Apparent Repulsion Research Articles

Can Counter-Intuitive Halogen Bonding Be Coulombic?

We use the term "counter-intuitive" to describe an intermolecular interaction in which the electrostatic potentials of the interacting regions of the ground-state molecules have the same sign, both positive or both negative. In the present work, we consider counter-intuitive halogen bonding with nitrogen bases, in which both the halogen σ-hole and the nitrogen lone pair have negative potentials on their molecular surfaces. We show that these interactions can be treated as Coulombic despite the apparent repulsion between the ground-state molecules, provided that both electrostatics and polarization are explicitly taken into account. We demonstrate first that the energies of 20 counter-intuitive interactions with four nitrogen bases can be expressed very well in terms of just two molecular properties: the electrostatic potential of the halogen σ-hole and the average polarizability of the nitrogen base. Then we show that the same two properties can also represent the energies of an expanded data base that includes the 20 counter-intuitive plus an additional 20 weak and moderately-strong intuitive halogen bonding interactions (in which the σ-hole potentials are now positive).

Apparent repulsion between equally and oppositely charged spherical polyelectrolytes in symmetrical salt solutions.

Ion-mediated interactions are very important for the properties of colloids and biomacromolecules such as nucleic acids and proteins. In this work, the ion-mediated interactions between equally and oppositely charged spherical polyelectrolytes (SPEs) in symmetrical divalent electrolytes have been investigated by Monte Carlo simulations, and an unexpected apparent repulsion was observed at high divalent salt concentration. Our investigations also show that the effective repulsion becomes more pronounced for SPEs with higher charge densities and for counterions with larger sizes and was found to be tightly accompanied with the over-neutralization to SPEs by condensed counterions and their release upon the approach of SPEs. Such attractive interaction can be reproduced by our proposed modified Poisson-Boltzmann model and is mainly attributed to the increase in the electrostatic repulsion between on charged SPE and the over-neutralized counterions around the other oppositely SPE with the approach of the two SPEs.

Luneburg-lens-like structural Pauli attractive core of the nuclear force at short distances

The nuclear force has been understood to have a repulsive core at short distances, similar to a molecular force, since Jastrow proposed it in 1951. The existence of the repulsion was experimentally confirmed from the proton-proton scattering 1S_0 phase shift, which becomes negative beyond 230 MeV. This repulsion is essential for preventing the nucleon-nucleon system from collapsing by attraction. The origin of the repulsion has been considered to be due to the Pauli principle, similar to the repulsion originally revealed in alpha-alpha scattering, in many studies including recent lattice QCD calculations. On the other hand, very recently it was shown that an inter-nuclear potential including alpha-alpha interactions has a Luneburg-lens-like attraction at short distances rather than repulsion. We show that the nuclear force with an attractive potential at short distances that reproduces the experimental phase shifts well has a Luneburg-lens-like structural Pauli attractive core (SPAC) at short distances and acts as apparent repulsion. The apparent repulsion is caused by the deeply embedded unobservable Pauli forbidden state similar to nucleus-nucleus potentials.

Hearing voices then seeing lips: Fragmentation and renormalisation of subjective timing in the McGurk illusion

Due to physical and neural delays, the sight and sound of a person speaking causes a cachophony of asynchronous events in the brain. How can we still perceive them as simultaneous? Our converging evidence suggests that actually, we do not. Patient PH, with midbrain and auditory brainstem lesions, experiences voices leading lipmovements by approximately 200 ms. In temporal order judgements (TOJ) he experiences simultaneity only when voices physically lag lips. In contrast, he requires the opposite visual lag (again of about 200 ms) to experience the classic McGurk illusion (e.g., hearing ‘da’ when listening to /ba/ and watching lips say [ga]), consistent with pathological auditory slowing. These delays seem to be specific to speech stimuli. Is PH just an anomaly? Surprisingly, neuro-typical individual differences between temporal tuning of McGurk integration and TOJ are actually negatively correlated. Thus some people require a small auditory lead for optimal McG but an auditory lag for subjective simultaneity (like PH but not as extreme), while others show the opposite pattern. Evidently, any individual can concurrently experience the same external events as happening at different times. These dissociative patterns confirm that distinct mechanisms for audiovisual synchronization versus integration are each subject to different neural delays. To explain the apparent repulsion of their respective timings, we propose that multimodal synchronization is achieved by discounting the average neural event time within each modality. Lesions or individual differences which slow the propagation of neural signals will then attract the average, so that relatively undelayed neural signals will be experienced as occurring relatively early.

Probing the active site tightness of DNA polymerase in subangstrom increments.

We describe the use of a series of gradually expanded thymine nucleobase analogs in probing steric effects in DNA polymerase efficiency and fidelity. In these nonpolar compounds, the base size was increased incrementally over a 1.0-A range by use of variably sized atoms (H, F, Cl, Br, and I) to replace the oxygen molecules of thymine. Kinetics studies with DNA Pol I (Klenow fragment, exonuclease-deficient) in vitro showed that replication efficiency opposite adenine increased through the series, reaching a peak at the chlorinated compound. Efficiency then dropped markedly as a steric tightness limit was apparently reached. Importantly, fidelity also followed this trend, with the fidelity maximum at dichlorotoluene, the largest compound that fits without apparent repulsion. The fidelity at this point approached that of wild-type thymine. Surprisingly, the maximum fidelity and efficiency was found at a base pair size significantly larger than the natural size. Parallel bypass and mutagenesis experiments were then carried out in vivo with a bacterial assay for replication. The cellular results were virtually the same as those seen in solution. The results provide direct evidence for the importance of a tight steric fit on DNA replication fidelity. In addition, the results suggest that even high-fidelity replicative enzymes have more steric room than necessary, possibly to allow for an evolutionarily advantageous mutation rate.

Comparison of figural aftereffect and figural retroactive-effect for the concentric circles figure.

The concentric circles figural aftereffect (FAE) and figural retroactive-effect (FRE) have been compared for different luminance contrast and temporal conditions (Adam, Heron, & Cooper, 1972; Adam, Mullin, & Sperry, 1969). This paper reports a comparison of the two phenomena for six diameters of the inducing figure, these ranging from four-tenths to nine-tenths of a standard 1.25-in. test figure diameter. Stimuli were drawn in black on white cards and presented tachistoscopically at a distance of 52 in. Both figures were 1.25 in. to S's left from a fixation point while comparison figures were the same distance to the right. Exposure durations of the inducing and test figures were 50 msec. and 200 msec. respectively, the second stimulus (test figure for FAE, inducing figure for FRE) immediately following the first (inducing figure for FAE, test figure for FRE). Two independent groups of 10 Ss each were used for measurement of the FAE and FRE, each S making judgments for all diameters of inducing figures. Other details of experimental method were essentially the same as those used in the earlier comparisons of FAE and FRE, and in previous FAE studies reported in This Journal (e.g., Adam & Ring, 1968). The FAE consistently displayed the typical apparent repulsion of test figure by the inducing figure. In the raw data for the FRE, however, a number of attraction effects were evident, these occurring most frequently when the ratio between diameters of the inducing and test figures was 8:lO. These attraction effects were replicated for a new group of 10 Ss and persisted when Ss were retested after an interval of one to two days. Conditions for their occurrence are currently under investigation.

Large-Scale Distribution of Solar Magnetic Fields.

Solar magnetograms covering a period of years were used to study the distribution of weak magnetic fields on the solar surface. The semiregular pattern of these background fields is the result of the expansion, weakening, and stretching by differential rotation of magnetic fields of old active regions and their interaction with neighboring fields, and of the continuing development of magnetic fields of new regions within the pattern. The net result is a slowly changing pattern of background fields which occupies at times during the solar cycle more than 50 per cent of the surface area of the Sun, as seen with the 23" resolution of the magnetograph. The interaction of nearby magnetic fields takes the form of the apparent attraction of features of the same polarity and the apparent repulsion of features of opposite polarity. Sometimes weak magnetic features covering a large area apparently disappear over a period of a few rotations. It is not clear from the observations what the mechanism of this disappearance could be. It is clear from the period near minimum that active regions are concentrated in complexes of activity whose location and development are clearly defined. The largest of these complexes, consisting of many active regions, result in the formation of Unipolar Magnetic Regions (UMR). The polar fields are discussed from the standpoint of the poleward drift of the UMR's.

XXIII. Distance correlations in an ideal Fermi-Dirac gas

Summary The antisymmetry of the total wave function of a Fermi-Dirac gas leads to an apparent repulsion between the gas particles. In other words, the average probability that two particles will be found at a distance r from one another falls to zero as r tends to zero. This pair distribution function, D(r), is calculated (§ 2) at all temperatures by means of a variation method founded on a minimum property of the free energy. It is not possible to make a straightforward calculation using Fermi-Dirac statistics on account of the integrals which arise. Instead we use an approximation which has already been employed to meet a similar difficulty in the problem of finding the influence of exchange energy on the specific heat of electrons in metals.

IV. The pressure upon the poles of the electric arc

Suspending one pole of a carbon arc and keeping the other pole fixed it was found that there was an apparent repulsion between them. There is in fact a pressure upon each electrode which tends to separate them. The first part of this paper is devoted to the experimental methods of estimating this pressure, the second to a discussion of its origin. Part I.—Experimental. Three series of observations have been made, the original observations and preliminary series by Duffield in 1912, the second series in conjunction with Burnham, and the third in conjunction with Davis. In spite of the very small forces examined the three series agree within reasonable limits. The general form of the apparatus (fig. 1) was the same in each series, though there were important differences in the dispositions of the carbons in different sets of experiments.

An hydrodynamic explanation of mitotic figures

The distinctly polar arrangement of the chromatin substance about the astral centers in dividing cells, combined with the pronounced curvature of the astral rays and of the spindle fibers, have demanded the assumption of some polar force as universally operative. On such an assumption it is of course necessary to assume further that astral centers represent either opposite or like poles. On the alternative of opposite poles, we should expect, with any force so far proposed, a configuration of astral rays simulating that of iron filings between opposite magnetic poles, coupled with a mutual attraction of the astral centers. On the other alternative, we should similarly expect a configuration of astral rays and spindle fibers simulating that of iron filings between like magnetic poles, coupled with a mutual repulsion of the astral centers. Actually, we have neither of these conditions, but instead, a configuratian like that of iron filings between opposite magnetic poles and at the same time an apparent repulsion between the astral centers or the centrosomes. This is not the case with the forces of attraction or repulsion existing between bodies oscillating or pulsating in a fluid medium. More specifically, if two spheres are pulsating synchronously and in opposite phase, or oscillating synchronously and in the same phase, they will repel one another, but at the same time the field between them will simulate the configuration of iron filings between opposite magnetic poles. If then we assume that the centrosomes are pulsating in opposite phase, or better, oscillating in the same phase, we will obtain the desired repulsion and at the same time have a configuration like that actually observed.

XXV. Experiments and reflections of Dr. Joachim Carradori de Prato on the apparent repulsion between some kinds of fluids observed by Draparnaud

XXV. <i>Experiments and reflections of Dr.</i> J<scp>oachim</scp> C<scp>arradori de</scp> P<scp>rato</scp> <i>on the apparent repulsion between some kinds of fluids observed by</i> D<scp>raparnaud</scp>