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International audience; Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude1. Until now, indications of 2νECEC decays have only been seen for two isotopes2,3,4,5, 78Kr and 130Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance6,7. The 2νECEC half-life is an important observable for nuclear structure models8,9,10,11,12,13,14 and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass15,16,17. Here we report the direct observation of 2νECEC in 124Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 1022 years (statistical uncertainty, 0.5 × 1022 years; systematic uncertainty, 0.1 × 1022 years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments18,19,20.

Upon exposure to ultra-intense, hard X-ray pulses, polyatomic molecules containing one heavy atom reach a much higher degree of ionization than do individual heavy atoms, contrary to previous assumptions. X-ray free-electron lasers offer many new applications such as the ability to structurally probe fast biological processes. This requires the use of hard and intense X-ray pulses, but the behaviour of matter under such conditions has not been fully explored. Artem Rudenko et al. show that when exposing small polyatomic molecules that contain one heavy atom to hard X-ray pulses with ultra-high intensities, the response is qualitatively different from what is seen in experiments carried out under less extreme conditions. The observed ionization of the molecule is considerably enhanced compared to that of an individual heavy atom under the same conditions, owing to ultrafast charge transfer within the molecule that replenishes the electrons removed from the heavy atom, enabling further ionization. Being able to account for this effect will aid further use of X-ray free-electron lasers for studying biological systems. X-ray free-electron lasers enable the investigation of the structure and dynamics of diverse systems, including atoms, molecules, nanocrystals and single bioparticles, under extreme conditions1,2,3,4,5,6,7. Many imaging applications that target biological systems and complex materials use hard X-ray pulses with extremely high peak intensities (exceeding 1020 watts per square centimetre)3,5. However, fundamental investigations have focused mainly on the individual response of atoms and small molecules using soft X-rays with much lower intensities8,9,10,11,12,13,14,15,16,17. Studies with intense X-ray pulses have shown that irradiated atoms reach a very high degree of ionization, owing to multiphoton absorption8,12,13,18, which in a heteronuclear molecular system occurs predominantly locally on a heavy atom (provided that the absorption cross-section of the heavy atom is considerably larger than those of its neighbours) and is followed by efficient redistribution of the induced charge14,15,16,17,19,20. In serial femtosecond crystallography of biological objects—an application of X-ray free-electron lasers that greatly enhances our ability to determine protein structure2,3—the ionization of heavy atoms increases the local radiation damage that is seen in the diffraction patterns of these objects21,22 and has been suggested as a way of phasing the diffraction data23,24. On the basis of experiments using either soft or less-intense hard X-rays14,15,16,17,18,19,25, it is thought that the induced charge and associated radiation damage of atoms in polyatomic molecules can be inferred from the charge that is induced in an isolated atom under otherwise comparable irradiation conditions. Here we show that the femtosecond response of small polyatomic molecules that contain one heavy atom to ultra-intense (with intensities approaching 1020 watts per square centimetre), hard (with photon energies of 8.3 kiloelectronvolts) X-ray pulses is qualitatively different: our experimental and modelling results establish that, under these conditions, the ionization of a molecule is considerably enhanced compared to that of an individual heavy atom with the same absorption cross-section. This enhancement is driven by ultrafast charge transfer within the molecule, which refills the core holes that are created in the heavy atom, providing further targets for inner-shell ionization and resulting in the emission of more than 50 electrons during the X-ray pulse. Our results demonstrate that efficient modelling of X-ray-driven processes in complex systems at ultrahigh intensities is feasible.

Polycyclic bio-active natural products that contain halogen atoms have been isolated from a number of different marine organisms. The biosynthesis of these natural products appears to be initiated by an electrophilic halogenation reaction at a carbon-carbon double bond via a mechanism that is similar to a proton-induced olefin polycyclization. Enzymes such as haloperoxidases generate an electrophilic halonium ion (or its equivalent), which reacts with the terminal carbon-carbon double bond of the polyprenoid, enantioselectively inducing a cyclization reaction that produces a halogenated polycyclic terpenoid. Use of an enantioselective halocyclization reaction is one possible way to chemically synthesize these halogenated cyclic terpenoids; although several brominated cyclic terpenoids have been synthesized via a diastereoselective halocyclization reaction that uses stoichiometric quantities of a brominating reagent, the enantioselective halocyclization of isoprenoids induced by a chiral promoter has not yet been reported. Here we report the enantioselective halocyclization of simple polyprenoids using a nucleophilic promoter. Achiral nucleophilic phosphorus compounds are able to promote the diastereoselective halocyclization reaction to give a halogenated cyclic product in excellent yields. Moreover, chiral phosphoramidites promote the enantioselective halocyclization of simple polyprenoids with N-iodosuccinimide to give iodinated cyclic products in up to 99% enantiomeric excess and diastereomeric excess. To the best of our knowledge, this is the first successful example of the enantioselective halopolycyclization of polyprenoids.

A GENERAL concensus of opinion among investigators has agreed that Boachardat's reagent is the most reliable among many tried for the cytochemical demonstration of alkaloids. It is a general alkaloid reagent, with no specificity, and consists of 1 per cent iodine dissolved in 1 per cent potassium iodide. It colours proteins very heavily. With practice, the red tint and fine grain of the alkaloid precipitate can be readily distinguished from the flocculent brown of the protein. Errera1 devised the further expedient of treating parallel preparations with a 5 per cent solution of tartaric acid in alcohol. This dissolves out the alkaloids so that a clear distinction should be apparent in alkaloid-containing tissues with and without such pre-treatment. No difference will be apparent if the iodine precipitation is due to protein.

THIS note describes the histological demonstration of mucin by Schiff's reagent following the action of periodic acid. Zenker-formol sections were passed to water, after iodine and hypo, and placed for two minutes in a 0.5 per cent solution of periodic acid in distilled water. The sections were then washed in tap and distilled water and placed in Schiff's reagent for fifteen minutes at room temperature. The customary rinsings in sulphurous acid, as for the Feulgen's test, followed, and the sections were dehydrated in alcohols and mounted in balsam after xylene.

SEVERAL methods have been used for the analysis of specific precipitates formed by the union of antigen and antibody and for the determination of the antigen-antibody ratio in these precipitates. Some investigators have used antigens containing a readily determinable element such as iron (haemoglobin1), copper (haemocyanin2), iodine (iodoalbumin3) or arsenic4, or an antigen containing a dye5. The precipitates are also usually analysed for protein nitrogen, and Heidelberger and his colleagues have, in particular, developed very accurate quantitative tests for the examination of serological precipitates6. N15 has also been used in investigations on the formation and the circulation of antibodies and in other immunological studies7.

O'Dowd et al. describe the formation of marine aerosols from biogenic iodine and the growth of these aerosols into cloud-condensation nuclei (CCN). Based on chamber and modelling results, the authors suggest that biogenic organic iodine compounds emitted from macroalgae may be responsible for coastal particle bursts and that production of these compounds in the open ocean could increase CCN there too. It has since been shown that coastal particles are more likely to be produced from the photooxidation of molecular iodine. Moreover, I contend that open-ocean particle production and cloud enhancement do not result from emissions of organic iodine at atmospheric levels. For iodine particles to affect cloud properties over the remote ocean, an additional source of iodine is necessary as organic precursors cannot be responsible.

The formation of marine aerosols and cloud condensation nuclei--from which marine clouds originate--depends ultimately on the availability of new, nanometre-scale particles in the marine boundary layer. Because marine aerosols and clouds scatter incoming radiation and contribute a cooling effect to the Earth's radiation budget, new particle production is important in climate regulation. It has been suggested that sulphuric acid derived from the oxidation of dimethyl sulphide is responsible for the production of marine aerosols and cloud condensation nuclei. It was accordingly proposed that algae producing dimethyl sulphide play a role in climate regulation, but this has been difficult to prove and, consequently, the processes controlling marine particle formation remains largely undetermined. Here, using smog chamber experiments under coastal atmospheric conditions, we demonstrate that new particles can form from condensable iodine-containing vapours, which are the photolysis products of biogenic iodocarbons emitted from marine algae. Moreover, we illustrate, using aerosol formation models, that concentrations of condensable iodine-containing vapours over the open ocean are sufficient to influence marine particle formation. We suggest therefore that marine iodocarbon emissions have a potentially significant effect on global radiative forcing.

BARTON1 has suggested that photoelectron interference patterns may be used directly as holograms to obtain atomic-resolution images of surface structures. Bulk structures have been obtained previously by this means from experimental patterns of high-energy Kikuchi (quasi-elastically scattered) and Auger electrons2,3. Here we test the feasibility of this technique for determination of surface structures using Auger intensity patterns obtained4,5 from iodine chemisorbed on a pseudomorphic silver monolayer on Pt{111}. By direct numerical holographic inversion, we obtain three-dimensional images which show that iodine adatoms are located in hollows of 3-fold symmetry on the surface. The images yield the site symmetry with good atomic resolution in the surface plane, but suffer from poor resolution along the Ag–I axis. We anticipate that data with better angular resolution obtained at low temperatures would improve the spatial resolution of such images.

Iodine atoms can be fitted with a chemical jacket to control the conversion of simple carbon chains into complex iodine-containing molecules. Previously, such reactions were only possible with enzymes.

O'Dowd et al. reply - McFiggans raises some interesting, but partly speculative, issues about the possibility of additional condensable-iodine-vapour (CIV) precursors being involved in marine aerosol formation from biogenic iodine emissions, and about the relative roles of iodine oxide and sulphuric acid in the marine new-particle formation process.

Iodine-containing emissions from marine algae can be converted by sunlight into aerosol particles. If this phenomenon occurs on a large scale, it could have significant effects on climate.

ONE of the most remarkable developments in quantum mechanics in recent years has been the discovery that when a system is moved adiabatically around a closed loop in parameter space there occurs, besides the familiar dynamical phase shift, an additional phase shift (sometimes referred to as 'Berry's phase) that is purely geometric in nature1–3. The dynamical phase shift, which results from the variation of the period of the oscillatory system with the change in parameters, is relatively easily understood and is proportional to the time over which the parameter change occurs. The geometric phase shift, on the other hand, is less intuitive and depends on the curvature of the surface in parameter space bounded by the closed path, but is independent of the time taken to traverse the circuit. Here we present evidence for time-independent geometric phase shifts in numerical solutions for a model of an oscillating chemical reaction. The conditions for the occurrence of such shifts seem to be sufficiently general that geometric phase effects should be experimentally observable in essentially all chemical oscillators as well as in biological networks such as the brain and the central nervous system, where phase control is of vital importance4.

Arising from: C. O'Dowd et al. Nature 417, 632–636 (2002); see O'Dowd et al. reply . O'Dowd et al.1 describe the formation of marine aerosols from biogenic iodine and the growth of these aerosols into cloud-condensation nuclei (CCN). Based on chamber and modelling results, the authors suggest that biogenic organic iodine compounds emitted from macroalgae may be responsible for coastal particle bursts and that production of these compounds in the open ocean could increase CCN there too. It has since been shown that coastal particles are more likely to be produced from the photooxidation of molecular iodine2. Moreover, I contend that open-ocean particle production and cloud enhancement do not result from emissions of organic iodine at atmospheric levels. For iodine particles to affect cloud properties over the remote ocean, an additional source of iodine is necessary as organic precursors cannot be responsible.

Accumulation of Radioactive Iodine in Thyroid Glands Subsequent to Nuclear Weapon Tests and the Accident at Windscale

In the Daguerre photographic process1–3, a polished silver-coated plate is exposed to iodine vapour, thereby forming a very thin film of silver iodide, usually about 30-nm thick4. After exposure, the image is developed by mercury vapour, which condenses preferentially where light has fallen, amalgamating with silver liberated from the silver iodide (and possibly with the substrate). The remaining silver iodide is dissolved with an aqueous photographic fixing solution (sodium thiosulphate). When viewed at most angles, a positive image of the scene is seen as areas of amalagam (diffusely reflecting), which appear white, on areas of polished silver (specularly reflecting), which appear black. The sharpness with which edges were reproduced in several old daguerreotypes led us to suspect that electrostatic processes might be involved, and we now report two experiments which confirm that.

THE basal metabolism is increased in many patients with lymphatic and myelogenous leukaemia. Elevated levels of plasma iodine have been found in various leukaemic cases, possibly indicating involvement of the thyroid gland1. It is thought that at lower pH values there is more free iodine, and so greater penetrability into cells and greater iodine activity2. Iodine fixation by human leukocytes requires energy3. Iodine alters the redox state of the cytoplasm and mitochondria, because the couple 2 I−→I2+2 e has a rather high redox potential: +0.535 V; iodine seems to be one of the oxidizing agents which can be used for ATP formation4. Flavin is the most likely oxidizing agent for a couple with such a high redox potential5.

THE usefulness of activation analysis for the estimation in vivo of total body sodium, chlorine and calcium in the human subject has recently been demonstrated1. Because of the remarkable concentration of iodine in the thyroid gland, irradiation of this organ by thermal neutrons should make it possible to estimate the total iodine content. To study this possibility we have carried out experiments with sheep, where the thyroid iodine content is of the same order of magnitude as in man.

THE level of protein-bound iodine in the serum of rats was found to undergo significant changes within 24 h after their thyroids were irradiated internally by iodine-131 at total doses above 150,000 rads1. In subsequent studies, it was found that labelled thyroglobulin is released from the thyroid on internal irradiation with doses above 250,000 rads2 and also at 130,000 rads3. No effect was, however, observed after the administration of 100 µc. iodine-131, which deliver to the thyroid a dose of about 15,000 rads2,3. Release of radioactivity from thyroids of dogs on external irradiation with 21,000 rads was also reported4. In experiments performed on animals so far, there is no evidence for an increase in the level of protein-bound iodine or in the release of thyroglobulin at doses lower than 20,000 rads. On the other hand, certain experimental results were reported which may indicate the release of thyroglobulin on irradiation even at lower doses. It was shown by bioassay that the thyroxino content of thyroids of mice diminished after doses of total body irradiation as low as 50 rads5. A slight increase in the labelled protein-bound iodine was observed in rats at total body irradiation of 450 rads 48 h after irradiation6; this effect was, however, interpreted as being due to an enhanced turn-over. On the other hand, it was stated in another investigation7 that the overall turn-over of the thyroid decreases on irradiation.

BLAU'S modification1 of the Leland and Foster2 method for separation of free iodides and iodotyrosines from thyroxine in n-butanol extracts has constituted the cornerstone of blood butanol-extractable iodine methods3,4 and facilitates the quantitative analysis of thyroxine in thyroid extracts1. The complete fractionation of iodinated compounds, however, requires the use of lengthy techniques of column or paper chromatography; the latter is not always easy to quantitate and is subject to considerable experimental error. Block et al.6 indicated that activated charcoal can quantitatively separate iodinated amino-acids from inorganic iodide, but fortunately no experimental details were given. We consequently worked out a method whereby Blau's alkaline reagent, containing iodide and iodotyrosines washed out of butanol extracts, is treated with modified charcoal to quantitatively adsorb the iodotyrosines and thus separate them from free iodide. The method is simple and is applicable to both chemical as well as to radioisotope studies.

Following the announement of the Windscite Reactor accident of Oct. 10, 1957, in apparatus was assembled to measure the trace uptake of iodine-131 in human thyroids and the concentration of iodine-131 and cesium-137 in milk. Data are tabulated covering a twomonth period. Where the intake of iodine-131 from milk could be estimated, a close association with the uptake of iodine-131 in the thyroid was observed. (C.H.)

Measurements were made of the gamma -ray dose rate over a lawn and a house in Leeds during Aug. and Sept. 1957, and were repeated on Oct. 19, 1957 following the Windscale Reactor accident of Oct. 10. Small increases in dose rate were observed. The measurements were continued for several months. Data are tabulated. The relation of the radioactivity to the iodine-131 activity in milk from the area is discussed. (C.H.)

DETERMINATION of the relative rates of physiological activity in free-ranging animals in different environments is important to the understanding of the interaction between an organism and its environment, as well as in comparative studies of energy flow in ecosystems. Methods should be used which permit determinations over a period of several days or longer so that most of the animal's daily activities are involved. Odum and Golley1 have expressed some of the points of view and methods applicable to the use of radio nuclides for this purpose.

SINCE the announcement by Mueller et al.1 of a technique for forming thin (

ALL forms of radioimmunoassay used at present include a procedure to separate free and bound tracer antigen when equilibrium is reached in the incubation mixture. This separation may involve precipitation by a second antibody1,2, electrophoresis3,4, chromatoelectrophoresis5, ion exchange6, solvent fractionation7, gel filtration8,9, salt precipitation10, or adsorption to charcoal11. The need to use such formal isolation procedures increases the length and complexity of the assay and introduces a number of possible sources of error.

Following the Windscale reactor accident of October 1957, measurements were made of iodine-131 levels in the thyroid gland of persons from various sections of the country. The measurements were made in a lowbackground laboratory using a crystal detector coupled to a photomultiplier tube. The measurements were repeated over a four-month period. Data are tabulated. An attempt is made to correlate the findings with the milk consumption of the subjects, the location of the dairies supplying the milk, and the passage of the atomic cloud over Britain. (C.H.)

Increase of the Rate of Release of Thyroidal Iodine-131 and of Circulating Thyrotrophic Activity at Early Stages of Propylthiouracil Treatment in the Rat

Separation of Antibody-Bound and Unbound Peptide Hormones Labelled with Iodine-131 by Talcum Powder and Precipitated Silica

Cytochemical and Immunochemical Analysis at the Electron Microscopy Level: Obtaining Contrasting Antibodies by Use of Iodine

USING the staining of cell-walls blue by an iodine in potassium iodide solution (0.3 gm. iodine and 1.6 gm. potassium iodide dissolved in 100 ml. water1), many plant species previously designated as containing amyloid in their seeds6 (for example, of the genera Iris, Polygonatum, Narcissus, Hydrophyllum) are excluded, while others show the amyloid reaction with this reagent. The species of the latter group are summarized in Table 1.