Effects Of Anion Binding Research Articles

Effect of anion binding on charge stabilization in a bis-fullerene–oxoporphyrinogen conjugate

Presence of strongly binding anion, F(-) stabilizes the photo-induced charge-separated states of a bis-fullerene-substituted oxoporphyrinogen due to the large shift in the oxidation potential of the oxoporphyrinogen moiety upon anion binding through hydrogen bonding at its core.

Highly effective electrochemical anion sensing based on oxoporphyrinogen

The effect of anion binding on the oxidation potential of an anion receptor, N 21, N 23-dibenzyl-5,10,15,20-(3,5-di- t-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen, 1 in o-dichlorobenzene is reported. The anion binding site of 1, at its inner pyrrolic amine hydrogens, is an integral part of the highly conjugated macrocycle, thus predicting larger potential shifts upon anion binding. Accordingly, cathodic shifts up to 600 mV are observed upon anion binding and such potential shifts correlate well with the anion binding constants.

Effect of anion binding on the thermal reverse reaction of bathoiodopsin: anion stabilizes two forms of iodopsin.

The thermal reactions of the bathoproduct of the long wavelength sensitive visual pigment iodopsin were investigated under various anionic and environmental conditions, to get an insight into the mechanism leading to the unusual thermal isomerization of the retinal chromophore from the trans to the 11-cis form at very low temperatures (-160 degrees C). The all-trans chromophore of the bathoiodopsin produced from iodopsin in the presence of chloride thermally reverted to the 11-cis form, while in the presence of nitrate it kept its all-trans configuration upon warming. Different protein environments, either in a detergent or in phosphatidylcholine (PC) liposomes, did not change the reaction characteristics of the bathoiodopsins under the two anionic conditions. However, reaction characteristics of bathoiodopsins produced in the absence of small anions were dependent on the environment. The trans-to-cis isomerization occurred upon warming of bathoiodopsin in the presence of detergent but not in liposomes. Spectral measurements revealed that iodopsin in the absence of small anions is a mixture of two spectrally distinct forms that exhibit absorption maxima and reaction characteristics similar to those of chloride-bound and nitrate-bound iodopsins, respectively. Thus, iodopsin exhibits two conformational states, each of which is stabilized by the binding of chloride and nitrate, respectively.

Effect of anion binding on iodopsin studied by low-temperature fourier transform infrared spectroscopy.

The effect of anion binding on iodopsin, the chicken red-sensitive cone visual pigment, was studied by measurements of the Fourier transform infrared spectra of chloride- and nitrate-bound forms of iodopsin at 77 K. In addition to the blue shift of the absorption maximum upon substituting nitrate for chloride, the C=C stretching vibrations of iodopsin and its photoproducts were upshifted 5-6 cm(-)(1). The C=NH and C=ND stretching vibrations were the same in wavenumber between the chloride- and nitrate-bound forms, indicating that the binding of either chloride or nitrate has no effect on the interaction between the protonated Schiff base and the counterion. The vibrational bands of iodopsin in the fingerprint and the hydrogen out-of-plane wagging regions were insensitive to anion substitution, suggesting that local chromophore interactions with the anions are not crucial for the absorption spectral shift. In contrast, bathoiodopsin in the chloride-bound form exhibited an intense C(14)H wagging mode, whose intensity was considerably weakened upon substitution of nitrate for chloride. These results suggest that binding of chloride changes the environment near the C(14) position of the chromophore, which could be one of the factors in the thermal reverse reaction of bathoiodopsin to iodopsin in the chloride-bound form.

The Stability of Tropomyosin at Acid pH: Effects of Anion Binding

The α-helical coiled-coil tropomyosin homodimer, ααTm, unfolds cooperatively withT1/2= 47°C, at neutral pH, 0.5 M NaCl. At pH 2, where each chain contains 55 positive charges, no cooperative unfolding occurs to at least 80°C. The NaCl and K2SO4dependence of thermal unfolding of ααTm and a less stable protein, ββTm, were studied with circular dichroism. For ααTm, 10 mM NaCl, or 0.5 mM K2SO4, was sufficient to increase the unfolding temperature by 80°C. For ββTm, similar concentrations of NaCl and K2SO4as for ααTm increased the unfolding temperature of the most stable domain. Titrations indicated that two to three anions bind preferentially to the ββTm intermediate. Thus anions bind to Tm at acid pH values to greatly stabilize the helix. But even in the absence of added salt, Tm is more stable at pH 2 than pH 7, suggesting destabilization by negatively charged amino acids.

The anion requirement for iron release from transferrin is preserved in the receptor-transferrin complex.

Rates of iron release from both sites of free transferrin at pH 7.4 are critically dependent upon ionic strength, because release appears to require binding of a simple nonchelating anion such as chloride to a kinetically active site of the protein. This site is distinct from the synergistic anion-binding site, occupancy of which is required for binding of iron to occur at all. Complexing of transferrin to its receptor also modulates release of iron, but in a more complex fashion. At extracellular pH, 7.4, receptor retards release, but at the pH of the endosome in which release occurs within the cell, 5.6, receptor accelerates release. The present study was undertaken to determine whether the kinetically active anion requirement is maintained at pH 5.6 and whether the effects of anion binding and receptor binding are independent of each other. A spectrofluorometric method was developed to monitor release of iron from C-terminal monoferric human transferrin and its complex with the transferrin receptor. At pH 5.6, as at pH 7.4, profiles of iron release to pyrophosphate from free and from receptor-complexed monoferric transferrin show curvilinear dependence on pyrophosphate concentration, consistent with a previously described kinetic scheme and suggestive of a similar release mechanism in all cases. Furthermore, at pH 5.6 release rates depend upon anion (chloride) concentration in free and in receptor-complexed transferrin as in free transferrin at pH 7.4, extrapolating nearly to zero as chloride concentration approaches zero. The enhancing effect of receptor on release is displayed at all concentrations of chloride tested,indicating that the release-promoting effects of receptor and chloride are independent of each other.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of anion binding on the conformations of the two domains of ovotransferrin.

A previous paper (Harris (1985) Biochemistry 24, 7412-7418) reported the occurrence of two classes of anion binding sites in transferrin. To evaluate the locations of the two anion binding sites in relation to the two major domains of transferrin we determined the binding constants of whole ovotransferrin and its two half-molecules by means of the difference UV spectroscopic technique. Anions induced strong negative absorbance at 245 nm in the order: citrate greater than phosphate greater than bicarbonate for whole ovotransferrin and the N-terminal half-molecule; and: phosphate greater than citrate greater than bicarbonate for the C-terminal half-molecule. The anion dissociation constants of the N-terminal half-molecule were consistent with lower dissociation constants, and those of the C-terminal half-molecule, with higher dissociation constants of whole ovotransferrin, indicating that the two classes of anion binding sites correspond to the binding sites in individual structural domains. Anion binding markedly protected the N-terminal half-molecule, but not the C-terminal half-molecule from digestion with trypsin and disulfide reduction with dithiothreitol. As to the far and near ultraviolet CD spectra data, however, there was no significant difference between in the presence and absence of an anion. Therefore, the binding of an anion would induce some conformational changes which were not reflected by the CD spectrum.

Effects of anion binding on the deprotonation reactions of halorhodopsin.

The retinal Schiff base of halorhodopsin deprotonates with a pKa of 7.4 in 0.5 M Na2SO4 in the dark. In the presence of various anions, such as chloride or nitrate, etc., the pKa is raised by up to 1.5 units. Analysis of the dependency of the pKa on anion concentration favors the model in which the anions do not bind to the positively charged Schiff base nitrogen, but to a site near it, and exert their effect on the pKa by direct (perhaps electrostatic) interaction. Adding nitrate, or one of several other anions, causes also a small blueshift in the visible absorption band of the chromophore. These effects on the pKa and the absorption band define an anion binding site in halorhodopsin, termed Site I. Chloride and bromide apparently bind in addition to another site, which is associated with a small red-shift of the absorption band and changes in the photocycle. This other anion binding site is termed Site II. Illumination of halorhodopsin samples results in the deprotonation of the Schiff base with a much lowered pKa, but at very low rates probably determined by the generation of a deprotonating photointermediate. Binding of Site I anions increases the pKa of deprotonation in the light also. The similarity of the responses of the apparent pKa in the dark and in the light to anion concentration suggests that anion binding to Site I influences deprotonation of the Schiff base similarly in the photointermediate and in the parent halorhodopsin molecule.

Interactions in cation permeation through the gramicidin channel. Cs, Rb, K, Na, Li, Tl, H, and effects of anion binding

As a prototype for binding and interaction in biological Na and K channels, the single channel conductances for Li, Na, K, Rb, Cs, H, and Tl and the membrane potentials for Tl-K mixtures are characterized for gramicidin A over wider concentration rangers than previously and analyzed using an "equilibrium domain" model that assumes a central rate-determining barrier. Peculiarities in the conductance-concentration relationship for TlF, TlNO3, and TlAc suggest that anions bind to Tl-loaded channels, and the theory is extended to allow for this. For concreteness, the selectivity of cation permeation is characterized in terms of individual binding and rate constants of this model, with the conclusions that the strongest site binds Cs greater than Rb greater than K greater than Na greater than Li, while the next strongest binds Na greater than K greater than Li greater than Rb greater than Cs. However, because Schagina, Grinfeldt, and Lev's recent finding of single filing (personal communication) indicates that the channel sites in gramicidin cannot be at equilibrium with the solution, and work in progress with Hägglund and Enos (Biophys. J. 21:26a. [Abstr.]) indicates that the simplest model adequate to account for the observed concentration-dependences of flux-ratio, conductance, I--V characteristic, and permeability has three barriers and four sites, some implications of additional rate-determining barriers at the mouth of the channel are discussed. The results are summarized using phenomenological "experimental" parameters that provide a model-independent way to represent that data concisely and which can be interpreted physically in terms of any desired model.