Transmembrane ion transport by calixarenes and their derivatives

Regulation of transmembrane ion transport is a vital aspect of bioinorganic chemistry. To understand how this highly selective process occurs, how it can become impaired and how impairment may be treated, model compounds are useful tools. Several systems are presently being explored but one of the most widely applicable combines a rigid macrocycle, capable of size-based ion recognition, with membrane-spanning substituents that allow the target ions to traverse a phospholipid bilayer. The calixarene class of macrocycles is ideally suited to this task. This article sets out the biological background to transmembrane ion transport, the methods available to study the phenomenon, examples of model compounds, and proposes areas of further study.     

Hydro- and silylcyanation of cholic acid derivatives. Synthesis of novel pincer ligands based on litocholic acid

Reactions of 3-oxo derivatives of cholic acid esters with acetone cyanohydrin and trimethylsilyl cyanide lead to exclusive or predominant formation of the corresponding 3-cyano-3-hydroxy and 3-cyano-3-trimethylsiloxy derivatives with axial orientation of the cyano group. Reduction of the cyano group in the addition products gives 3-aminomethyl derivatives which can be used to obtain pincer ligands containing two cholane fragments.     

Determination of critical micellar concentrations of two monoketo derivatives of cholic acid

Critical micellear concentrations (CMC) were determined for two novel promoters of membrane permeability—7-monoketocholic acid (7-MKC) and 12-monoketocholic acid (12-MKC), using two non-invasive (¹H NMR relaxation experiment and conductometry) and two invasive (spectral shift and partition coefficient of the probe molecule) methods. Studies by the former methods suggest the different aggregation abilities of the investigated bile acid derivatives. In an aqueous solution, 7-MKC has a somewhat lower CMC value (43 mM) than 12-MKC (50 mM). Further, it was found that, in addition to primary micelles, 7-MKC forms also secondary micelles. In the experiments with probe (hydrophobic) molecules, the aggregation properties of investigated bile acids did not differ in water, whereas the presence of urea altered the aggregation of 7-MKC.Based on the CMC value, 7-MKC is more hydrophobic than 12-MKC. The apparent hydrophobicity of 7-MKC is a consequence of the formation of secondary micelles, shifting the monomer equilibrium to the direction of primary micelles, which is manifested as a decrease in the CMC value.     

Determination of critical micellar concentrations of cholic acid and its keto derivatives

The critical micellar concentration (CMC) values of keto derivatives of cholic acid (3,12-dihydroxy-7-oxo-5β-cholanoic acid, 3,7-dihydroxy-12-oxo-5β-cholanoic acid, 12-hydroxy-3,7-dioxo-5β-cholanoic acid, 3-hydroxy-7,12-dioxo-5β-cholanoic acid, 3,7,12-triketo-5β-cholanoic acid) and cholic acid itself, were determined. Replacement of hydroxyl groups in cholic acid molecule with keto groups yields the derivatives whose CMC values increase with increase in the number of keto groups introduced. The CMCs of derivatives with the same number of keto groups but at different positions do not differ significantly. The relationship between the number of keto groups in the molecule of cholic acid keto derivatives and CMC value can be described by the following equation: CMC = 43 number of keto groups + 14.667. The effect of NaCl concentration on CMC increases with increase in the number of keto groups.     

胆甾类分子钳对氨基酸衍生物的对映选择性识别

用差紫外光谱滴定法考察了以脱氧胆酸作spacer的手性分子钳1～3对一系列α-氨基酸甲酯的对映选择性识别性能。结果表明,分子钳1和2与客体氨基酸甲酯形成1:1型超分子配合物,并显示较好的手性识别能力。分钳3对所考察的氨基酸甲酯均没有明显的识别作用。讨论了主-客体间尺寸/形状匹配、几何互补等因素对形成超分子配合物的影响,并利用计算机模拟作辅助手段对实验结果和现象进行了解释。     

胆酸酰氧基膦酸酯衍生物的合成及抗肿瘤活性

以胆酸和亚磷酸酯为原料,在缩合剂二环己基碳二亚胺(DCC)和4-二甲基吡啶(DMAP)的催化下进行酯化反应,合成了10个未见报道的胆酸酰氧基膦酸酯衍生物,所有目标化合物均经过TG,IR,1 H NMR,31P NMR和HRMS对其进行了结构确认.利用MTT法测定了目标化合物的抗肿瘤活性,结果显示:目标化合物4H,4I,4J对人肝癌细胞(HepG2)表现出较好的增殖抑制作用.     

Antifungal evaluation of cholic acid and its derivatives on Candida albicans by microcalorimetry and chemometrics

Multi-hydroxyl amines including tris(hydroxymethyl)aminomethane (Tris), serinol and ethanolamine were selected as weak affinity ligands using a rapid screening by quartz crystal microbalance (QCM) biosensor. Based on the specific recognition between the ligands and two proteins, lysozyme (LZM) and cytochrome c (Cyt c), a weak affinity chromatography method was developed for specific separation of the two proteins. The frontal analysis results showed that the apparent dissociation constants (K_D) of ligand–protein complexes were all in the order of weak affinity (10⁻⁴ M). By weak affinity columns modified with the three multi-hydroxyl amines individually, LZM and Cyt c were baseline separated as retarded peaks from non-specific protein and each other in a single cycle of loading and eluting. Moreover, the Tris-modified column typically showed the satisfactory repeatability and stability as a new type of weak affinity columns. The present strategy composed of QCM selecting and affinity chromatography separating was promising to extend the variety of weak affinity ligands and develop inexpensive specific affinity methods for separation and purification of multiple proteins on one single column.     

某些胆酸及去氧胆酸肟类化合物的合成

从胆酸和去氧胆酸出发,经过不同反应,将甾核以及支链上的取代基分别转化为羰基及酯基,然后和盐酸羟胺反应引进肟基,合成了六个新的甾体肟类化合物:7-羟基-12-氧代-3-肟基胆酸甲酯(5)、7,12-二氧代-3-肟基胆酸甲酯(6)、12-氧代-3,7-二肟基胆酸甲酯(7)、3,7,12-三肟基胆酸甲酯(8)、12-氧代-3-肟基去氧胆酸甲酯(12)、3,12-二肟基去氧胆酸甲酯(13)。通过NMR和IR对合成物5~10的结构进行了表征。     

Anionic facial amphiphiles from cholic acid

[structure:see text] Anionic facial amphiphiles have been prepared from cholic acid. These compounds offer antipodes of recently reported cationic amphiphiles derived from cholic acid. The synthesis of the anionic amphiphiles was accomplished in few steps from a common intermediate. In contrast to many other anionic facial amphiphiles, the cholic acid derived amphiphiles appeared to aggregate at relatively low concentration.     

Antifungal evaluation of cholic acid and its derivatives on Candida albicans by microcalorimetry and chemometrics

In the last few years, several fungus infections caused by multidrug-resistant pathogenic agents have got tremendous emergence and prevalence. Screening for novel antifungal agents is in great demand, but traditional microbiological techniques are far from sufficient to meet that requirement. In this study, a non-invasive and non-destructive microcalorimetric method was performed to investigate the antifungal activities of cholic acid (CA) and its derivatives, glycocholic acid (GCA) and taurocholic acid (TCA) on the multiplying and non-multiplying metabolism of Candida albicans. Then, the heat-flow power-time curves of C. albicans growth affected by different concentrations of CA, GCA and TCA were studied by similarity analysis (SA), the quantitative thermokinetic parameters from these curves were analyzed by multivariate analysis of variance (MANOVA) and principal component analysis (PCA). By comparing the values of two main parameters, P₂ (the heat-flow output power of the highest peak) and Q₂ (the heat output of the second exponential growth phase) of C. albicans, it could be found that CA had the strongest antifungal activity among the three steroid compounds, which might be used as a potential antifungal agent in the future. This study provided a useful method and idea of microcalorimetry with chemometrics to efficiently evaluate the antifungal activities of bile acid derivatives, giving some references for screening out new antifungal agents.However, it has to be stressed that all these experiments are carried out in vitro and they still require clinical validation.     

Transmembrane ion conductance by an acyclic bolaamphiphile

[reaction in text] An acyclic bolaamphiphile forms ion channels in vesicles and planar bilayer membranes that are closely similar to the channels formed by related bismacrocyclic compounds. Thus the function of these ion channels does not depend on synthetically difficult macrocyclic subunits.     

Synthesis of heterosteroids. First synthesis of oxa steroid from cholic acid

We wish to report here a new and efficient partial synthesis of 3-oxa-5β-steroid, the first oxa steroid synthesized from cholic acid.     

Straightforward synthesis of cholic acid stabilized loop mimetics

We here report on a new straightforward strategy for the synthesis of cyclic cholic acid–peptide conjugates. A solid-phase synthesis method is presented in which a selected anti-lysozyme CDR3 fragment, Asp-Ser-Thr-Ile-Tyr-Ala-Ser-Tyr-Tyr-Glu-Ser, is immobilized onto a steroidal cholic acid derived scaffold in order to yield a loop-like structure. Therefore, part of the desired sequence, that is, Ser-Tyr-Tyr-Glu-Ser, is introduced, at the C12 position of the scaffold. Subsequently, the remainder of the envisaged sequence is introduced at C3 via a Cu-catalyzed cyclo-addition reaction. Finally, amide bond formation delivers the desired cyclic peptidosteroid. This new synthetic strategy offers an easy and short access to cyclic peptidosteroids via convergent peptide ligation and macrocyclization.     

胆酸含量的近红外分析数学模型

本文应用近红外技术研究了快速测定胆酸含量的方法.通过测定胆酸在10000~4000cm-1范围内的近红外透射光谱,基于偏最小二乘(PLS)算法,建立了胆酸含量的数学模型.以校正均方差(RMSEC)和相关系数(R)为指标,确定了用于建模的最优近红外波段和光谱预处理方法,并基于此模型预测了9个样品.结果显示,建模效果良好,...     

Polymer-catalyzed aminolysis of covalently imprinted cholic acid derivative

A cholic acid template bearing an acrylate functional group was copolymerized with acrylamides to generate a series of cross-linked imprint polymers. Subsequent release from the matrix by means of an aminolysis reaction showed a remarkable catalysis by the polymer surface.     

Artificial ion channels showing rectified current behavior.

Voltage-dependent artificial ion channels 3 and 4 were synthesized. Two cholic acid derivatives were connected through a m-xylylene dicarbamate unit at 3-hydroxyl groups. Asymmetries were introduced by terminal hydrophilic groups, carboxylic acid and phosphoric acid for 3 and hydroxyl and carboxylic acid for 4. Under basic conditions, these headgroups in 3 and 4 are expected to be dissociate into -1/-2 (pH 8.2) and 0/-1 (pH 7.2), respectively. Single ion channel properties were examined by a planar bilayer lipid membrane method under symmetrical 500 mM KCl at pH 8.2 or 7.2. When 3 and 4 were introduced into the bilayer membrane under application of positive voltage (a positive-shift method), the current values at positive applied voltage were larger than the corresponding ones at the negative applied voltage. The current-voltage plots were fitted by curves through a zero point to show clear rectification properties. The direction of rectification could be controlled by positive- or negative-shift methods. Vectorial alignment of terminal headgroup charges by the voltage-shift incorporation is essential for giving voltage-dependent rectified ion channels.     

Ligand-gated synthetic ion channels

Supramolecular pi-stack architecture is fundamental in DNA chemistry but absent in biological and synthetic ion channels and pores. Here, a novel rigid-rod pi-stack architecture is introduced to create synthetic ion channels with characteristics that are at the forefront of rational design, that is, ligand gating by a conformational change of the functional supramolecule. Namely, the intercalation of electron-rich aromatics is designed to untwist inactive electron-poor helical pi-stacks without internal space into open barrel-stave ion channels. Conductance experiments in planar lipid bilayers corroborate results from spherical bilayers and molecular modeling: Highly cooperative and highly selective ligand gating produces small, long-lived, weakly anion selective, ohmic ion channels. Structural studies conducted under conditions relevant for function provide experimental support for helix-barrel transition as origin of ligand gating. Control experiments demonstrate that minor structural changes leading to internal decrowding suffice to cleanly annihilate chiral self-organization and function.     

DNA-nanotube artificial ion channels

There is considerable interest in developing chemical devices that mimic the function of biological ion channels. We recently described such a device, which consisted of a single conically shaped gold nanotube embedded within a polymeric membrane. This device mimicked one of the key functions of voltage-gated ion channels: the ability to strongly rectify the ionic current flowing through it. The data obtained were interpreted using a simple electrostatic model. While the details are still being debated, it is clear that ion-current-rectification in biological ion channels is more complicated and involves physical movement of an ionically charged portion of the channel in response to a change in the transmembrane potential. We report here artificial ion channels that rectify the ion current flowing through them via this "electromechanical" mechanism. These artificial channels are also based on conical gold nanotubes, but with the critical electromechanical response provided by single-stranded DNA molecules attached to the nanotube walls.