Showing papers on "Chemical binding published in 1974"

Modified transport substrates as probes for intramembrane gradients.

Abstract: Probes for measuring energetic gradients across biological membranes have generally been selected for their chemical inertness with respect to the membrane. For example, the distribution of the chloride ion or of the dibenzyldimethylammonium ion across the red blood cell membrane may serve to report the transmembrane potential gradient if their movements respond to that force and to no other; 5,5-dimethyl-2,4-oxazolidinedione (DMO) will report the cytoplasmic pH only if its movement responds only to the pH gradient and to no other gradient. The same requirements apply to ion-ionophore combinations that are used for similar purposes. If any of these test substances were to encounter specific uphill transport in traversing the membrane, an invalid measurement of the desired gradient would result. Probes of this type, of course, provide only a statistical measurement of the energetic gradient, measured all the way across the membrane and averaged for its total area. For the sake of clarity, we deliberately sidestep the possibility that we could obtain information on local gradients by avoiding any energytransferring reactivity between the probe and components of the membrane. We will develop here the possibility of using quite another type of probe, one whose crossing of the membrane occurs only through the chemical binding and release events implicit in active transport.'. 2 Such probes should be hydrophilic solutes, often zwitterions, sometimes with a net charge, so that their passage by diffusion through the lipoid membrane substance is negligible in relation to their passage by specific transport. Their molecular structure must combine two sets of features: (1) the structural features necessary to keep transport restricted to the given system, which may or may not supply a vectorial force in one or the other direction; and (2) a structural feature that causes a response to a membrane gradient. Control substrates that lack this feature will then provide the reference standard, by comparison with which the response is recognized. Such test substances we will call gradient-sensing substrates for transport. Application of a membrane probe that combines these features would invite only confusion if the membrane also permitted their passive permeation. But when migration is limited to the pathway described by the process of active transport, then we may achieve probes that sense gradients specific to the interval between successive topographic points on that pathway. Rather than obtaining another

Biological activity and electronic structure of the aflatoxins.

Abstract: In theoretical studies of aromatic hydrocarbons, Pullman and Pullman (1969) used the molecular orbital method to correlate electronic structure with biological activity They suggested that the interaction between carcinogens and their molecular receptors must occur through the K region of the carcinogenic molecule and involve a strong chemical binding of the type of an addition reaction In the present work the electronic structures of aflatoxins B(1), G(1), 4-20 dehydro B(1) and of versicolorin A have been determined by the simple Huckel molecular orbital method using a computer, in order to see whether the correlation between electronic structure and biological activity is applicable to these compounds also Calculations show that the 2-3 pi-bond, which has the highest bond order of the aflatoxin molecules, should be the most susceptible to electrophilic attack and is the most probable location of the K region This is in agreement with the experimental observation of Dutton and Heathcote (1968) that aflatoxins B(1) and G(1) hydrate rapidly in dilute acid to the hydroxyaflatoxins B(2a) and G(2a) with an apparent total loss of carcinogenicity The calculations also show that aflatoxins B(1) G(1) and M(1) have no suitable site for an L region and this probably accounts for their highly carcinogenic nature

Chemical models of the hydrophobic interaction: Apparent molal volumes and heat capacities of three symmetrical bolaform electrolytes and their homologous monomers in aqueous solution at 25°C

Abstract: The densities and volumetric specific heats of aqueous solutions of Bu3NHBr, Pent3NHCl, and three diazonium salts, HN−Oct3−NHBr2, HN−Dec3−NHCl2, and Bu3N−Oct−NBu3Br2, have been measured at 25°C. From these data, the apparent molal volumes φv and apparent molal heat capacities φc have been calculated and are reported here. In the series of compounds chosen, the diazonium (higher homologs) can be regarded as dimers of the alkyl-substituted ammonium ions (lower homologs), and these systems are examined as chemical models for the hydrophobic interaction. With the three homologous pairs studied here, the chemical model predicts that the strong interaction (limitingly, chemical binding) of two hydrocarbon chains in water leads to a major decrease in both φv and φc of the interacting solutes, ca.−22 cm3-mole−1 and −200 J-oK−1-mole−1. These predictions constitute limiting behavior — useful, but not sufficient, to explain the observed concentration dependence of φv and φc in aqueous solutions of the lower homologs Bu3NHBr, Pent3NHCl, and Bu4NBr. An explanation for the concentration dependence of φc is suggested with reference to ultrasonic relaxation data.

Chemisorption on (001), (110) and (111) Nickel Surfaces

Abstract: Comparative features of the chemisorption of reactive gases on the three principal low index clean single crystal faces of nickel and interpretations based on several typical atom-surface interaction models are presented and discussed. The conclusions based on measurements of microscopic parameters are reviewed in terms of data correlated from LEED patterns, computational analysis of LEED spectra, fine structure of Auger peak shapes and variations in work function changes as a function of coverage, exposure and heat treatment. The observed adsorption features are used to distinguish chemisorption phenomena in terms of differences in chemical binding, surface structure and charge transfer. Chemical overlayer systems involving well ordered single surface phases formed on unperturbed substrates most easily lend themselves to quantitative crystallographic analysis. Some comparisons of the results of this study to other experimental and theoretical studies on clean and chemisorbed nickel surfaces are considered.

Appearance potential spectrum of barium

Abstract: When the conventional tungsten filament in an appearance potential spectrometer was replaced by an oxide coated filament, barium contamination was detected on the iron specimen being examined. The spectrum obtained showed prominent M structure of barium in addition to weak iron L structure. Even after the work function correction is applied, the barium edges are shifted by several eV from the respective energy levels quoted in the literature. This is presumably the effect of chemical binding of the barium atoms on the surface. The iron and the barium spectra showed slight changes in structure when the specimen was heated in situ.

In Situ Localization of Proline in Oral Bacteria and on Lingual Epithelium

Abstract: During a study of proline utilization by mouse lingual epithelial cells (MACCALLUM, Anat Rec 175: 380, 1973), some temporal aspects of the incorporation of the free amino acid by oral bacteria and the appearance of a proline-containing surface coat on oral epithelial cells were determined. Weanling C58 mice were injected intraperitoneally with 166 microcuries/gm of Lproline-2, 3_3H (> 40 curies/mM) and were killed 30 minutes or 1, 4, or 24 hours later. Numerous bacteria demonstrated incorporated proline 30 minutes and one hour after injection (Fig 1). The mode of entrance of the free amino acid into the oral cavity is uncertain because the methods used in this study were designed to eliminate the chemical binding of free amino acids to tissue structures. Two obvious routes of entrance to the oral cavity are the salivary glands and gingival sulci. There is at least some fluid that contains free amino acids emanating from gingival sulci (WEINSTEIN ET AL, Periodontics 5: 161, 1967). Salivary gland fluid