Complex formation of monovalent cations with biofunctional ligands

Switching between Ligand-to-Ligand Charge-Transfer, Intraligand Charge-Transfer, and Metal-to-Ligand Charge-Transfer Excited States in Platinum(II) Terpyridyl Acetylide Complexes Induced by pH Change and Metal Ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2''-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.

Effect of ionizing radiation on artificial (planar) lipid membranes. II. The ion carriers valinomycin and nonactin as probes for radiation induced structural changes of the membrane

Planar lipid membranes in the presence of the ion carriers valinomycin or nonactin were irradiated with 14 MeV electrons from a linear accelerator. A large increase of the membrane conductance by up to more than two orders of magnitude was found. The effect is virtually abolished either at high pH, or in the absence of oxygen, or in the presence of the radical scavenger ethanol. A further prerequisite for the effect is the presence of unsaturated fatty acid residues. A kinetic analysis of the carrier transport model based on current-voltage curves and on voltage-jump relaxation experiments was performed as a function of radiation dose. Only the translocation rate constant, kMS, of the charged carrier-ion complex was found to be influenced by irradiation. The effect is interpreted as an increase of the polarity (dielectric constant) of the membrane interior induced by the presence of polar products of lipid peroxidation. A combined action of OH- and HO2-radicals seems to be responsible for the phenomena. At large radiation doses (greater than or equal to 10(3) Gy) a reduction of the membrane conductance was observed. This is interpreted as an increased microviscosity, possibly caused by cross-linking of fatty acid residues. Ion carriers represent sensitive probes of radiation induced membrane damage.

Relaxation studies of ion transport systems in lipid bilayer membranes

Relaxation techniques have been widely used in kinetic studies of chemical reactions in homogeneous solution (Eigen & DeMayer, 1963). The principle of this method is well known: an external variable such as temperature or pressure is suddenly changed and the time course of a state parameter of the system such as concentration is recorded as it approaches a new steady value. Relaxation techniques can also be used for studying the rate of elementary processes in membranes. This method has proved particularly useful for the investigation of ion transport systems (ion carriers, channels, pumps) in artificial planar bilayer membranes. In this review we describe different relaxation techniques which have been developed for this purpose during the last years, as well as applications to a number of ion transport systems.

Open chain crown-type polyethers and pyridinophane cryptands act as ionophores upon frog motor nerve and isolated rat heart cells

Frog motor nerves and isolated heart cells from neonatal rats were incubated with solutions of open chain crown-type polyether or pyridinophane cryptand. The following alterations in membrane excitability and energy consumption were found: 1. The non-cyclic ligand stabilizes the resting potential of the frog nerve and reduces the pulsation rate of heart muscle cells. It is reversibly bound at the cell surface and does not affect the energy metabolism of the heart cells. (formula: see text) 2. The cryptand 1,12-dioxo-2,11-diaza-5,8,21,24-tetraoxa[12-8(2,11)] (2,6)-pyridinophane) ([2.2.1py]-diamide) is irreversibly bound by the tissues. It facilitates the depolarization of the nerve and shows a positively chronotropic effect upon the heart muscle cells. Single treatment of the cell cultures with 10 microgram [2.2.1py]-diamide per ml medium increased the activities of lactate dehydrogenase and of creatine kinase. When the cell cultures were treated three times at 24 h intervals with 10 microgram complexone/ml, the creatine kinase activity of the heart muscle cells decreased by about 40%. The physiological properties of the ligands are correlated with the stability of their alkali metal ion complexes and with the rate constants of complex formation. It is concluded that [2.2.1py]-diamide can act as a passive carrier for Na+ K+.