Multiple Stoichiometries Research Articles

Formation and Physicochemical Properties of Crystalline and Amorphous Salts with Different Stoichiometries Formed between Ciprofloxacin and Succinic Acid

Multi-ionizable compounds, such as dicarboxylic acids, offer the possibility of forming salts of drugs with multiple stoichiometries. Attempts to crystallize ciprofloxacin, a poorly water-soluble, amphoteric molecule with succinic acid (S) resulted in isolation of ciprofloxacin hemisuccinate (1:1) trihydrate (CHS-I) and ciprofloxacin succinate (2:1) tetrahydrate (CS-I). Anhydrous ciprofloxacin hemisuccinate (CHS-II) and anhydrous ciprofloxacin succinate (CS-II) were also obtained. It was also possible to obtain stoichiometrically equivalent amorphous salt forms, CHS-III and CS-III, by spray drying and milling, respectively, of the drug and acid. Anhydrous CHS and CS had melting points at ∼215 and ∼228 °C, while the glass transition temperatures of CHS-III and CS-III were ∼101 and ∼79 °C, respectively. Dynamic solubility studies revealed the metastable nature of CS-I in aqueous media, resulting in a transformation of CS-I to a mix of CHS-I and ciprofloxacin 1:3.7 hydrate, consistent with the phase diagram. CS-III was observed to dissolve noncongruently leading to high and sustainable drug solution concentrations in water at 25 and 37 °C, with the ciprofloxacin concentration of 58.8±1.18 mg/mL after 1 h of the experiment at 37 °C. This work shows that crystalline salts with multiple stoichiometries and amorphous salts have diverse pharmaceutically relevant properties, including molecular, solid state, and solubility characteristics.

Counting membrane-embedded KCNE β-subunits in functioning K + channel complexes

Ion channels are multisubunit proteins responsible for the generation and propagation of action potentials in nerve, skeletal muscle, and heart as well as maintaining salt and water homeostasis in epithelium. The subunit composition and stoichiometry of these membrane protein complexes underlies their physiological function, as different cells pair ion-conducting alpha-subunits with specific regulatory beta-subunits to produce complexes with diverse ion-conducting and gating properties. However, determining the number of alpha- and beta-subunits in functioning ion channel complexes is challenging and often fraught with contradictory results. Here we describe the synthesis of a chemically releasable, irreversible K(+) channel inhibitor and its iterative application to tally the number of beta-subunits in a KCNQ1/KCNE1 K(+) channel complex. Using this inhibitor in electrical recordings, we definitively show that there are two KCNE subunits in a functioning tetrameric K(+) channel, breaking the apparent fourfold arrangement of the ion-conducting subunits. This digital determination rules out any measurable contribution from supra, sub, and multiple stoichiometries, providing a uniform structural picture to interpret KCNE beta-subunit modulation of voltage-gated K(+) channels and the inherited mutations that cause dysfunction. Moreover, the architectural asymmetry of the K(+) channel complex affords a unique opportunity to therapeutically target ion channels that coassemble with KCNE beta-subunits.

Structural aspects of a series of cation radical salts of tetrathiotetracene (TTT) with 2-alkoxy-1,1,3,3-tetracyanoallyl anions (RO-TCA–; R = Me, Et, Prn, Bun)

Eight cation radical salts of tetrathiotetracene (TTT) with a series of 2-alkoxytetracyanoallyl anions (RO-TCA−; R = Me, Et, Prn, Bun) were prepared by electrocrystallisation. All of them were non-magnetic semiconductors. Seven crystal structures were determined. The physical and structural aspects of the salts were well categorised by their stoichiometries of donor to anion, 3 ∶ 2, 1 ∶ 1 and 1 ∶ 2. The 1 ∶ 2 salts had alternating stacks of dication of TTT and anion molecules, which is uncommon for TTT salts. Molecular structures were also studied by PM3 and RHF/6-31G* calculations. One of the fascinating features of RO-TCAs is their structural flexibility, which gives rise to multiple stoichiometries with varied physical properties.

Structures and physical properties of radical cation salts based on alkyloxytetracyanoallylide anion

Various radical cation salts of a series of 2-alkyloxytetracyanoallylide anions (RO-TCA; R=Me, Et, Pr, Bu) were investigated to examine the structure-property relationship. TTT salts were classified into three groups based on their crystal and elctronic structures which were essentially determined by the stoichiometries; 3:2, 1:1 and 1:2. The crystal structures of 7 TTT salts were solved, which contained the first examples of TTT 2+ structure. BEDT-TTF salts of α′ and β″ type were obtained. All the a′ type salts were semiconducting, however, MeO-TCA salt turned into a metallic one after keeping it for a month at room temperature or for a day at 330 K. RO-TCAs possess structural flexibility, which is essential to provide multiple stoichiometries and similar packing motives of donor.

MinK-KvLQT1 Fusion Proteins, Evidence for Multiple Stoichiometries of the Assembled IsKChannel

IsK, a slowly activating delayed rectifier K+ current through channels formed by the assembly of two channel proteins KvLQT1 and MinK, modulates the repolarization of cardiac action potentials. Mutations that map to the KvLQT1 and minK genes account for more than 50% of an inherited cardiac disorder, the Long QT syndrome (Splawski, I., Tristani-Firouzi, M., Lehmann, M. H., Sanguinetti, M. C., and Keating, M. T. (1997) Nat. Genet. 17, 338-340). Despite the importance of these channels to human cardiac function, the molecular basis of their uniquely slow gating properties as well as the stoichiometry and interaction sites of these two subunits are still unclear. We have constructed several fusion channel proteins to begin investigating the stoichiometry of these two subunits and the role of voltage-dependent subunit assembly in channel gating. Functional properties of these constructs were measured using whole cell patch clamp recordings of transiently transfected Chinese hamster ovary cells. The constructs we tested are as follows: MK24 (C terminus of MinK linked to N terminus of KvLQT1); KK40 (a tandem homodimer of KvLQT1); and MKK44 (C terminus of MinK linked to N terminus of KK40). In control experiments (no DNA, control DNA, or only MinK), no time-dependent K+ current was observed. Expression of KvLQT1 or KK40 produced currents that activate and inactivate in a voltage-dependent manner as reported by others for KvLQT1. In contrast, expression of MK24 and MKK44 elicited current with activation kinetics and voltage dependence very similar to native IsK and identical to currents expressed by cells co-transfected with independent MinK and KvLQT1 cDNA. Expression of MK24 plus additional MinK significantly slows current kinetics. Our data raise the possibility 1) of multiple MinK/KvLQT1 stoichiometries and 2) indicate that uniquely slow kinetics of IsK channels is due to voltage-dependent conformational changes of the channel protein and not to assembly of channel subunits.

Stoichiometry and cluster specificity of copper binding to metallothionein: homogeneous metal clusters

Experiments were done to define the stoichiometry of binding of Cu(I) to metallothionein (MT) and to determine its sites of binding in mixed-metal species. Spectrophotometric titrations of rabbit liver Cd7-MT 2, apoMT, and Cd4-alpha-domain with Cu(I) revealed endpoints of 3-4, 4 and 8, and 4 and 6-7 added Cu(I)/mol of MT for the three species respectively. Observed endpoints depended on conditions of the titration and the wavelength chosen for absorbance measurement. Nevertheless, from metal and sulphydryl analyses of titrated proteins that were pretreated with Chelex-100 to remove metal ions from solution, almost all of the cadmium was displaced from Cd7-MT by the addition of 7 Cu(I)/mol of MT. Similarly, 4 Cu(I)/mol of Cd4-alpha-domain completely displaced bound cadmium. The Cu4-alpha-domain was converted into a Cu6-alpha species upon addition of two equivalents of Cu(I)/mol of alpha-domain. Reaction of Cd7-MT with 7, 12 and 20 Cu/mol of MT, followed by reaction with Chelex resin, generated protein samples in each case with about 7 Cu/mol of MT. 111Cd-NMR analysis of the reaction of 111Cd7-MT with Cu(I) showed that nearly co-operative one-for-one replacement of 111Cd occurred and that the beta-domain cluster reacted before the alpha-domain cluster. Two mixed-metal MTs with Cu to Zn ratios approximating 3 to 4 and 6 to 4 were isolated from calf liver. After substitution of Zn with 111Cd, NMR spectra of each protein showed that 111Cd was confined almost completely to the alpha-domain. By inference, about 3 or 6 Cu were bound in the beta-domain of these proteins. Supporting this segregation of metal ions into domains, reaction of Cu6, Zn4-MT with nitrilotriacetate removed zinc exclusively, whereas reaction of Cu6,Cd4-MT with 4,7-phenylsulphonyl-2,9-dimethyl-1,10-phenanthroline extracted only Cu(I). Proteolytic digestion of both products followed by gel filtration demonstrated that Cu(I) and Cd were bound to fragments of the intact protein. Finally, reaction of rabbit liver 111Cd7-MT 2 with Cu10-MT 2 resulted in interprotein metal exchange in which 111Cd-moved from the beta- to the alpha-domain according to NMR analysis. In contrast with the prevalent view that six Cu(I) bind to each domain of MT, the present results show that Cu(I) binds to MT with a minimum stoichiometry of about 7 Cu(I)/mol of MT and can bind to the alpha-domain with stoichiometries of 4 or 6 Cu(I)/mol of MT. Although MTs interacting with 12 or 20 Cu(I)/mol of MT are less stable than that binding about 7 Cu(I)/mol, it appears that MT can bind Cu(I) in multiple stoichiometries.

Binding characteristics of Hoechst 33258 with calf thymus DNA, poly[d(A-T)] and d(CCGGAATTCCGG): multiple stoichiometries and determination of tight binding with a wide spectrum of site affinities

Equilibrium binding experiments using fluorescence and absorption techniques have been performed throughout a wide concentration range (1 nM to 30 microM) of the dye Hoechst 33258 and several DNAs. The most stable complexes found with calf thymus DNA, poly[d(A-T)], d(CCGGAATTCCGG), and d(CGCGAATTCGCG) all have dissociation constants in the range (1-3) X 10(-9) M-1. Such complexes on calf thymus DNA occur with a frequency of about 1 binding site per 100 base pairs, and evidence is presented indicating a spectrum of sequence-dependent affinities with dissociation constants extending into the micromolar range. In addition to these sequence-specific binding sites on the DNA, the continuous-variation method of Job reveals distinct stoichiometries of dye-poly[d(A-T)] complexes corresponding to 1, 2, 3, 4, and 6 dyes per 5 A-T base pairs and even up to 1 and 2 (and possibly more) dyes per backbone phosphate. Models are suggested to account for these stoichiometries. With poly[d(G-C)] the stoichiometries are 1-2 dyes per 5 G-C pairs in addition to 1 and 2 dyes per backbone phosphate. Thermodynamic parameters for formation of the tightest binding complex between Hoechst 33258 and poly[d(A-T)] or d-(CCGGAATTCCGG) are determined. Hoechst 33258 binding to calf thymus DNA, chicken erythrocyte DNA, and poly[d(A-T)] exhibits an ionic strength dependence similar to that expected for a singly-charged positive ion. This ionic strength dependence remains unchanged in the presence of 25% ethanol, which decreases the affinity by 2 orders of magnitude. In addition, due to its strong binding, Hoechst 33258 easily displaces several intercalators from DNA.

Synthesis of Biethylenedithiolylene-Tetrathiafulvalene Donors (BEDT-TTF) and Electrochemical Preparation of their Charge Transfer Complexes

Abstract Various synthetic procedures for preparing BEDT-TTF are critically examined. Starting from readily available thiapendione, BEDT-TTF can be made in a high yield, simple, two-step synthesis. Electrochemical crystal growth of charge transfer complexes of BEDT-TTF typically involves the formation of multiple phases and stoichiometries. In the case of perrhenate (ReO- 4) at least five different complexes have been identified. Controlled preparation of the superconducting phase (BEDT-TTF2ReO4) remains an elusive goal.

Crystal chemistry of alpha-boron derivatives

Abstract : Assignment of stoichiometry to interstitial compounds is discussed. It is shown that multiple stoichiometries can occur within the same structural framework where more than one type of interstitial opening is available. These principles are used to clarify the confusion which currently exists in the isotypic series of interstitial compounds derived from alpha boron. With respect to occupancy of interstitial sites three distinct stoichiometries can exist in this isotypic group; namely, B12 X2Y (B12X3), B12X2 and B12X. Data are presented on (1) a new form of boron carbide (B12C2), (2) a new alpha boron derivative containing interstitial aluminum (B12C2A1), (3) identification and differentiation of two boron subphosphides (B12P2 and B13P2), (4) preparation of boron subarsenide, and (5) relationship between metal dodecaborides and alpha boron derivatives. (Author)