抑菌剂与DNA作用方式的研究方法

防腐剂是食品工业中不可缺少的物质, 但是食品防腐剂仍存在种类少、耐药性和安全性等问题, 新型抑菌剂的研发及其作用机理研究已经成为研究者们关注的重点。脱氧核糖核苷酸(DNA)是抑菌剂发挥作用的重要靶点, 抑菌剂与DNA作用可以抑制微生物的生长繁殖。抑菌剂与DNA的作用方式有直接结合和间接影响, 直接结合方式有静电结合、沟槽结合和嵌插结合, 探究两者作用方式的研究方法对明确抑菌机理具有重要意义。本文综述了抑菌剂与DNA结合方式的研究方法,包括光谱法、电化学方法和热力学方法等常规的方法, 分子对接、分子动力学模拟及单分子力谱技术等新兴方法, 为抑菌机理的研究、新型抑菌剂的开发及在食品工业中的应用提供思路。     

金属离子Cu2+、Ca2+和Mg2+对抗蚜威与DNA结合作用的影响

在人体生理酸度(pH7.4)下,运用紫外、荧光光谱法和DNA 熔点实验研究抗蚜威与小牛胸腺DNA 的作用方式以及Cu2+、Ca2+ 和Mg2+ 分别对两者结合的影响。溴化乙锭(EB)竞争实验和DNA 熔点测定结果表明,抗蚜威主要是通过嵌插方式与DNA 碱基发生作用。3 种金属离子均能与抗蚜威络合,络合物的生成使体系的紫外吸收峰强度和形状发生改变,并能不同程度地猝灭DNA-EB 复合物的荧光;Cu2+、Ca2+ 的存在使DNA- 抗蚜威复合物的结合常数呈现先减弱后增强的趋势,而Mg2+ 的参与能增强两者之间的结合。由此推断出,金属离子对抗蚜威与DNA 结合的影响主要取决于金属离子与DNA 的碱基和磷酸基团间结合的相对亲和比。     

Modeling the interactions between humics, ions, and mineral surfaces

Model VI and the NICA-Donnan model are two successful models presently available for describing metal ion binding by humic materials. Both models deal with the complexity of the underlying processes (intrinsic heterogeneity, partial correlation between affinity distributions, variable stoichiometry, electrostatics) in a pragmatic way. The parameters of the NICA-Donnan model and their determination are discussed. The current interpretation of the ion-specific "non-ideality" parameters, ni, found in the NICA-Donnan model, emphasizes their role in determining the stoichiometry for the competitive binding of ions. The ratio nM/nH is reflected in the corresponding exchange ratios and the pH dependence of the metal ion binding. Experimental complications occur in testing models. Although Model VI and the NICA-Donnan models have much in common, there are differences that may become more apparent as the models are more widely tested. Recently ion binding to complexes of humics and oxides has been described by combining the NICA-Donnan model (ion binding to humics) with the CD-MUSIC model (ion binding to oxides). The impact of humic to oxide binding on both cation and anion binding is briefly discussed.     

The nuclear actin-related protein Act3p/Arp4p is involved in the dynamics of chromatin-modulating complexes

Chromatin remodelling and histone-modifying complexes govern the modulation of chromatin structure. While components of these complexes are diverse, nuclear actin-related proteins (Arps) have been repeatedly found in these complexes from yeast to mammals. In most cases, Arps are required for functioning of the complexes, but the molecular mechanisms of nuclear Arps have as yet been largely unknown. The Arps and actin, sharing a common ancestor, are supposed to be highly similar in the three-dimensional structure of their core regions, including the ATP-binding pocket. The Arp Act3p/Arp4p of Saccharomyces cerevisiae exists within the nucleus, partly as a component of several high molecular mass complexes, including the NuA4 histone acetyltransferase (HAT) complex, and partly as uncomplexed molecules. We observed that mutations in the putative ATP-binding pocket of Act3p/Arp4p increased its concentration in the high molecular mass complexes and, conversely, that an excess of ATP or ATPgammaS led to the release of wild-type Act3p/Arp4p from the complexes. These results suggest a requirement of ATP binding by Act3p/Arp4p for its dissociation from the complexes. In accordance, a mutation in the putative ATP binding site of Act3p/Arp4p inhibited the conversion of the NuA4 complex into the smaller piccoloNuA4, which does not contain Act3p/Arp4p and exhibits HAT activity distinct from that of NuA4. Although the in vitro binding activity of ATP by recombinant Act3p/Arp4p was found to be rather weak, our observations, taken together, suggest that the ATP-binding pocket of Act3p/Arp4p is involved in the function of chromatin modulating complexes by regulating their dynamics.     

Binding Characteristics and Protective Capacity of Cyanidin‐3‐Glucoside and its Aglycon to Calf Thymus DNA

The binding characteristics and protective capacity of cyanidin (Cy) and cyanidin‐3‐glucoside (C3G) to calf thymus DNA were explored for the first time. The Cy and C3G gave a bathochromic shift to the ultraviolet‐visible spectra of the DNA, indicating the formation of the DNA‐Cy and DNA‐C3G complexes. The complexes were formed by an intercalative binding mode based on the results of the fluorescence spectra and competitive binding analysis. Meanwhile, the Cy and C3G protected the DNA from the damage induced by the hydroxyl radical. The binding capacity and protective capacity of the C3G were stronger than that of the Cy. Furthermore, the formation of the DNA‐anthocyanin complexes was spontaneous when the hydrogen bond and hydrophobic force played a key role. Hence, the Cy and C3G could protect the DNA automatically from the damage induced by the hydroxyl radical.     

Does caffeine bind to metal ions?

The complex formation capacity of caffeine, a highly-consumed tea and coffee component, was determined for Ca, Mg, Fe, Zn, Pb, Mn, Co and Cr metal ions. The binding constants of metal ion–caffeine complexes for the metals chosen were determined spectrophotometrically. The results were compared with the known stability constants of metal ion–EDTA complexes, EDTA being known for its high metal binding capacity. Furthermore, iron chelating activity of caffeine, using the ferrozine reference method, was studied and compared with that of EDTA. The results showed very little complex formation capacity of caffeine with binding constants of 29.6, 22.4, 59, 396, 55, 9.3, 83 and 592 M⁻¹ for Ca, Mg, Fe, Zn, Pb, Mn, Co and Cr metal ions, respectively, in contrast to that of EDTA. The iron chelating activity of caffeine was also found to be 6%, which was considered to be quite low compared with EDTA.     

Spectroscopic investigation of ciprofloxacin speciation at the goethite-water interface

We investigated ciprofloxacin (a fluoroquinolone antibiotic) speciation as a function of pH in aqueous solution and in the presence of dissolved ferric ions and goethite using ATR-FTIR and UV-vis spectroscopy. The presence of dissolved and surface bound ferric species induced the deprotonation of the ciprofloxacin carboxylic acid group at pH < pKa1. The resultant ciprofloxacin zwitterions appeared to interact via both carboxylate oxygens to form bidentate chelate and bridging bidentate complexes within colloidal iron oxide-ciprofloxacin precipitates and bidentate chelates on the goethite surface. However, the structure of the aqueous ferric-ciprofloxacin complexes remains unclear. Our evidence for bidentate chelates (involving only the carboxylate oxygens) on the goethite surface was distinct from previous IR studies of fluoroquinolone sorption to metal oxides that have proposed surface complexes involving both the keto and the carboxylate groups. We find that the distinct ciprofloxacin surface complex proposed at the goethite-water interface may be a result of differences in metal oxide mineralogy or assignment of the carboxylate antisymmetric stretch in the metal oxide-fluoroquinolone spectra.     

Metal-polysaccharide complexes—Part I

Many factors, including linear charge density, effective nuclear charge of the cation, basicity of the donor group, chelation, intermolecular binding of two or more donor groups by a single cation and steric fit, contribute—or possibly contribute—to the formation and stability of metal ion-polysaccharide complexes. In order to form a proper foundation for studying and interpreting metal ion-polysaccharide interactions, the author introduces into his treatise on polysaccharide complexes much current information concerning the interaction of metal ions with simple unidonor and simple multidonor ligands. For convenience of discussion, the subject of polysaccharide complexes is divided into two categories: interactions of neutral polysaccharides, such as cellulose and starch, and interactions of anionic polysaccharides, such as alginate, pectate, mucopolysaccharides and carrageenan. On the one hand, neutral polysaccharides form weak complexes with cations in neutral or non-alkaline media. Only in alkaline solution is there any great affinity between cation and donor. On the other hand, anionic polysaccharides have a strong affinity for metal counterions, even at low concentrations. Attempts are made to provide reasonable explanations for the formation of these complexes and for the often large differences in affinity between metals for a particular polysaccharide or between polysaccharides for a particular metal. In some instances, rheological behaviour of polysaccharide systems can be related to structural features of macromolecular complexes.     

An evaluation of DGT performance using a dynamic numerical model

A numerical model of the transport and dynamics of metal complexes in the resin and gel layers of a DGT (diffusive gradients in thin films) device was developed and used to investigate how the chelating resin and metal-ligand complexes in solution affect metal uptake. Decreasing the stability constant or concentration of the binding resin increases the competition for free metal ions by ligands in solution, lowering the rate of mass uptake. Such effects would be rarely observed for moderately or strongly binding resins (K> 10(12)), including Chelex, which out-compete labile ligands in solution. With weakly binding resins, strongly bound solution complexes can diffuse into the resin layer before a measurable amount of dissociation occurs, such that concentrations of bound metal at the rear and front surfaces of the resin layer are equal. With more strongly binding resins, metal mainly binds to the front surface of the resin. Only complexes with the largest binding constants penetrate the gel layer containing Chelex, buttheir lack of lability means thatthe DGT sensitivity to the complex is, in any case, very low. The slow diffusion of complexes, such as those of fulvic acids, which increases the time required to establish steady state, compromises the use of the simple DGT equation. Errors are negligible for 24 h deployments, when diffusive layer thicknesses are less than 1 mm, but 3 day deployments are required to ensure accuracy with 2.4 mm thick layers. The extent to which the commonly used equation, that accounts for the concentration and diffusion of metal-complex species, overestimates DGT uptake if the rate of dissociation is slow, was estimated.     

Heavy metal sorption at the muscovite (001)-fulvic acid interface

The role of fulvic acid (FA) in modifying the adsorption mode and sorption capacity of divalent metal cations on the muscovite (001) surface was evaluated by measuring the uptake of Cu(2+), Zn(2+), and Pb(2+) from 0.01 m solutions at pH 3.7 with FA using in situ resonant anomalous X-ray reflectivity. The molecular-scale distributions of these cations combined with those previously observed for Hg(2+), Sr(2+), and Ba(2+) indicate metal uptake patterns controlled by cation-FA binding strength and cation hydration enthalpy. For weakly hydrated cations the presence of FA increased metal uptake by approximately 60-140%. Greater uptake corresponded with increasing cation-FA affinity (Ba(2+) ≈ Sr(2+) < Pb(2+) < Hg(2+)). This trend is associated with differences in the sorption mechanism: Ba(2+) and Sr(2+) sorbed in the outer portion of the FA film whereas Pb(2+) and Hg(2+) complexed with FA effectively throughout the film. The more strongly hydrated Cu(2+) and Zn(2+) adsorbed as two distinct outer-sphere complexes on the muscovite surface, with minimal change from their distribution without FA, indicating that their strong hydration impedes additional binding to the FA film despite their relatively strong affinity for FA.     

甘氨酸及其小肽配合物的pH电位法研究

利用ｐＨ 电位法研究了甘氨酸及其二、三肽与锌、铜元素的配位作用,测定了相应的配合稳定常数,结果表明,氨基酸和二、三肽的配合行为不同,而小肽之间配位作用基本相似．ｐＨ 电位法研究还表明,铜与肽的配合过程中存在着明显的晶体场稳定化能效应,表现在配合滴定过程中出现特征颜色变化（双缩脲反应）．     

紫外分光光度法与荧光光谱法探究呈味核苷酸与EGCG及其蛋白络合物的相互作用

为了探究IMP、GMP是否与EGCG及其蛋白络合物发生相互作用从而影响呈味特性，利用紫外分光光度法和荧光光谱法测定光谱的变化，结果表明: IMP、GMP与EGCG会发生相互作用，改变EGCG的光谱特性，且只存在一种结合方式，GMP与EGCG相互作用的效应强于IMP与EGCG的相互作用，两种呈味核苷酸1:1混合时对EGCG影响最大；IMP、GMP均不单独与BSA发生相互作用，但两者同时存在时会对BSA产生静态荧光淬灭，结合常数为0.6634，结合位点数为0.88；与EGCG蛋白络合物相互作用中，GMP单独存在时只与EGCG反应，IMP、GMP同时存在时以与蛋白的相互作用为主，结合常数为1.1054，结合位点数为1.49。此研究可更清晰地了解茶汤的呈味机制，为茶叶品质提高及茶饮料风味改善提供理论基础。     

荷叶碱与ct-DNA相互作用的光谱法研究

本实验采用荧光光谱、紫外- 可见光谱和盐效应等手段研究荷叶碱与小牛胸腺DNA(ct-DNA)之间的相互作用,并在pH7.4 的Tris - HCl 介质中, 以中性红为荧光探针,对其作用机理进行研究。结果表明：在生理条件下(pH7.4),荷叶碱与DNA 发生作用方式为混合方式,嵌插与沟槽作用是两种主要作用方式,DNA 对荷叶碱荧光猝灭属于静态猝灭,测得其结合常数为1.396 × 106 L/mol。     

花色苷与蛋白质相互作用的研究进展

花色苷作为一种功能活性物质备受关注,但因稳定性差、生物利用率低限制了其应用。蛋白质作为人体必需的三大营养素之一,具有较好的生物相容性和生物可降解性,常用于食品加工中,但其功能性质有待改善。食品组分中的蛋白质与花色苷通常是共存的成分,很容易发生相互作用,作用方式包括非共价和共价作用,其中非共价作用方式最为普遍,包括氢键、疏水相互作用、静电相互作用、范德华力。本文总结了花色苷-蛋白质相互作用的研究进展,包括蛋白质对花色苷稳定性和生物利用率的影响、花色苷对蛋白质功能性质的影响,以及复合物功能产品的应用现状,以期为花色苷与蛋白质在食品深加工中的应用提供理论参考。     

Binding constants of mercury and dissolved organic matter determined by a modified ion exchange technique

Ion-exchange techniques have been widely used for determining the conditional stability constants (logK) between dissolved organic matter (DOM) and various metal ions in aqueous solution. An exception is mercuric ion, Hg2+, whose exceedingly strong binding with reduced sulfur or thiol-like functional groups in DOM makes the ion exchange reactions difficult. Using a Hg-selective thiol resin, we have developed a modified ion-exchange technique which overcomes this limitation. This technique allows not only the determination of binding constants between Hg2+ and DOM of varying origins, but also the discrimination of complexes with varying coordination numbers [i.e., 1:1 and 1:2 Hg:thiol-ligand (HgL) complexes]. Measured logK values of four selected DOM isolates varied slightly from 21.9 to 23.6 for 1:1 HgL complexes, and from 30.1 to 31.6 for 1:2 HgL(2) complexes. These results suggest similar binding modes that are likely occurring between Hg2+ and key thiolate functional groups in DOM particularly at a relatively low Hg to DOM ratio. Future studies should further elucidate the nature and precise stoichiometries of binding between Hg2+ and DOM at environmentally relevant concentrations.     

Interaction of copper ion with some food dyes.

The nature of the complexes formed by Cu²⁺ with some food dyes has been studied spectrophotometrically under varied conditions. The results indicated formation of 1:1 metal: dye species at pH 4.5 and of 2:1 metal: dye species at higher pH. The latter result is unexpected and surprising since in most cases azo dyes form 1:1 or 1:2 metal: dye species and no example of a metal complex having more than one metal atom per dye molecule has so far been reported. This curious observation has been rationalised in terms of a salt forming Cu²⁺ ion and a coordinated Cu²⁺ ion per dye molecule; tentative structures for these species have also been proposed. It is hoped this novel finding will stimulate further investigations into the metal-dye complexes.     

STUDIES ON HAZE PRECURSORS IN BEERS AND WORTS

Detailed studies have been carried out on the fractionation of worts and beers on Sephadex and Bio-gel columns with high degrees of cross-linking. On G.25 Sephadex, the excluded fraction contains much of the total protein in the form of complexes with polyphenols bound in such a way that they do not interact with the column material. Continued elution with aqueous solvents gives many low molecular weight wort components and in addition some protein-polyphenol complexes which are retarded by relatively weak adsorption on the column. These compounds have been further studied by rechromatography in urea to dissociate the complexes and after reaction with excess flavonoid. Since the adsorbed compounds interact with the column through hydrogen bonding of active phenolic bonds, it may be that these are also the compounds which cross-link with other protein complexes in the beer to build up haze particles. Molybdate and alkali can be used subsequently to elute the free tannin and flavonoid molecules which are strongly absorbed.     

Mechanisms underlying the dualistic mode of action of major soy isoflavones in relation to cell proliferation and cancer risks

Isoflavones are phytoestrogens that have been linked to both beneficial as well as adverse effects in relation to cell proliferation and cancer risks. The present article presents an overview of these seemingly contradicting health effects and of mechanisms that could be involved in this dualistic mode of action. One mechanism relates to the different ultimate cellular effects of activation of estrogen receptor (ER) α, promoting cell proliferation, and of ERβ, promoting apoptosis, with the major soy isoflavones genistein and daidzein activating especially ERβ. A second mode of action includes the role of epigenetics, including effects of isoflavones on DNA methylation, histone modification and miRNA expression patterns. The overview presented reveals that we are only at the start of unraveling the complex underlying mode of action for effects of isoflavones, both beneficial or adverse, on cell proliferation and cancer risks. It is evident that whatever model system will be applied, its relevance to human tissues with respect to ERα and ERβ levels, co‐repressor and co‐activator characteristics as well as its relevance to human exposure regimens, needs to be considered and defined.     

The interactions between anthocyanin and whey protein: A review

Anthocyanins (ACN) are natural pigments that produce bright red, blue, and purple colors in plants and can be used to color food products. However, ACN sensitivity to different factors limits their applications in the food industry. Whey protein (WP), a functional nutritional additive, has been shown to interact with ACN and improve the color, stability, antioxidant capacity, bioavailability, and other functional properties of the ACN-WP complex. The WP's secondary structure is expected to unfold due to heat treatment, which may increase its binding affinity with ACN. Different ACN structures will also have different binding affinity with WP and their interaction mechanism may also be different. Circular dichroism (CD) spectroscopy and Fourier transform infrared (FTIR) spectroscopy show that the WP secondary structure changes after binding with ACN. Fluorescence spectroscopy shows that the WP maximum fluorescence emission wavelength shifts, and the fluorescence intensity decreases after interaction with ACN. Moreover, thermodynamic analysis suggests that the ACN-WP binding forces are mainly hydrophobic interactions, although there is also evidence of electrostatic interactions and hydrogen bonding between ACN and WP. In this review, we summarize the information available on ACN-WP interactions under different conditions and discuss the impact of different ACN chemical structures and of WP conformation changes on the affinity between ACN and WP. This summary helps improve our understanding of WP protection of ACN against color degradation, thus providing new tools to improve ACN color stability and expanding the applications of ACN and WP in the food and pharmacy industries.