Mechanism of ion transport through lipid bilayer-membranes mediated by peptide cyclo-(d-Val-l-Pro-l-Val-d-Pro)3

The cyclic dodecapeptide PV, cyclo-(d-Val-l-Pro-l-Val-d-Pro)₃, a structural analogue of the ion-carier valinomycin, increase the cation permeability of lipid bilayer membranes. This paper reports the results of two types of relaxation experiments, namely relaxation of the membrane current after a voltage jump and decay of the membrane voltage after a charge pulse in lipid bilayer membranes exposed to PV. From the relaxation data, the rate constant for the translocation of the ion carrier complex across the membrane, as well as the partition coefficient of the complex between water and membrane solution interface were computed and found to be about one order of magnitude less than the comparable values for valinomycin (Val). Furthermore, the dependence of the initial membrane conductivity on ion concentration was used to evaluate the equilibrium constant, K, of complexation between PV and some monovalent cations in water. The values of K yield the following selectivity sequence of PV: Na⁺ < NH₄⁺ < K⁺ < Cs⁺ < Rb⁺. These and earlier results are consistent with the idea that PV promotes cation movement across membranes by the solution complexation mechanism which involves complexation between ion and carrier in the aqueous phase and transport of the carrier across the membrane. In the particular form of the solution complexation mechanism operating here, the PV present in the PV-cation complex carrying charge across the membrane derives from the side from which the current is flowing (cis-mechanism). As shown previously, valinomycin, in contrast to PV, acts by an interfacial complexation mechanism in which the Val in the Val-cation complex derives from the side toward which current is flowing (trans-mechanims). The comparison of the kinetic properties of these two closely related compounds yields interesting insights into the relationship between chemical structure and function of ion carriers.     

Silk fibroin model,poly(l-alanylglycyl-l-alanylglycyl-l-alanylglycyl-l-seryglycine)

l-Alanylglycyl-l-alanylglycyl-l-alanylglycyl-l-serylglycine and its pentachlorophenyl ester methanesulphonate have been synthesized as monomers for the preparation of silk fibroin model polypeptide. The former octapeptide was polymerized with diphenylphosphorylazide (DPPA) and triethylamine in DMSO or in HMPA—pyridine, and the latter octapeptide pentachlorophenylester was polymerized by adding triethylamine in DMSO to give poly(l-alanylglycyl-l-alanylglycyl-l-alanylglycyl-l-serylglycine). This sequential polypeptide gave a similar i.r. pattern to the crystalline part of Bombyx mori silk fibroin, which indicated antiparallel β-conformation. Dialysis of the solution of this polymer in 60%, aqueous LiBr against water gave mainly the polymer of α-form. O.r.d. measurements suggest that this polypeptide exists as a random structure in dichloroacetic acid on in 60% aqueous LiBr.     

Conformational studies on synthetic polypeptides: poly (d-phenylglycine) and poly(d-cyclohexylglycine)

Poly(d-phenylglycine) and poly(d-cyclohexylglycine) containing phenyl and cyclohexyl rings bound to the α-carbon of the polypeptide chain, have been synthesized. Circular dichroism measurements show that both polymers undergo a conformational transition from the random-coil form to an ordered form, upon addition of water, ethanol or trifluoroethanol to sulphuric acid solutions. Solid state measurements indicate that the ordered structures of poly(d-phenylglycine) and poly(d-cyclohexylglycine) are of the β-type. While for the former the antiparallel arrangement is predominant, for the latter there seems to be a greater tendency towards the parallel form. The ordered form of poly(d-cyclohexylglycine) is slightly more stable than the corresponding form of poly(d-phenylglycine) in all the above solvent systems. This can be interpreted in terms of stronger non covalent bond formation in the former polypeptide. Our results have been compared with literature on poly(l-phenylalanine) and poly(l-cyclohexylalanine).     

Alkaline decomposition of d-xylose-1-C, d-glucose-1-C, and d-glucose-6-C

The distribution of radioactivity in the three- and four-carbon saccharinic acids, lactic acid and 2,4-dihydroxybutyric acid, obtained from d-xylose-1-¹⁴C, d-glucose-1-¹⁴C, and d-glucose-6-¹⁴C, was measured. The relative importance of the various mechanisms for forming 2,4-dihydroxybutyric acid was determined. Recombination of two-carbon fragments was found to be an important mechanism at the high alkalinity and temperature employed.     

Skeletal muscle [(V)\dot]O2 \dot V_{O_2 } in rat and lemming: Effect of blood flow rate

Summary The rate of oxygen consumption ( ) by skeletal muscle was investigated in isolated perfused hindlimbs of laboratory rats and lemmings (Lemmus). In both species, increased in proportion to blood flow rate, even at flow rates 4–5 times above resting level. The slope of the line relating to skeletal muscle blood flow was significantly greater in the lemming than in the rat. This may be related to the inverse relationship between body weight and metabolic rate. These data support the hypothesis that in small animals a dependent relationship exists between blood flow and skeletal muscle .     

Reaction of FcCCo3(CO)9 with 2,3-bis(diphenylphosphino)maleic anhydride (bma). X-ray diffraction structure and redox properties of

Cyclic dipeptide cyclo(l- or d-Glu-l-His) carrying an anionic site and a nucleophilic site has been synthesized and used as a catalyst for the solvolysis of cationic esters in aqueous alcohols. In the solvolysis of 3-acyloxy-N-trimethylanilinium iodide (S⁺_n, n = 2 and 10) and Cl^?H₃N⁺(CH₂)₁₁COOPh(NO₂), no efficient nucleophilic catalysis was observed. On the other hand, in the solvolysis of Gly-OPh(NO₂)·HCl, Val-OPh(NO₂)·HCl and Leu-OPh(NO₂)·HCl a very efficient general base-type catalysis by cyclo(l-Glu-l-His) was observed. In particular, with the latter two substrates the catalysis by cyclo(l-Glul-His) was more efficient than that by imidazole, although the catalysis was not enantiomer-selective. The diastereomeric cyclic dipeptide cyclo(d-Glu-l-His) was almost inactive under the same conditions. Confomation of cyclo(l- or d-Glu-l-His) in aqueous solution was investigated and the structure/catalysis relationship is discussed.     

Solvolysis of charged active esters of carboxylic acids with cyclo (l- or d-Glu-l-His)

Cyclic dipeptide cyclo(l- or d-Glu-l-His) carrying an anionic site and a nucleophilic site has been synthesized and used as a catalyst for the solvolysis of cationic esters in aqueous alcohols. In the solvolysis of 3-acyloxy-N-trimethylanilinium iodide (S⁺_n, n = 2 and 10) and Cl^?H₃N⁺(CH₂)₁₁COOPh(NO₂), no efficient nucleophilic catalysis was observed. On the other hand, in the solvolysis of Gly-OPh(NO₂)·HCl, Val-OPh(NO₂)·HCl and Leu-OPh(NO₂)·HCl a very efficient general base-type catalysis by cyclo(l-Glu-l-His) was observed. In particular, with the latter two substrates the catalysis by cyclo(l-Glul-His) was more efficient than that by imidazole, although the catalysis was not enantiomer-selective. The diastereomeric cyclic dipeptide cyclo(d-Glu-l-His) was almost inactive under the same conditions. Confomation of cyclo(l- or d-Glu-l-His) in aqueous solution was investigated and the structure/catalysis relationship is discussed.