Presence Of Such Complexes Research Articles

Selective phosphate removal by lanthanum carbonate @ anion exchange resin with fixed-bed column: RSM for process optimization and DFT for mechanistic study

Lanthanum carbonate @ anion exchange resin (LC@AER) has shown promise in adsorption processes due to its high maximum adsorption capacity and excellent stability in comparison with other lanthanum-based adsorbents. Nevertheless, there is a lack of significant investigation in evaluating the collective effect of the most influential parameters and determining the molecular complexes between the adsorbent and phosphate. This work aims to comprehensively evaluate and optimize the phosphate adsorption performance in fixed-bed columns and to determine the most favourable molecular configurations. Firstly, a response surface model (RSM) was developed to describe the phosphate adsorption performance in fixed-bed columns under the collective effect of selected independent variables, including influent pH, co-existing sulfate concentration, and empty bed contact time (EBCT). The RSM analysis reveals the significant effects of influent pH and co-existing sulfate concentration on phosphate adsorption. Moreover, the model achieves a correlation coefficient of >0.9, demonstrating its suitability for describing the experimental data. Secondly, various phosphate-lanthanum carbonate configurations were formulated, and the respective adsorption energies were compared using density functional theory (DFT) calculations. Subsequently, bidentate mononuclear complexes are determined as the most favourable configurations. The successful matching of the simulated peak locations with the experimental deconvoluted spectra further confirms the presence of such complexes in the system. Overall, this work provides an improved foundation and underlayer theory for potential future scale-up work on phosphate adsorption over LC@AER.

Multi-stimuli Photo and Redox-active Nanostructured Mesoporous Silica Films on Transparent Electrodes.

Silica matrices hosting transition metal guest complexes may offer remarkable platforms for the development of advanced functional devices. We report here the elaboration of ordered and vertically oriented mesoporous silica thin films containing covalently attached tris(bipyridine)iron derivatives using a combination of electrochemically assisted self-assembly (EASA) method and Huisgen cycloaddition reaction. Such a versatile approach is primarily used to bind nitrogen-based chelating ligands such as (4-[(2-propyn-1-yloxy)]4'-methyl-2,2'-bypiridine, bpy') inside the nanochannels. Further derivatization of the bpy'-functionalized silica thin films is then achieved via a subsequent in-situ complexation step to generate [Fe(bpy)2 (bpy')]2+ inside the mesopore channels. After giving spectroscopic evidences for the presence of such complexes in the functionalized film, electrochemistry is used to transform the confined diamagnetic (S=0) species to paramagnetic (S=1/2) oxidized species in a reversible way, while blue light irradiation (λ=470 nm) enables populating the short-lived paramagnetic (S=2) excited state. [Fe(bpy)2 (bpy')]2+ -functionalized ordered films are therefore both electro- and photo-active through the manipulation of the oxidation state and spin state of the confined complexes, paving the way for their integration in optoelectronic devices.

ON COMPLEXATION IN PROCESS OF CHEMICAL COPPER PLATING FROM SOLUTIONS CONTAINING POLYVINYLPYRROLIDONE

A formation of multifunctional composite coatings with a copper matrix by chemical vapor deposition is an effective method of modifying the surface of metals and non-metals and the increase in the deposition rate of coatings. It is important that the copper complex compounds are formed during chemical vapor deposition coatings and are included in the coatings composition. During the work by methods of thermogravimetric analysis and IR spectroscopy the composite coatings with copper matrix obtained from solutions containing copper sulphate and polyvinylpyrrolidone were studied. The results of thermogravimetry and IR spectroscopy of these components were presented. Samples of composite coatings for IR spectroscopy were obtained as a suspension of the coatings in vaseline oil. Deposited coating was scraped off from the substrate, and then was triturated in an agate mortar with vaseline oil until they formed a stable not stratified suspension. These suspensions were analyzed. The samples for thermogravimetry containing copper sulfate and polyvinylpyrrolidone were obtained by mixing these components. The following proofs of interaction in polyvinylpyrrolidone - copper sulphate system with forming a complex of polyvinylpyrrolidone - copper and includings of this complex into the coating were obtained. The differences of thermogravimetric curves of the mixture CuSO4 5H2O and polyvinylpyrrolidone were established. They are softening temperature increasing in the polymer material on 30 ... 35 °С towards to the softening temperature of polyvinylpyrrolidone, the appearance of the endothermic process, probably associated with the interaction of the polyvinylpyrrolidone with the copper, and the increase in the thermal stability of the polymer component of the system on 50-60 °С, which can be also with the result of complexation of polyvinylpyrrolidone - copper. The estimated structure of this complex, which reflects the results of the IR-spectrogram of the composite coatings analysis was presented. The IR spectra have absorption bands with wave numbers corresponding to the stretching vibrations of C-H bonds in the aromatic ring of polyvinylpyrrolidone, the stretching vibrations of the group – CH2 - and C-N bonds, which are also typical for the polyvinylpyrrolidone. This indicates the presence of polyvinylpyrrolidone in the coating or in its complex compound. In the spectra of the coatings the absorption bands in the range of 1750-1250 cm-1 are presented, which correspond to the stretching vibrations of C=O in the aromatic part of polyvinylpyrrolidone compound, in particular, in the group -CONHR, but they are shifted on about 50 cm-1 with respect to the same bands in the spectra of polyvinylpyrrolidone solution. Such shift is typical for the complex compounds of polyvinylpyrrolidone - copper which caused the C=O bond deformation, which occurs during the formation of additional bond of oxygen and copper atoms: C=O ∙∙∙ Cu, and indicates the presence of such complexes in a coating.Forcitation:Serbinovskiy M.Yu., Vasil'yeva N.A., Popova O.V. On complexation in process of chemical copper plating from solutions containing polyvinylpyrrolidone. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 2. P. 26-32.

Detection of drug specific circulating immune complexes from in vivo cynomolgus monkey serum samples

BackgroundAdministration of a biotherapeutic can result in the formation of anti-drug antibodies (ADAs). The resulting ADA can potentially form immune complexes (ICs) with the drug leading to altered pharmacokinetic (PK) profiles and/or adverse events. Furthermore the presence of such complexes may interfere with accurate PK assessment, and/or detection of ADA in immunogenicity assays. Here, we present two assays to detect the presence of drug–ADA immune complexes in cynomolgus monkeys. ResultsSerum samples were analyzed for IC formation in vivo. 8/8 tested animals were positive for drug specific IC. Depending on the time point tested 4/8 or 7/8 animals tested positive for ADA during drug dosing. All 8 animals were confirmed positive for ADA during the washout phase, indicating drug interference in the bridging assay. Relative amount of IC over time was determined and its correlation with PK and ADA was then assessed. Multivariate data analysis demonstrates good correlation between signals obtained from the anti-drug and FcγRIIIa based capture assays, although due to its biological characteristic FcγRIIIa based assay captured only a subset of drug specific IC. In one animal IC remained in circulation even when the drug levels decreased below detection limit. ConclusionResults from this study indicate the presence of IC during administration of an immunogenic biotherapeutic. Potential application of these assays includes detection of ADA in an IC during high drug levels. The results on the kinetics of IC formation during ADA response can complement the understanding of PK and ADA profiles. Moreover, the presence of IC indicates possible ADA interference in standard PK assays and potential underestimation of total drug exposure in toxicology studies. In addition this study also highlights the need to understand downstream in vivo consequences of drug–ADA IC as no animals under investigation developed adverse events.

Binding of oxindole-Schiff base copper(II) complexes to DNA and its modulation by the ligand

Previous studies on copper(II) complexes with oxindole-Schiff base ligands have shown their potential antitumor activity towards different cells, inducing apoptosis through a preferential attack to DNA and/or mitochondria. Herein, we better characterize the interactions between some of these copper(II) complexes and DNA. Investigations on its binding ability to DNA were carried out by fluorescence measurements in competitive experiments with ethidium bromide, using plasmidial or calf-thymus DNA. These results indicated an efficient binding process similar to that observed with copper(II)-phenanthroline species, [Cu( o-phen) 2] 2+, with binding constants in the range 3 to 9 × 10 2 M − 1 . DNA cleavage experiments in the presence and absence of distamycin, a recognized binder of DNA, indicated that this binding probably occurs at major or minor groove, leading to double-strand DNA cleavage, and being modulated by the imine ligand. Corroborating these data, discrete changes in EPR spectra of the studied complexes were observed in the presence of DNA, while more remarkable changes were observed in the presence of nucleotides (AMP, GMP, CMP or UMP). Additional evidence for preferential coordination of the copper centers to the bases guanine or cytosine was obtained from titrations of these complexes with each nucleotide, monitored by absorption spectral changes. Therefore, the obtained data point out to their action as groove binders to DNA bases, rather than as intercalators or covalent cross-linkers. Further investigations by SDS PAGE using 32P-ATP or 32P-oligonucleotides attested that no hydrolysis of phosphate linkage in DNA or RNA occurs, in the presence of such complexes, confirming their main oxidative mechanism of action.

Studies on the dissociation of botulinum neurotoxin type A complexes

The neurotoxins produced by the various strains of the anaerobic bacterium Clostridium botulinum naturally occur associated with complexing proteins which serve to protect the neurotoxins from the harsh environment of the mammalian gastrointestinal tract during bacterial invasion of the host. Three different complex species with the discrete sizes 19S (900 kDa, LL complex), 16S (500 kDa, L complex) and 12S (300 kDa, M complex) may be isolated from C. botulinum type A cultures. However, to affect their target cells these complexes must dissociate releasing the free 150 kDa neurotoxin.This study assesses the stability of these Clostridium botulinum neurotoxin serotype A (BoNT/A) complexes and identifies factors which influence their dissociation. The knowledge gained with purified toxin complexes was subsequently employed to analyze the presence of such complexes in the freeze or spray-dried commercial BoNT/A products Botox and Dysport in comparison to the complexing protein free product Xeomin.Purified 900 kDa and 500 kDa toxin complex preparations show a pH and time dependent release of the 150 kDa neurotoxin with a half-life of less than a minute at pH values >7.0. At pH values of 6.25 or less, the complexes are stable. Furthermore, dilution of concentrated 900 kDa complexes leads to dissociation into 500 kDa, neurotoxin containing complexes. Addition of sodium chloride as contained in isotonic saline leads to further disruption of these complexes resulting in the release of the free 150 kDa neurotoxin.Examination of the commercial botulinum neurotoxin products Botox and Dysport using the same analytical procedures leads to the same conclusion: the dilution, drying and reconstitution processes of these products lead to a complete dissociation of 900 kDa complexes and 85% or more of neurotoxin are present in free form. ConclusionBoNT A toxin complexes have evolved to quickly respond to specific environmental changes by efficient release of the neurotoxin. During pharmaceutical production and reconstitution of BoNT A products, the same principles effect the quantitative dissociation of 900 kDa complexes and release of free neurotoxin prior to injection into target tissues.

Efficient Plasmid DNA Cleavage by a Mononuclear Copper(II) Complex

The Cu(II) complex of the ligand all-cis-2,4,6-triamino-1,3,5-trihydroxycyclohexane (TACI) is a very efficient catalyst of the cleavage of plasmid DNA in the absence of any added cofactor. The maximum rate of degradation of the supercoiled plasmid DNA form, obtained at pH 8.1 and 37 degrees C, in the presence of 48 microM TACI.Cu(II), is 2.3 x 10(-3) s(-1), corresponding to a half-life time of only 5 min for the cleavage of form I (supercoiled) to form II (relaxed circular). The dependence of the rate of plasmid DNA cleavage from the TACI.Cu(II) complex concentration follows an unusual and very narrow bell-like profile, which suggests an high DNA affinity of the complexes but also a great tendency to form unreactive dimers. The reactivity of the TACI.Cu(II) complexes is not affected by the presence of several scavengers for reactive oxygen species or when measured under anaerobic conditions. Moreover, no degradation of the radical reporter Rhodamine B is observed in the presence of such complexes. These results are consistent with the operation of a prevailing hydrolytic pathway under the normal conditions used, although the failure to obtain enzymatic religation of the linearized DNA does not allow one to rule out the occurrence of a nonhydrolytic oxygen-independent cleavage. A concurrent oxidative mechanism becomes competitive upon addition of reductants or in the presence of high levels of molecular oxygen: under such conditions, in fact, a remarkable increase in the rate of DNA cleavage is observed.

Iron Complexes in Carbohydrate Chemistry

Formation and synthetic applications of carbohydrate-iron complexes possessing C-metal bond are discussed. Some of these intermediates are stable and can be isolated and characterized. In many instances, however, the sugar carbon-iron complexes exhibit a limited stability and are directly converted into the final products without their prior isolation and characterization. Nevertheless, the presence of such complexes can be inferred from spectral data. In this review the emphasis is put on the applications of carbohydrate-iron complexes in the synthesis of a less common sugar derivatives. Thus, de novo synthesis of mycaminose, an antibiotic sugar, is described employing an intermediary amino-dienone tricarbonyliron complex. Sodium (η5-cyclopentadienyl)-dicarbonyl-iron served as a source for the CO group insertion into derivatives of methyl β-D-glucopyranoside. A general method of deoxysugar synthesis was proposed starting from monosaccharide aldehydes and anion obtained from (η5-cyclopentadienylcarbonyl- triphenylphosphine-acetyl-iron (“acetyl-iron”). In that manner several derivatives of deoxy-pentoses, - hexoses as well as a series of 6-deoxy heptoses were obtained. Reactions of acetyl- and propionyl-iron anions with hexofuranose epoxides led to the heptofuranose lactones. Also, reactions of sugar aldehydes with anion derived from “(methylthioacetyl)iron” are described. The review presents the literature from 1979 (first applications of iron complexes in carbohydrate chemistry) to 2001, inclusively, 39 references. Keywords: iron complexes, carbohydrate chemistry, carbohydrate-iron complexes, (methylthioacetyl), mycaminose, antibiotic sugar, amino-dienone tricarbonyliron

S100–ANNEXIN COMPLEXES: SOME INSIGHTS FROM STRUCTURAL STUDIES

Several annexins have been shown to bind proteins that belong to the S100 calcium-binding protein family. The two best-characterized complexes are annexin II with p11 and annexin I with S100C, the former of which has been implicated in membrane fusion processes. We have solved the crystal structures of the complexes of p11 with annexin II N-terminus and of S100C with annexin I N-terminus. Using these structural results, as well as electron microscopy observations of liposome junctions formed in the presence of such complexes (Lambert et al., 1997 J Mol Biol 272, 42–55), we propose a computer generated model for the entire annexin II/p11 complex.

Diffusivity of arsenic interstitials in GaAs studied by sulfur in-diffusion

Sulfur in-diffusion, which is governed by the kick-out mechanism, is a suitable tool to study the diffusion on the arsenic sublattice of GaAs. Sulfur diffuses for higher surface concentrations under non-equilibrium conditions of arsenic self-interstitials (IAs). The formation of extrinsic dislocation loops has been observed. The loop formation increases the concentration of Ga vacancies, which form complexes with S donors. The presence of such complexes has been investigated by cathodoluminescence, positron annihilation, and electrochemical capacity–voltage profiling. Taking into account the concentration of As interstitials agglomerated in faulted loops, the analysis of the sulfur profile can provide both the diffusion constant and the equilibrium concentration of IAs.

Chromium speciation in tannery effluent—I. An assessment of techniques and the role of organic Cr(III) complexes

Three methods of chromium speciation were assessed in relation to possible interferences from Cr(III)-organic complexes. They were, namely: 1,5-diphenyl-carbazide (DPC) spectrophotometry, organic extraction with methyl isobutyl ketone (MIBK) and co-precipitation with iron and bismuth salts. Initially, a number of organic ligands, i.e. proteins, amino acids and organic acids were reacted with Cr(III) and only those complexes with solubility levels above that of ionic Cr(III) were assessed further. The spectrophotometric method was found to suffer minor interferences which were attributable to a non-specific turbidity effect. In comparison, two methods of MIBK extraction were found to suffer severe interferences with Cr(III)-protein complexes. Also, co-precipitation with iron salts resulted in high levels of contamination by Cr(III)-organic complexes in the ionic chromium fractions. Due to the poor solubility of bismuth salts, this method was discarded. The results demonstrate that chromium speciation in the presence of organic complexes can lead to erroneous results. However, with recognition of the presence of such complexes, corrections can be made. Since the co-precipitation behaviour of organic Cr(III) appears to be relatively specific, a scheme is described for the speciation of Cr(III) in tannery contaminated waterways. In addition, it is recommeded that a separate determination of Cr(VI) is conducted using a modification of the DPC method.

Non-transferrin-bound iron in plasma or serum from patients with idiopathic hemochromatosis: Characterization by high performance liquid chromatography and nuclear magnetic resonance spectroscopy.

The nature of non-transferrin-bound iron in the plasma or serum of iron-overloaded hemochromatosis patients was studied by high performance liquid chromatography (HPLC) and high resolution nuclear magnetic resonance (NMR). 500-MHz proton Hahn spin-echo NMR spectra of plasma or serum, combined with the use of the iron chelator desferrioxamine, suggests complexation of iron ions with citrate and a possible involvement of acetate. Addition of FeCl3 to hemochromatosis samples broadened the NMR signals from citrate. HPLC analysis rigorously confirmed the presence of an iron-citrate complex in ultrafiltrates of plasma or serum studies with added FeCl3 or desferrioxamine supported this conclusion. It is proposed that non-transferrin-bound iron in the plasma of iron-overloaded patients exists largely as complexes with citrate and possibly also as ternary iron-citrate-acetate complexes. The presence of such complexes would account for the ability of non-transferrin-bound iron to be measurable by the bleomycin assay and for its rapid clearance from the circulation by the liver.

Pathways of kinin formation and role in allergic diseases

We have developed new assays for the assessment of the plasma kinin forming system which have increased sensitivity and specificity. We utilize double-antibody ELISA assays for quantitation of complexes of activated Hageman factor-C1 inhibitor, kallikrein-C1 inhibitor, and kallikrein-α 2-macroglobulin which reflect activation of each enzyme. The fraction of cleaved high-molecular-weight kininogen is determined by immunoblotting using a monoclonal antibody to the light chain, and bradykinin is determined by radioimmunoassay. Activation of the Hageman factor-dependent pathway of kinin formation can occur when plasma is in contact with initiating surfaces or when C1 inhibitor function is diminished. The latter mechanism can occur in hereditary angioedema, in which the protein is absent or dysfunctional, or when plasma is diluted so that the effect of inhibitors is diminished and Hageman factor autoactivation is facilitated. Thus apparent “spontaneous” generation of bradykinin is seen upon incubation of plasma of hereditary angioedema patients under conditions in which normal plasma is unaffected. Studies of late-phase reactions have used a cutaneous model in which induced blisters are unroofed and challenged with antigen or buffer control using chambers which can be changed hourly. A time course of mediator release is obtained by assay of the blister fluids. Whereas most histamine is released during the first half hour, significantly elevated levels of activated Hageman factor and kallikrein complexes with C1 inactivator are seen in antigen-challenged sites between 4 and 6 hr. The presence of such complexes correlated with the presence of late-phase reactions rather than the histamine values or the magnitude of the immediate reaction. Although late-phase reactions are characterized by cellular infiltration, release of a variety of inflammatory low-molecular-weight mediators, and deposition of fibrin, activation of the Hageman factor-dependent pathway of kinin formation is also likely to be contributory.

Effect of the ionic composition of the solution on the polyvinyl alcohol adsorption on the surface of al 2o 3

The adsorption of polyvinyl alcohol (PVA) from solutions of various ionic composition has been studied on alumina. In the presence of sodium dedecylsulphate (SDS) the complexes PVA-SDS are probably formed at the interface oxide-solution. The interacaction between PVA and SDS at the interface leads to increase of PVA adsorption. However the presence of such complexes in the solution causes stepwise decrease of polyvinyl alcohol adsorption. The observed changes of surface charge density of alumina are in accord with the adsorption data. The estimated free energy of PVA adsorption on the surface of alumina is less than −9.7kJ/mole.

Plasma from rheumatoid arthritis patients does not contain abnormally high levels of α2‐macroglobulin–proteinase complexes

We measured the levels of alpha 2-macroglobulin (alpha 2M)-proteinase complexes in the plasma of 18 patients with classic rheumatoid arthritis, 11 age-matched patients with noninflammatory back pain and osteoarthritis, and 8 healthy volunteers. In contrast with previous reports, we found no evidence of alpha 2 M-proteinase complexes in the plasma samples from individuals in any of the groups. In our assays, all activity that might have been the result of the presence of such complexes in the plasma samples proved instead to be an artifact attributable to contamination of the anti-alpha 2M antibody immobilized on the AffiGel solid phase with a trypsin-like proteinase. When the contaminating activity was eliminated by pretreatment of the antibody with 1 mM diisopropyl fluorophosphate, no degradation of substrate was detected with any of the plasma samples. However, the ability of the solid-phase assay to detect and quantitate alpha 2M-proteinase complexes when they are present was confirmed in control experiments with plasma samples to which performed alpha 2M-trypsin complexes had been added, or in which alpha 2M-kallikrein complexes had been generated by activation of Hageman factor (coagulation factor XII). We therefore conclude that neither normal plasma nor that from rheumatoid arthritis patients contains measurable amounts of alpha 2M-proteinase complexes.

Complex formation in solid deuterium in the presence of a ion

Using the Metropolis scheme, we have carried out a Monte Carlo simulation of solid deuterium in the neighbourhood of a [Formula: see text] ion. The simulation is performed using a [Formula: see text] potential, which is based on the theoretical study of the stationary-point geometries of [Formula: see text] conformers by Yamaguchi et al. Complexes similar to [Formula: see text] clusters, as observed in the supersonic nozzle-beam experiments of van Deursen and Reuss, are found to form in the solid. Infrared absorption in solid deuterium due to the presence of such complexes is discussed.

Complex Formation between Cyclic Urethane and Zinc Chloride: Its Effect on Polyrotaxane Synthesis

Abstract The complex formation between cyclic urethane based on diethylene glycol and hexamethylene diisocyanate and ZnCl2 has been investigated by the NMR method. It has been established that coordination proceeds only through oxygen atoms of cyclic urethane. Its ability to aggregate into ordered clusterlike complexes under the influence of ZnCl2 has been proved, The effect of clusterlike complexes on styrene polymerization in the presence of such complexes and pure cyclourethane, leading to polyrotaxane formation, has also been investigated. X-ray diffractograms of extracted reaction products show the maxima typical for both polystyrene and cyclic urethane. The analogous investigation of products obtained in the presence of the linear analogue of cyclic urethane and its complexes with ZnCl2 prove that polystyrene and urethane are not chemically bonded. The number of cyclic molecules corresponding to one polymer chain in the polyrotaxanes obtained has been calculated. It was established that this number ...

Quantum Chemical Interpretation of the Photoinitiated Autoxidation of Sulphite Catalysed by Ferric Ions

Abstract Autoxidation of sulphite is the first well known chain reaction proceeding through free radicals in solutions. Dark and photochemical autoxidation of sulphite were stated to take place by the same mechanism except in the initial steps. It was found that the formation of sulphite radicals in photoinitiated autoxidation of sulphite which is a metal catalysed reaction as well as the classical thermal reaction is due to the absorption of photons by ferrisulphite complexes rather than sulphite itself. Confirmation of this finding was achieved spectrophotometrically as well as by the fact that such complexes are specified by very high stability constants. The present work is a further evidence for the presence of such complexes and to cover the gap present before in the literature data. Quantum mechanical calculations using the extended Hückel molecular orbital method has proved that the range of absorption by sulphite lies in a region at which sulphite alone does absorb at all, and the absorption process is thus due to the formation of ferrisulphite complexes.

Circulating soluble immune complexes containing pseudomonas antigens in cystic fibrosis.

In order to investigate whether circulating immune complexes containing Pseudomonas aeruginosa antigens mediate pulmonary damage in cystic fibrosis, we studied lung function, serum immune complex levels, and immunoglobulin concentrations in relationship to chronic pseudomonas colonisation in 69 affected children. Sixteen of the children with cystic fibrosis had increased levels of immune complexes which contained pseudomonas antigens. There was no significant relationship between lung function corrected for the effect of chronic pseudomonas colonisation and the presence of such complexes or increased levels of complexes detected by Cl1 binding or raised serum immunoglobulin concentrations. Our results suggest that these abnormalities in cystic fibrosis are secondary effects of chronic infection and they do not provide evidence for immune complex mediated lung damage in this disease.

The development of estrogen receptor systems in the rat brain: Perinatal development

The estrogen receptor system of the rat brain is not detected at 15 and 19 days of gestation, even though estrogen receptors can be measured in the maternal hypothalamus. Estrogen receptors are first detected around the 21st day of gestation after which they increase rapidly in the perinatal period and reach a plateau around postnatal day 6. Estrogen receptors are found predominantly in the limbic brain (hypothalamus, preoptic area, amygdala) and in the cerebral cortex, and they are present in very low levels in a sample consisting of the midbrain, brain stem and cerebellum. Development of cortical estrogen receptors is retarded by a few days compared to those in the limbic brain, but then is extremely rapid so that cortical receptor levels are equal to those in the limbic block on or about postnatal day 6. As early as estrogen receptors can be detected in the fetus (day 21 of gestation), they are occupied by endogenous estradiol in male but not in female fetuses. This is measured by the occurrence of cell nuclear estrogen receptor complexes which can be detected by an exchange assay. The presence of such complexes in male limbic brain tissue is consistent with the presence of testicular androgen in the calculation and with the existence in the brain of aromatizing enzymes capable of converting testosterone to estradiol. The absence of such complexes from the female limbic brain is consistent with the protective role of alpha-fetoprotein in scavenging circulating maternal estradiol and keeping it from reaching intracellular receptor sites. The results are discussed with respect to the timing of the critical period for brain sexual differentiation and related maturational events.