Effect of carboxylic acid side chains on the absorption maximum of visual pigments

The proposal that the absorption maximum of the visual pigments is governed by interaction of the 11-cis-retinal chromophore with charged carboxylic acid side chains in the membrane-embedded regions of the proteins has been tested by mutating five Asp and Glu residues thought to be buried in rhodopsin. Changing Glu113 to Gln causes a dramatic shift in the absorption maximum from 500 nanometers to 380 nanometers, a decrease in the pKa (acidity constant) of the protonated Schiff base of the chromophore to about 6, and a greatly increased reactivity with hydroxylamine. Thus Glu113 appears to be the counterion to the protonated Schiff base. Wavelength modulation in visual pigments apparently is not governed by electrostatic interaction with carboxylate residues, other than the counterion.     

Transducin activation by rhodopsin without a covalent bond to the 11-cis-retinal chromophore

Rhodopsin and the visual pigments are a distinct group within the family of G-protein-linked receptors in that they have a covalently bound ligand, the 11-cis-retinal chromophore, whereas all of the other receptors bind their agonists through noncovalent interactions. The retinal chromophore in rhodopsin is bound by means of a protonated Schiff base linkage to the epsilon-amino group of Lys-296. Two rhodopsin mutants have been constructed, K296G and K296A, in which the covalent linkage to the chromophore is removed. Both mutants form a pigment with an absorption spectrum close to that of the wild type when reconstituted with the Schiff base of an n-alkylamine and 11-cis-retinal. In addition, the pigment formed from K296G and the n-propylamine Schiff base of 11-cis-retinal was found to activate transducin in a light-dependent manner, with 30 to 40% of the specific activity measured for the wild-type protein. It appears that the covalent bond is not essential for binding of the chromophore or for catalytic activation of transducin.     

Chemistry. Superacids--it's al lot about anions

Superacids are powerful agents for protonating weak bases, with applications ranging from synthetic chemistry to batteries and fuel cells. DesMarteau explains in his Perspective that many superacids have undesirable properties, because after proton donation, the resulting anion may interact strongly with the protonated base or the solvent. Reed et al. report the discovery of a superacid that overcomes this problem. In the anion, charge is distributed efficiently over many atoms, reducing unwanted interactions.     

Ionic mechanism of cholinergic inhibition in molluscan neurons

Acetylcholine, the inhibitory transmitter to the so-called H-neurons of molluscs, produces its effect by increasing the permeability of the subsynaptic membrane to chloride ions. The change in permeability gives rise to a net influx of this anion, which hyperpolarizes the neuron. The presence of an outward pump of chloride ions is postulated to account for the required electrochemical gradient. The participation of potassium ions in this inhibitory phenomenon was not detected.     

Widespread genetic switches and toxicity resistance proteins for fluoride

Most riboswitches are metabolite-binding RNA structures located in bacterial messenger RNAs where they control gene expression. We have discovered a riboswitch class in many bacterial and archaeal species whose members are selectively triggered by fluoride but reject other small anions, including chloride. These fluoride riboswitches activate expression of genes that encode putative fluoride transporters, enzymes that are known to be inhibited by fluoride, and additional proteins of unknown function. Our findings indicate that most organisms are naturally exposed to toxic levels of fluoride and that many species use fluoride-sensing RNAs to control the expression of proteins that alleviate the deleterious effects of this anion.     

壳寡糖衍生物对烟草花叶病的田间防效试验

测定了壳寡糖、宁南霉素、水杨醛壳寡糖希夫碱、5-溴水杨醛壳寡糖希夫碱、水杨醛壳寡糖希夫碱膦酸酯、5-溴水杨醛壳寡糖希夫碱膦酸酯对烟草花叶病的抑制效果,结果表明:这些药剂对烟草花叶病毒(TMV)均具有一定的诱导抗病效应,其中以5-溴壳寡糖希夫碱膦酸酯药剂的防治效果较好。     

壳寡糖席夫碱对感染烟草花叶病毒烟株叶绿素含量的影响

测定5种壳寡糖席夫碱3个浓度对烟草花叶病毒的抑制效果,结果显示,壳寡糖席夫碱对烟草花叶病毒(TMV)具有一定的诱导抗病效应,其中100μg/mL的3,5-二氯水杨醛壳寡糖席夫碱药剂的效果最好。并用该药剂对豫烟5号进行处理,测定了其对烤烟叶片中叶绿素含量变化的影响。结果表明,壳寡糖席夫碱可以有效诱导烟草抗烟草花叶病毒活性,处理后的烟草叶片中的叶绿素和胡萝卜素含量较阳性对照有较大的提高。     

手性Salen金属配合物修饰碳糊电极的制备及对甲醛的电催化氧化

以3,5-二叔丁基水杨醛和(1R,2R)-环己二胺为原料,合成了(R,R)-N,N-二(3,5-二叔丁基水杨醛)-1,2环己二胺钴配合物(Salen-Co(Ⅱ)配合物),并以此作为化学修饰剂,利用直接混合法将化学修饰剂、粘合剂和石墨粉按一定比例研磨混合制作成西夫碱钴配合物修饰碳糊电极(Salen-Co/CPE).实验表明,该西夫碱钴配合物修饰碳糊电极对甲醛具有良好的电催化作用.利用计时电流法,在优化的实验条件下,氧化峰电流与甲醛的浓度在3.0×10-2～1.2×10-1mol/L的范围内具有良好的线性关系,检出限为3.33×10-3mol/L.     

Bacterial rhodopsin: evidence for a new type of phototrophy in the sea

Extremely halophilic archaea contain retinal-binding integral membrane proteins called bacteriorhodopsins that function as light-driven proton pumps. So far, bacteriorhodopsins capable of generating a chemiosmotic membrane potential in response to light have been demonstrated only in halophilic archaea. We describe here a type of rhodopsin derived from bacteria that was discovered through genomic analyses of naturally occuring marine bacterioplankton. The bacterial rhodopsin was encoded in the genome of an uncultivated gamma-proteobacterium and shared highest amino acid sequence similarity with archaeal rhodopsins. The protein was functionally expressed in Escherichia coli and bound retinal to form an active, light-driven proton pump. The new rhodopsin exhibited a photochemical reaction cycle with intermediates and kinetics characteristic of archaeal proton-pumping rhodopsins. Our results demonstrate that archaeal-like rhodopsins are broadly distributed among different taxa, including members of the domain Bacteria. Our data also indicate that a previously unsuspected mode of bacterially mediated light-driven energy generation may commonly occur in oceanic surface waters worldwide.     

The anion paradox in sodium taste reception: resolution by voltage-clamp studies

Sodium salts are potent taste stimuli, but their effectiveness is markedly dependent on the anion, with chloride yielding the greatest response. The cellular mechanisms that mediate this phenomenon are not known. This "anion paradox" has been resolved by considering the field potential that is generated by restricted electrodiffusion of the anion through paracellular shunts between taste-bud cells. Neural responses to sodium chloride, sodium acetate, and sodium gluconate were studied while the field potential was voltage-clamped. Clamping at electronegative values eliminated the anion effect, whereas clamping at electropositive potentials exaggerated it. Thus, field potentials across the lingual epithelium modulate taste reception, indicating that the functional unit of taste reception includes the taste cell and its paracellular microenvironment.     

Non-Watson-Crick G.C and A.T base pairs in a DNA-antibiotic complex

The structure of a DNA octamer d(GCGTACGC) cocrystallized with the bisintercalator antibiotic triostin A has been solved. The DNA forms an unwound right-handed double helix. Four base pairs are of the Watson-Crick type while four are Hoogsteen base pairs, including two A.T and two G.C base pairs. This is the first observation in an oligonucleotide of Hoogsteen G.C base pairs where the cystosine is protonated. It is likely that these also occur in solutions of DNA complexed to this antibiotic.     

Electron-driven acid-base chemistry: proton transfer from hydrogen chloride to ammonia

In contrast to widely familiar acid-base behavior in solution, single molecules of NH3 and HCl do not react to form the ionic salt, NH+4Cl-, in isolation. We applied anion photoelectron spectroscopy and ab initio theory to investigate the interaction of an excess electron with the hydrogen-bonded complex NH3...HCl. Our results show that an excess electron induces this complex to form the ionic salt. We propose a mechanism that proceeds through a dipole-bound state to form the negative ion of ionic ammonium chloride, a species that can also be characterized as a deformed Rydberg radical, NH4, polarized by a chloride anion, Cl-.     

Receptors: localization and specificity of binding of serotonin in the central nervous system

Formation of a Schiff base between the ethylamine residue of serotonin and an appropriate carbonyl residue at the receptor site may be among the forces holding serotonin onto the receptor. Reduction of this imine may provide a means of permanently labeling receptors as a preliminary to their isolation.     

Peroxidation of subcellular organelles: formation of lipofuscinlike fluorescent pigments

Lipid peroxidation of subcellular organelles gives fluorescent products which have fluorescence and excitation spectra similar to those of lipofuscin pigments. Fluorescence and excitation spectra and total fluorescence in the 460-nanometer region are useful for qualitative identification and quantitative measurement of the Schiff base product, a molecular damage site of lipid peroxidation which develops during some aging processes.     

Monomeric cobalt-oxygen complexes

Some Schiff base complexes of cobalt(II) form stable cobalt-oxygen species in solution instead of the usual cobalt-oxygen-cobalt bridged complexes. These one-to-one adducts were isolated as crystalline solids and characterized by means of elemental analysis, magnetic properties, and molecular weight determinations.     

A new charge-mosaic membrane from a multiblock copolymer

A charge-mosaic membrane was prepared from a pentablock copolymer of the BABCB type by selectively introducing anion and cation exchange groups into the microseparated phases. The three-layer lamellar structure of the starting pentablock copolymer film was not disturbed by the modifications. The membrane obtained was highly permeable only to sodium chloride in mixed aqueous solutions of sodium chloride and organic species of low molecular weight, such as sucrose. Marked pH-dependent permeabilities were also observed for amino acids.     

Collagen has a discrete family of reactive hydroxylysyl and lysyl side-chain amino groups

Several cross-linking substances which derive from lysine have been isolated from hydrolyzates of collagen and elastin. This observation suggests that certain lysyl side chains exhibiting enhanced reactivity and unique configuration may participate in cross-linking. In native collagen there is a small family of lysyl and hydroxylysyl side chains which exhibit a high propensity of Schiff base formation.     

5-硝基水杨醛双缩二氨基硫脲Schiff碱的合成·表征与抑菌活性研究

[目的]合成对称的5-硝基水杨醛双缩二氨基硫脲Shiff碱,并测试其抑菌活性。[方法]目标化合物经红外、元素、核磁、质谱确证其化学结构,并用管碟法测试其抑菌活性。[结果]4种不同浓度对2种细菌均有一定程度的抑菌效果,但对革兰氏阳性金黄色葡萄球菌的抑菌效果优于大肠杆菌。[结论]该新化合物对革兰氏阳性金黄色葡萄球菌有较好的抑制效果。     

Charge-mosaic membranes: enhanced permeability and negative osmosis with a symmetrical salt

Charge-mosaic membranes are prepared by embedding a single layer of alternating cation and anion exchange beads in silicone resin. Membranes made in an identical manner but containing only one type of exchanger serve as controls. The mosaic membranes are 50 to 100 times more permeable to potassium chloride than the controls; and furthermore they give rise to net volume flow from concentrated to dilute solutions of potassium chloride in the absence of a pressure gradient ("negative osmosis"), whereas the controls exhibit normal osmotic behavior. The negative reflection coefficients of the mosaics suggest potential applications in desalination.     

Synaptic activation of an electrogenic sodium pump

An identified molluscan interneuron mediates different cholinergic synaptic actions by increasing the conductance of its follower cells to different ions. We have now found that this interneuron also mediates a new class of synaptic actions which does not involve a conductance change but the activation of an electrogenic sodium pump. This synaptic action results in a prolonged inhibitory synaptic potential which is dependent on metabolism and is selectively blocked by cooling and ouabain. In cells which have this synaptic potential, part of the resting membrane potential is also maintained by an electrogenic sodium pump. The same transmitter, acetylcholine, can independently stimulate both a chloride ion conductance and a sodium pump mechanism in the same follower cell by acting on two different postsynaptic receptors.