Valinomycin cation transport

In what follows, the phenomenology of carrier transport will be briefly reviewed along with the mechanism of the Valinomycin model of carrier transport. The development of the molecular structure of Valinomycin will be considered in some detail, since the key to the dramatic selectivity of Valinomycin is thought to reside in the energetics of the molecular structure. Confidence in an understanding of the molecular structure of the Valinomycin-cation complex becomes tantamount to confidence in the presented basis of ion selectivity. 

A more detailed study of transport processes in solvent polymeric membranes was initiated recently.72 One aim was to get information on the distribution within the membrane of the carrier and the cation transported after a steady state has built up during an electrodialysis experiment. A further objective was the demonstration of a relaxation of the concentration gradients of both carrier and cation. To this end the transport properties of solvent polymeric membranes containing the carrier l4C-valinomycin (66 wt.% dioctyladipate, 33 wt.% polyvinyl chloride, 1 wt.%, JC-valinomycin) in contact with aqueous solutions of -,H-a-phenylethylammonium chloride were studied. 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

Fig. 11. Plot of initial cation transport rates for various carrier/alkali picrate pairs versus equilibrium extraction constants log Ke the points are experimental data, the curve is calculated [6.1, 6.4,6.17]. For analytical reasons the Ke values were determined in conditions different from those of the transport experiments the carriers are cryptands (for [2.2.C5] and [2.1.C5] see structures in [6.1]) dibenzo-18-crown-6, DB18-6 and valinomycin, VAL picrate, P.

Valinomycin is an ionophore but with different properties from gramicidin A (a) it specifically transports K+, and no other ion, when inserted into membranes or vesicles (b) it can transport K+ only above the phase transition temperature of the membrane, whereas cation transport by gramicidin A is insensitive to temperature. 

Table 17. Rate constants r, k-Q, ms> ks of valinomycin and trinactin mediated cation transport through glycerol monooleate/n-decane bilayer membranes at 25 °C (cf. Fig. 18)...

Ion pair transport is also possible by incorporating the anion and cation transporters as independent components to the lipid bilayer, using a dual host approach. Using combinations or cocktails of different drugs is a commonly used strategy in biology. This approach has been demonstrated by using combinations of classical cation transporters such as valinomycin and calix[4]pyrrole derivatives such as 75. 

Transport of metal picrate across a chloroform liquid membrane was examined in a U-shaped tube at 25°. The source aqueous phase (pH 7.2) contains a metal chloride and picric acid. The alkali metal cations used were Na". K, and Rb, and the alkaline earth metal cations Mg " ", Ca ", and Ba ". The chloroform phase contains an ion carrier. The second aqueous phase was adjusted to pH 7.2. Linear octapeptides Boc-[Gly-L-Lys(Z)-Sar-L-Pro]2 0H (LGLSP2-0H) and Boc-[Gly-L-Lys(Z)-Sar-L-Pro]2-OCH3 (LGLSP2-0M), cyclic tetrapeptide c-[Gly-L-Lys(Z)-Sar-L-Pro] (CGLSPl), and linear tetrapeptide Boc-Gly-L-Lys(Z)-Sar-L-Pro-OH (LGLSPl-OH) were used as related petidic carriers. In addition, valinomycin and dibenzo-18-crown-6 were employed as a ionophore. The amount of cation transported to the second aqueous phase through membrane was determined spectrophotometrically at 355 nm. 

The use of an external cationic transporter to assist in the transport of anions by certain carriers has been explored by Gale and others recently. This was demonstrated by Gale in the case the triazole-strapped calix[4]pyrrole 36, which was combined with the potassium carrier valinomycin. The combination led to enhanced transport of chloride anions across POPC membranes (cf. Fig. 12.23) [71]. The overall transport process was consistent with a KVC1 symport mechanism. This dual host approach was employed by Sessler, Gale, Shin, and coworkers in the case of the pyridine-diamide strapped caUx[4]pyrrole Cl transporter 37 (cf. Fig. 12.23) [72]. It was found that the combination of 37 and... 

MacrotetroHdes of the valinomycin group of electrically neutral antibiotics form stable 1 1 complexes with alkaH metal ions that increase the cation permeabiHty of some biological and artificial lipophilic membranes. This solubiHzation process appears to have implications in membrane transport research (30) (see Antibiotics, peptides). 

With respect to the carrier mechanism, the phenomenology of the carrier transport of ions is discussed in terms of the criteria and kinetic scheme for the carrier mechanism the molecular structure of the Valinomycin-potassium ion complex is considered in terms of the polar core wherein the ion resides and comparison is made to the Enniatin B complexation of ions it is seen again that anion vs cation selectivity is the result of chemical structure and conformation lipid proximity and polar component of the polar core are discussed relative to monovalent vs multivalent cation selectivity and the dramatic monovalent cation selectivity of Valinomycin is demonstrated to be the result of the conformational energetics of forming polar cores of sizes suitable for different sized monovalent cations. 

There appear to be two major ways by which ionophores aid ions to cross membrane barriers. Ionophores such as valinomycin and nonactin enclose the cation such that the outside of the complex is quite hydro-phobic (and thus lipid-soluble). The transport behaviour thus involves binding of the cation at the membrane surface by the antibiotic, followed by diffusion of the complexed cation across the membrane to the opposite surface where it is released. Such carrier type ionophores can be very efficient, with one molecule facilitating the passage of thousands of ions per second. A prerequisite for efficient transport by this type of ionophore is that both the kinetics of complex formation and dissociation be fast. 

Impetus was given to work in the field of selective cation complex-ation by the observation of Moore and Pressman (5) in 1964 that the macrocyclic antibiotic valinomycin is capable of actively transporting K+ across mitochondrial membranes. This observation has been confirmed and extended to numerous macrocyclic compounds. There is now an extensive literature on the selective complexation and transport of alkali metal ions by various macrocyclic compounds (e.g., valinomycin, mo-nactin, etc.) (2). From solution spectral (6) and crystal X-ray (7) studies we know that in these complexes the alkali metal cation is situated in the center of the inwardly oriented oxygen donor atoms. Similar results are found from X-ray studies of cyclic polyether complexes of alkali metal ions (8) and barium ion (9). These metal macrocyclic compound systems are especially noteworthy since they involve some of the few cases where alkali metal ions participate in complex ion formation in aqueous solution. 

Five membranes (thickness, 40 /am) were stacked and the concentration of ligand and cation in each membrane was measured before and immediately after the transport experiment as well as 5 days after restacking the membranes. Since a concentration gradient of valinomycin developed (Fig. 11, f = 3hr), which decayed almost completely after a relaxation period (Fig. 11, l = 5 days), the a-phenylethylammonium cation had obviously been transported by a carrier mechanism. During the transport process a cation profile built up (Fig. 12, t = 3 hr) that had the same trend as the ligand profile. This cation gradient disappeared after some time (Fig. 12, t = 5 days). 

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Alkali metal transport in biochemistry is a vital process in maintenance of cell membrane potentials of use, for example, in nerve signal transduction and is at the core of some of the early work on artificial ionophores that mimic natural ion carriers such as valinomycin. Ionophore mediated ion transport is much slower than transport through cation and anion ion channel proteins, however. 

---