-Molecule Kinetic Investigation of Cocaine-Dependent Split-Aptamer Assembly.

Aptamers are short nucleic-acid biopolymers selected to have high affinity and specificity for protein or small-molecule target analytes. Aptamers can be engineered into split-aptamer biosensors comprising two nucleic acid strands that coassemble as they bind to a target, resulting in a large signal change from attached molecular probes (e.g., molecular beacons). The kinetics of split-aptamer assembly and their dependence on target recognition are largely unknown; knowledge of these kinetics could help in design and optimization of split-aptamer biosensors. In this work, we measure assembly kinetics of cocaine-dependent split-aptamer molecules using single-molecule fluorescence imaging. Assembly is monitored between a DNA strand tethered to a glass substrate and solutions containing the other strand tagged with a fluorescent label, with varying concentrations of the cocaine analyte. Dissociation rates are measured by tracking individual molecules and measuring their bound lifetimes. Dissociation-time distributions are biexponential, possibly indicating different folded states of the aptamer. The dissociation rate of only the longer-lived complex decreases with cocaine concentration, suggesting that cocaine stabilizes the long-lived aptamer complex. The variation in the slow dissociation rate with cocaine concentration is well described with an equilibrium-binding model, where the dissociation rate approaches a saturation limit consistent with the dissociation-equilibrium constant for cocaine-binding to the split aptamer. This single-molecule methodology provides a sensitive readout of cocaine-binding based on the dissociation kinetics of the split aptamer, allowing one to distinguish target-dependent aptamer assembly from background assembly. This methodology could be used to study other systems where target association affects the stability of aptamer duplexes.

The geological and structural evolution of the long-lived Miocene-Pleistocene La Hoyada Volcanic Complex in the geodynamic framework of the Central Andes, Argentina

The long-lived La Hoyada Volcanic Complex (LHVC) is located in the east of Cordillera de San Buenaventura in Argentina. This region is important for understanding of the geodynamic evolution of the Southern Puna, since it is located in its southern edge, where the transition between normal and low-angle subduction occurs. The aim of this work is to contribute to the knowledge of the volcanism of the Southern Puna through the analysis of the stratigraphy and geological evolution of LHVC. Fieldwork and image analysis have led to a new lithostratigraphic scheme, that identifies lithosomes and unconformity-bounded stratigraphic units defining the stratigraphic architecture of the LHVC. New 40Ar/39Ar ages further constrain the geological evolution of this complex. The resulting stratigraphic scheme combines the lithostratigraphic units into lithosomes and synthemic units, with the identification of at least six superimposed eruptive centers and their products during the LHVC evolution. A new stratigraphic framework for the eastern Cordillera de San Buenaventura, composed by four supersynthems separated by unconformities that reflect periods of quiescence of the volcanic activity, catastrophic destructive events and changes in the magmatic plumbing system, is proposed: 1) San Buenaventura supersynthem (ca. 9–7 Ma), including mainly explosive rhyolitic volcanism; 2) Lower La Hoyada supersynthem (ca. 7–<4.63 Ma), comprising andesitic-dacitic lava flows and domes characterized by a pervasive compressive tectonic deformation; 3) Upper La Hoyada supersynthem (4.63–1.37 Ma), mainly represented by arc-related, calc-alkaline, andesitic-dacitic effusive products and subordinated pyroclastic and epiclastic deposits; and 4) Purulla supersynthem (<1.37 Ma), which comprises volcanic products with bimodal composition related to a Pleistocene extensional volcanotectonic fault system. The application of unconformity-based stratigraphic units provides a powerful tool for understanding the volcanic evolution of eroded long-lived volcanic complexes, with an effective reconstruction of the temporal and spatial relationships among constructive and destructive phases of the volcano history, correlated with the regional volcanic, magmatic and tectonic evolution of the southern Puna.

Intermolecular dissociation energies of 1-naphthol·n-alkane complexes.

Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we have determined accurate intermolecular dissociation energies D0 of supersonic jet-cooled intermolecular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane, propane, and n-butane. Experimentally, the smaller alkanes form a single minimum-energy structure, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation energies D0(S0) for the complexes with propane and n-butane (isomers A and B) were bracketed within ±0.5%, being 16.71 ± 0.08 kJ/mol for S = propane and 20.5 ± 0.1 kJ/mol for isomer A and 20.2 ± 0.1 kJ/mol for isomer B of n-butane. All 1NpOH·S complexes measured previously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weak SEP-R2PI bands are observed above the putative dissociation onset for the methane and ethane complexes. We attribute these bands to long-lived complexes that retain energy in rotation-type intermolecular vibrations, which couple only weakly to the dissociation coordinates. Accounting for this, we find dissociation energies of D0(S0) = 7.98 ± 0.55 kJ/mol (±7%) for S = methane and 14.5 ± 0.28 kJ/mol (±2%) for S = ethane. The D0 values increase by only 1% upon S0 → S1 excitation of 1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, and ωB97X-D predict that the n-alkanes bind dispersively to the naphthalene "Face." The assignment of the complexes to Face structures is supported by the small spectral shifts of the S0 → S1 electronic origins, which range from +0.5 to -15 cm-1. Agreement with the calculated dissociation energies D0(S0) is quite uneven, the B97-D3 values agree within 5% for propane and n-butane, but differ by up to 20% for methane and ethane. The ωB97X-D method shows good agreement for methane and ethane but overestimates the D0(S0) values for the larger n-alkanes by up to 20%. The agreement of the B3LYP-D3 D0 values is intermediate between the other two methods.

IR, Raman, and Vibrational Optical Activity Spectra of Methyl Glycidate in Chloroform and Water: The Clusters-in-a-liquid Solvation Model.

Solvent effects, in particular those involving water as the solvent, are of significant interest to the chemistry and physics communities. IR, vibrational circular dichroism (VCD), Raman, and Raman optical activity (ROA) spectra of methyl glycidate in two very different solvents, namely CCl4 and water, have been measured experimentally and simulated theoretically. The observed spectra in CCl4 could be well modelled using the polarizable continuum model for the solvent, whereas the situation is much different in water. The experimental VCD spectrum of methyl glycidate in water reveals strong induced VCD signatures in the water bending region, indicating the presence of the relatively long-lived methyl glycidate-watern complexes. We applied the clusters-in-a-liquid approach to identify the dominant methyl glycidate-water1,2 complexes which are the long-lived species responsible for all the spectra observed in water. We examined the influences of solvent dielectric environment and the hydrogen-bonding interactions on the conformational distribution of methyl glycidate. The geometry optimizations, frequency calculations, IR, VCD, Raman and ROA intensity calculations were performed at the B3LYP/6-311++G(2d,p) and aug-cc-pVTZ levels of theory with D3BJ dispersion correction. It is particularly satisfying to note that the clusters-in-a-liquid approach has captured all main experimental features in IR, VCD, Raman and ROA spectra of methyl glycidate in water.

Dynamics of the C(3P) + Ethylene Reaction: A Trajectory Surface Hopping Study.

The direct dynamics trajectory surface hopping (DDTSH) method has been employed to study the reaction of C(3P) with ethylene (C2H4). Our trajectory simulations show that at a reagent collision energy of 7.36 kcal/mol, there are two possible product channels: propargyl (H2CCCH) + H and carbene (CH2) + acetylene (HCCH). Estimated branching ratios based on trajectory propagations indicate that propargyl radical formation is the dominant channel contributing (94.1 ± 5.2) % of the overall products formation with (5.9 ± 1.7)% contribution from the minor CH2 + HCCH channel. These findings are consistent with earlier experimental observations and theoretical predictions that propargyl (H2CCCH) formation is the dominant channel for the C(3P) + C2H4 collision reaction. Our trajectory simulations, however, unravel five distinctly different dynamical pathways, unlike earlier experimental and theoretical predictions of only two pathways proposed for the formation of propargyl radical, and three different dynamics are followed for the CH2 + HCCH channel (this channel was not detected experimentally). The computed translational energy distribution for the propargyl + H channel is narrower and showed peak maximum at a lower energy compared to the experimental one. While the center of mass product angular distribution based on our trajectory propagation is nearly isotropic in nature indicating formation of long-lived intermediate complexes, the experimental one was reported to be backward-forward distributed with more intensity in the forward direction indicating the formation of an osculating complex. Our trajectory surface hopping calculations confirm that the effect of intersystem crossing (ISC) is not important for the title reaction presumably because of weak spin-orbit coupling values (<10 cm-1) for the (C + C2H4) system. No trace of cyclic products formation was obtained from our trajectory simulations, which however was predicted to be a minor (2%) product channel, experimentally.

Direct Observation of MUTYH and the Cancer-Associated Y150C Variant Binding to OG: A Mismatches at the Single Molecule Level

Of the four DNA bases, guanine is the most likely to be damaged by reactive oxygen species. Oxidized guanine (specifically, 8-oxoguanine) mispairs with adenine (instead of cytosine) during DNA replication (an OG:A mismatch), ultimately resulting in genomic G -> T transversion mutations. MUTYH is a mammalian DNA repair glycosylase that locates and removes the adenine in these OG:A mismatches. Y150C is the mouse homolog of a human MUTYH variant that is associated with colorectal adenomas and carcinomas. Structural information indicates that tyrosine 150 is analogous to the critical “wedge residue” present in many other DNA glycosylases, which is essential for lesion recognition. Here, we directly observe, at the single molecule level in vitro, fluorescently tagged wildtype (WT) MUTYH and the Y150C variant while they scan suspended DNA “tightropes” comprised of concatenated, linearized plasmids that contain specifically positioned OG:A mismatches every 2700 base pairs. In bulk assays, Y150C demonstrates reduced catalytic activity. Although we find that both the WT and Y150C diffuse along undamaged DNA with the same diffusion constant (1x105nm2/s) as they scan for damage, Y150C remains bound to and scans the undamaged DNA for a much shorter period of time than WT (3.9s and 32.7s, respectively). Both the WT and Y150C recognize and pause at OG:A mismatches, suggesting that Y150C lesion recognition is not impaired. However, once recognized Y150C pauses at a mismatch for only a short time (median 1.9s), while the WT enzyme remains bound at these mismatches for at least the duration of our 5-minute recordings. Taken together, our data show that mutation of the tyrosine “wedge residue” does not impair MUTYH's ability recognize and pause at OG:A mismatches, although the variant is unable to form a long-lived complex.

New 4-Amino-1,2,3-Triazole Inhibitors of Indoleamine 2,3-Dioxygenase Form a Long-Lived Complex with the Enzyme and Display Exquisite Cellular Potency.

Indoleamine-2,3 dioxygenase 1 (IDO1) has emerged as a central regulator of immune responses in both normal and disease biology. Due to its established role in promoting tumour immune escape, IDO1 has become an attractive target for cancer treatment. A novel series of highly cell potent IDO1 inhibitors based on a 4-amino-1,2,3-triazole core have been identified. Comprehensive kinetic, biochemical and structural studies demonstrate that compounds from this series have a noncompetitive kinetic mechanism of action with respect to the tryptophan substrate. In co-complex crystal structures, the compounds bind in the tryptophan pocket and make a direct ligand interaction with the haem iron of the porphyrin cofactor. It is proposed that these data can be rationalised by an ordered-binding mechanism, in which the inhibitor binds an apo form of the enzyme that is not competent to bind tryptophan. These inhibitors also form a very tight, long-lived complex with the enzyme, which partially explains their exquisite cellular potency. This novel series represents an attractive starting point for the future development of potent IDO1-targeted drugs.

A long-lived cuprous bis-phenanthroline complex for the photodynamic therapy of cancer.

Copper is an earth-abundant and a biologically essential metal that offers a promising alternative to noble metals in photochemistry and photobiology. In this work, a series of sterically encumbered Cu(i) bis-phenanthroline complexes were investigated for their use in photochemotherapy (PCT). It was found that Cu(dsbtmp)2+ [dsbtmp = 2,9-disec-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline] (compound 3), which possessed the longest excited state lifetime, exhibited significant in vitro photocytotoxicity on A375 (human malignant melanoma) and A549 (human lung carcinoma) cell lines. Fluorescence imaging demonstrated the significant uptake and localization of compound 3 in a perinuclear fashion. A comet assay indicated the induction of DNA damage in the dark. The DNA breaks were significantly amplified upon photoactivation. The light-induced enhancement of cytotoxicity was associated with the formation of reactive oxygen species (ROS), a known intermediate in photodynamic therapy (PDT). This successful demonstration of photocytotoxicity using long-lived cuprous phenanthroline paves the way to exploit this class of photosensitizers for PDT applications.

Full dimensional potential energy surface and low temperature dynamics of the H2CO + OH → HCO + H2O reaction.

A new method is proposed to analytically represent the potential energy surface of reactions involving polyatomic molecules capable of accurately describing long-range interactions and saddle points, needed to describe low-temperature collisions. It is based on two terms, a reactive force field term and a many-body term. The reactive force field term accurately describes the fragments, long-range interactions among them and the saddle points for reactions. The many-body term increases the desired accuracy everywhere else. This method has been applied to the OH + H2CO → H2O + HCO reaction, giving a barrier of 27.4 meV. The simulated classical rate constants with this potential are in good agreement with recent experimental results [Ocaña et al., Astrophys. J., 2017, submitted], showing an important increase at temperatures below 100 K. The reaction mechanism is analyzed in detail here, and explains the observed behavior at low energy by the formation of long-lived collision complexes, with roaming trajectories, with a capture observed for very long impact parameters, >100 a.u., determined by the long-range dipole-dipole interaction.

Manipulating Cohesin Levels in Live Mouse Oocytes.

The cohesin complex is essential for chromosome segregation in mitosis and meiosis. Cohesin is a tripartite protein complex that holds sister chromatids together from DNA replication until anaphase. In mammals, meiotic DNA replication occurs in oogonia of embryos and chromosome segregation occurs in oocytes of sexually mature females. Sister chromatid cohesion establishment and chromosome segregation are thus separated by months in the mouse and decades in the human. The meiotic cohesin complex that maintains sister chromatid cohesion must therefore hold replicated sisters together for a long time in oocytes. Remarkably, this is achieved by establishing cohesion exclusively in prenatal oocytes. Meiotic cohesion in females ismaintained without detectable turnover and cohesinis therefore thought to be a long-lived protein complex. Nevertheless, the lifespan of cohesin molecules is limited as chromosomal cohesin levelsdecline with maternal age. The age-related loss of cohesin and weakened cohesion correlate with an age-related increase in chromosome missegregation of meiosis I oocytes that results in aneuploid eggs. Therefore, loss of chromosomal cohesin has been proposed to be a leading cause of the maternal age effect. To better understand cohesin deterioration in oocytes, it is crucial to gain insights into mammalian cohesion establishment and maintenance mechanisms by manipulating cohesin in live oocytes.This chapter describes techniques that address the manipulation of meiotic cohesin levels in mouse oocytes. First, we describe how cohesin can be efficiently removed from meiotic chromosomes by injecting mRNA encoding TEV protease in live oocytes expressing cohesin with engineered TEV recognition sites, followed by imaging. Secondly, we describe how cohesin expression can be induced during different stages of oocyte development using genetically modified mouse strains. In particular, we describe how to determine the deletion timing of germline-specific Cre recombinases using β-galactosidase staining of fetal ovaries. Lastly, we provide guidance on how to quantify cohesin levels on metaphase I chromosome spreads.

Cell imaging of dopamine receptor using agonist labeling iridium(iii) complex.

Dopamine receptor expression is correlated with certain types of cancers, including lung, breast and colon cancers. In this study, we report luminescent iridium(iii) complexes (11-14) as intracellular dopamine receptor (D1R/D2R) cell imaging agents. Complexes 11 and 13, which are conjugated with a dopamine receptor agonist, showed superior cell imaging characteristics, high stability and low cytotoxicity (>100 μM) in A549 lung cancer cells. siRNA knockdown and dopamine competitive assays indicated that complexes 11 and 13 could selectively bind to dopamine receptors (D1R/D2R) in A549 cells. Fluorescence lifetime microscopy demonstrated that complex 13 has a longer luminescence lifetime at the wavelength of 560-650 nm than DAPI and other chromophores in biological fluids. The long luminescence lifetime of complex 13 not only provides an opportunity for efficient dopamine receptor tracking in biological media, but also enables the temporal separation of the probe signal from the intense background signal by fluorescence lifetime microscopy for efficient analysis. Complex 13 also shows high photostability, which could allow it to be employed for long-term cellular imaging. Furthermore, complex 13 could selectively track the internalization process of dopamine receptors (D1R/D2R) in living cells. To the best of our knowledge, complex 13 is the first metal-based compound that has been used to monitor intracellular dopamine receptors in living cells.

Statistical analysis of dimer formation in supersaturated metal vapor based on molecular dynamics simulation

We discuss the problem of dimer formation during the homogeneous nucleation of atomic metal vapor in an inert gas environment. We simulated nucleation with molecular dynamics and carried out the statistical analysis of double- and triple-atomic collisions as the two ways of long-lived diatomic complex formation. Close pair of atoms with lifetime greater than the mean time interval between atom–atom collisions is called a long-lived diatomic complex. We found that double- and triple-atomic collisions gave approximately the same probabilities of long-lived diatomic complex formation, but internal energy of the resulted state was essentially lower in the second case. Some diatomic complexes formed in three-particle collisions are stable enough to be a critical nucleus.

Long-lived complexes and signatures of chaos in ultracold K2 +Rb collisions

Lifetimes of complexes formed during ultracold collisions are of current experimental interest as a possible cause of trap loss in ultracold gases of alkali-dimers. Microsecond lifetimes for complexes formed during ultracold elastic collisions of K2 with Rb are reported, from numerically-exact quantum-scattering calculations. The reported lifetimes are compared with those calculated using a simple density-of-states approach, which are shown to be reasonable. Long-lived complexes correspond to narrow scattering resonances which we examine for the statistical signatures of quantum chaos, finding that the positions and widths of the resonances follow the Wigner-Dyson and Porter-Thomas distributions respectively.

A generalized quantitative antibody homeostasis model: maintenance of global antibody equilibrium by effector functions.

The homeostasis of antibodies can be characterized as a balanced production, target-binding and receptor-mediated elimination regulated by an interaction network, which controls B-cell development and selection. Recently, we proposed a quantitative model to describe how the concentration and affinity of interacting partners generates a network. Here we argue that this physical, quantitative approach can be extended for the interpretation of effector functions of antibodies. We define global antibody equilibrium as the zone of molar equivalence of free antibody, free antigen and immune complex concentrations and of dissociation constant of apparent affinity: [Ab]=[Ag]=[AbAg]=KD. This zone corresponds to the biologically relevant KD range of reversible interactions. We show that thermodynamic and kinetic properties of antibody–antigen interactions correlate with immunological functions. The formation of stable, long-lived immune complexes correspond to a decrease of entropy and is a prerequisite for the generation of higher-order complexes. As the energy of formation of complexes increases, we observe a gradual shift from silent clearance to inflammatory reactions. These rules can also be applied to complement activation-related immune effector processes, linking the physicochemical principles of innate and adaptive humoral responses. Affinity of the receptors mediating effector functions shows a wide range of affinities, allowing the continuous sampling of antibody-bound antigen over the complete range of concentrations. The generation of multivalent, multicomponent complexes triggers effector functions by crosslinking these receptors on effector cells with increasing enzymatic degradation potential. Thus, antibody homeostasis is a thermodynamic system with complex network properties, nested into the host organism by proper immunoregulatory and effector pathways. Maintenance of global antibody equilibrium is achieved by innate qualitative signals modulating a quantitative adaptive immune system, which regulates molecular integrity of the host by tuning the degradation and recycling of molecules from silent removal to inflammatory elimination.

Sterically encumbered mixed sandwich compounds of uranium(III): Synthesis and reactivity with small molecules

A series of uranium(III) mixed sandwich complexes with sterically demanding CpR ligands, of the type [U(COTTIPS2)(CpR)] (CpR = CptBu (C5H4tBu), CptBu2 (C5H3tBu2-1,3), CptBu3 (C5H2tBu3-1,2,4), CpTIPS2 (C5H3(SiiPr3)2-1,3), CpMe4Bz (C5Me4CH2Ph), IndMe6 (C9HMe6) and IndMe7 (C9Me7), and COTTIPS2 = C8H6(SiiPr3)2-1,4), have been synthesised and their X-ray crystal structures determined. The reactivity of these complexes with CO and CO2 is reported, including the squarate complex [U(COTTIPS2)(IndMe6)]2(μ-C4O4), IR data on the long-lived carbonyl complex [U(COTTIPS2)(IndMe7)(CO)] and the carbonate complex [U(COTTIPS2)(CptBu)]2(μ-η1:η2-CO3). The Solid-G algorithm has been to assess the steric properties of these and previously reported mixed-sandwich complexes in the solid state and correlate these properties with the observed reactivity.