Fractional Site Occupancy Research Articles

Cold Alchemi: Impurity Atom Site Location and the Temperature Dependance of Dechannelling

ABSTRACTRecent applications of the “Atom Location by Channelling Enhanced Microanalysis” (ALCHEMI) are summarised. This is a quantitative method for determining the sites of impurity atoms in crystals using an electron microscope and X-ray analysis system. Since the fractional site occupancies are given in terms of measured X-ray counts alone, it involves no adjustable parameters. New experiments have been performed on the temperature dependence of characteristic X-ray production under channelling conditions for InP and GaAs. We conclude that low temperatures will give more accurate results for “ALCHEMI” in some materials due to the reduction in diffuse inelastic phonon scattering, and that the study of this temperature dependence may provide new information from small areas or particles on the correlations amongst atom motions due to thermal vibration.

Fluorescence energy transfer between cobra alpha-toxin molecules bound to the acetylcholine receptor.

An approach was developed with steady state fluorescence energy transfer measurements to examine the spatial relationship between the two alpha-toxins bound to the acetylcholine receptor. By taking advantage of the slow dissociation rates of alpha-toxins (Naja naja siamensis 3) from the receptor and of the equal probability with which alpha-toxins bind to the two alpha-toxin-binding sites, we derived an equation which allows prediction of a "true" efficiency of transfer based on the relationship between fractional site occupancy and the observed transfer efficiency ascertained from donor quenching. Using this approach, we examined the efficiency of energy transfer between two fluorescently labeled alpha-toxins, N epsilon-fluorescein isothiocyanate lysine 23 alpha-toxin and monolabeled tetramethylrhodamine isothiocyanate alpha-toxin bound to the receptor from the Torpedo californica electric organ. Significantly greater (32 versus 14%) energy transfer was observed with the membrane-associated than with the solubilized receptor, suggesting that transfer between fluorophores on separate receptor molecules is greater than that occurring intramolecularly between the two sites on the receptor. The magnitude of the distances calculated from the intrareceptor energy transfer efficiency combined with the considerable inter-receptor energy transfer indicate that the fluorophores would reside on the outer perimeter of the receptor molecule rather than near the central axis perpendicular to the plane of the membrane.

An Order-Disorder Theory of the Cubic-Trigonal Phase Transition in a Superionic Conductor RbAg4I5 Crystal

An order-disorder theory of the first-order cubic-trigonal transition in a superionic conductor RbAg 4 I 5 crystal is presented. Under a suitable choice of parameter values the fractional site occupancy by silver ions could be reasonably reproduced. The temperature dependence of an elastic stiffness constant c 44 is discussed.

Interconversion between different states of affinity of the human growth hormone receptor on rat hepatocytes: effects of fractional site occupancy on receptor availability.

Isolated rat hepatocytes accumulate a slowly dissociable human growth hormone (hGH) binding fraction with incubation time. Slowly dissociable [125I]hGH is receptor bound, intact and immunocompetent. Fifty-six percent of the bound hormone was slowly dissociable within 3 min of the initiation of hGH-hepatocyte incubation. Subsequently, the proportion of slowly dissociable [125I]hGH increased at the expense of the rapidly dissociable fraction. This suggested that binding induced interconversion between different states of affinity of the hGH receptor. Preincubation with hGH diminished the capacity of hepatocytes to subsequently bind [125I]hGH. Receptor occupancy resulting from accumulation of slowly dissociable hGH accounted for 37 and 62% of the decreased binding after preincubation with 0.79 and 7.9 nM hGH, respectively. Fractional receptor occupancy, among but distinguishable from other processes, may account for the inverse relationship between site number and applied hormone concentration. Addition of hGH to the medium of [125I]-hGH-hepatocyte incubates increased the extent of loss of label from hepatocytes. The progressive retention of intact [125I]-hGH by hepatocytes with site occupancy and invariant receptor affinity subsequent to fractional saturation was inconsistent with negative cooperativity. A mechanism in which hGH diminished reassociation of [125I]hGH with available sites during dissociation was consistent with the available binding data. The interrelationship between peptide hormone in rapid and slow equilibrium with the medium is of fundamental importance in modulating receptor binding and availability.

Ein einfacher Weg, Strukturinformation aus den diffusen Reflexen von Kristallen mit eindimensionaler Lagenfehlordnung zu erhalten

One-dimensionally disordered crystals consisting of layers that are equal with one another but stacked with displacement faults show sharp reflexions and diffuse streaks on their diffraction patterns. From the sharp reflexions alone an averaged structure (with fractional site-occupation factors) can be deduced. To obtain the real layer structure, the diffuse streaks have to be of considered. A simple experimental and mathematical procedure is described which enables their use without the need of measuring integrated streak intensities or streaks analysing the diffuse intensity profiles. Essentially, one has to measure the intensities of equal portions Δζ at equal positions ζ on different diffuse streaks; the values can be used directly, as they are proportional to the squared value of the layer structure factor. Additionally, a procedure is described for the measurement of the diffuse streak intensity profile on a four-circle diffractometer.

Free radical‐catalyzed reaction of the acrylamidomethyl ether of cotton

AbstractA limited number of acrylamidomethyl ether groups can be introduced into cotton. We can assume that the reactive groups form a lattice and that when two or more adjacent sites are occupied, these neighboring vinyl groups will react with each other when a free radical catalyst is applied to them. A monomer group isolated from others by empty neighboring sites will remain unconverted. If each site has Z neighbors, Z being the coordination number, the conversion can be calculated as a function of the fractional site occupancy and of Z. The expression derived from the lattice model is mathematically equivalent to an expression which is based on an excluded volume model. In the latter, it is assumed that each reactive monomer has a reactive shell around it. If another monomer is placed within this shell, the two can react with each other. In terms of this model the parameter Z is defined as the ratio between the volume of the reactive shell and of the volume occupied by the monomer. The latter model offers some mathematical advantages and allows the calculation of both conversion and degree of polymerization from Z and the fractional site occupancy. It is shown that the ultimately achievable conversion and the degree of polymerization increases with Z and with site occupancy. Acrylamidomethylated cotton samples were treated with increasing amounts of ammonium persulfate and the conversion was determined. The data could be extrapolated to infinite initiator concentration and Z was found to have a value of 3.5–4. Since the reaction between double bonds on neighborings sites of a single cellulose chain would have a coordination number equal to 2, the fact that Z > 2 indicates that reaction between double bonds attached to different cellulose chains must occur. The fraction of links which are intermolecular is (Z‐2)/Z, being 43–50%. This result is an excellent agreement with previous data showing that only 42% of the crosslinks contribute to the increased resilience of cotton. It is likely that the termination of the polymerization reaction involves the combination of two radicals. Consequently, each polymeric or oligomeric chain contains two sulfate groups when ammonium persulfate is used as initiator. The molecular weight of these chains can be calculated from the mole ratio between the converted monomers and sulfur. The molecular weights determined this way agreed reasonably well with the molecular weight values calculated from the theory with Z = 4.