Interactions of monovalent cations with sodium channels in squid axon. I. Modification of physiological inactivation gating

Inactivation of Na channels has been studied in voltage-clamped, internally perfused squid giant axons during changes in the ionic composition of the intracellular solution. Peak Na currents are reduced when tetramethylammonium ions (TMA+) are substituted for Cs ions internally. The reduction reflects a rapid, voltage-dependent block of a site in the channel by TMA+. The estimated fractional electrical distance for the site is 10% of the channel length from the internal surface. Na tail currents are slowed by TMA+ and exhibit kinetics similar to those seen during certain drug treatments. Steady state INa is simultaneously increased by TMA+, resulting in a "cross-over" of current traces with those in Cs+ and in greatly diminished inactivation at positive membrane potentials. Despite the effect on steady state inactivation, the time constants for entry into and exit from the inactivated state are not significantly different in TMA+ and Cs+. Increasing intracellular Na also reduces steady state inactivation in a dose-dependent manner. Ratios of steady state INa to peak INa vary from approximately 0.14 in Cs+- or K+-perfused axons to approximately 0.4 in TMA+- or Na+-perfused axons. These results are consistent with a scheme in which TMA+ or Na+ can interact with a binding site near the inner channel surface that may also be a binding or coordinating site for a natural inactivation particle. A simple competition between the ions and an inactivation particle is, however, not sufficient to account for the increase in steady state INa, and changes in the inactivation process itself must accompany the interaction of TMA+ and Na+ with the channel.     

Interactions of monovalent cations with sodium channels in squid axon. II. Modification of pharmacological inactivation gating

The time-, frequency-, and voltage-dependent blocking actions of several cationic drug molecules on open Na channels were investigated in voltage-clamped, internally perfused squid giant axons. The relative potencies and time courses of block by the agents (pancuronium [PC], octylguanidinium [C8G], QX-314, and 9-aminoacridine [9-AA]) were compared in different intracellular ionic solutions; specifically, the influences of internal Cs, tetramethylammonium (TMA), and Na ions on block were examined. TMA+ was found to inhibit the steady state block of open Na channels by all of the compounds. The time-dependent, inactivation-like decay of Na currents in pronase-treated axons perfused with either PC, 9-AA, or C8G was retarded by internal TMA+. The apparent dissociation constants (at zero voltage) for interaction between PC and 9-AA with their binding sites were increased when TMA+ was substituted for Cs+ in the internal solution. The steepness of the voltage dependence of 9-AA or PC block found with internal Cs+ solutions was greatly reduced by TMA+, resulting in estimates for the fractional electrical distance of the 9-AA binding site of 0.56 and 0.22 in Cs+ and TMA+, respectively. This change may reflect a shift from predominantly 9-AA block in the presence of Cs+ to predominantly TMA+ block. The depth, but not the rate, of frequency-dependent block by QX-314 and 9-AA is reduced by internal TMA+. In addition, recovery from frequency-dependent block is not altered. Elevation of internal Na produces effects on 9-AA block qualitatively similar to those seen with TMA+. The results are consistent with a scheme in which the open channel blocking drugs, TMA (and Na) ions, and the inactivation gate all compete for a site or for access to a site in the channel from the intracellular surface. In addition, TMA ions decrease the apparent blocking rates of other drugs in a manner analogous to their inhibition of the inactivation process. Multiple occupancy of Na channels and mutual exclusion of drug molecules may play a role in the complex gating behaviors seen under these conditions.     

Spectroscopic, mass spectrometry, and semiempirical investigation of a new ester of Monensin A with ethylene glycol and its complexes with monovalent metal cations

A new ester of Monensin A with ethylene glycol (MON2) has been synthesized by a new method and its ability to form complexes with Li+, Na+, and K+ cations has been studied by ESI MS, 1H and 13C NMR, FT-IR, and PM5 semiempirical methods. It is demonstrated that MON2 forms stable complexes of 1:1 stoichiometry with monovalent metal cations. The structures of the complexes are stabilized by intramolecular hydrogen bonds in which the OH groups are always involved. In the structure of MON2 the oxygen atom of the C=O ester group is involved in very weak bifurcated intramolecular hydrogen bonds with two hydroxyl groups, whereas in the complexes of MON2 with monovalent metal cations the C=O ester group is not engaged in any intramolecular hydrogen bonds. The structures of the MON2 and its complexes with Li+, Na+, and K+ cations are visualized and discussed in detail.     

Transition metal halide salts of N-methylmorpholine: Synthesis, crystal structures and magnetic properties of N-methylmorpholinium salts of copper(II), cobalt(II) and manganese(II)

A family of complexes of N-methylmorpholinium metal halides has been synthesized and analyzed using X-ray crystallography and temperature dependant magnetic susceptibility. The compounds produced include (nmmH)₂[CuCl₄] (1), (nmmH)₂[CoCl₄] (2), (nmmH)₂[MnCl₄] (3), (nmmH)₂[CuBr₄] (4), (nmmH)₂[CoBr₄] (5), and (nmmH)₂[MnBr₄] (6) [nmmH = N-methylmorpholinium]. The compounds are all isomorphous and exhibit weak one dimensional antiferromagnetic interactions, with a contribution from single-ion anisotropy in the case of the cobalt complexes.     

Synthesis and structures of lithium salts of two cluster iodoarsenate(III)s and a periodohexaantimonate(III)

Treatment of the lithium β-diketiminate Li[{N(C₆H₃-2,6)C(Me)}₂CH] with an equivalent portion of arsenic(III) iodide unexpectedly afforded [Li(thf)₄]₂[As₈I₂₆] (1) in modest yield. Another new cluster anion featured in [Li(thf)₄]₂[Li(thf)₆][As₇I₂₄]·1.5(thf) (2) was obtained in excellent yield from lithium iodide and four equivalents of AsI₃ in thf. Using the same stoichiometry and solvent, but with SbI₃ in place of AsI₃, furnished in good yield [Li(thf)₄]₄[Sb₆I₂₂]·2(thf) (3). The structures of the new crystalline orange (1, 3) or orange/red (2) salts was established by single crystal X-ray diffraction. Each anion of 1-3 may be factorised into respectively [(AsI₂)₆(AsI)₂(μ₂-I)₁₀(μ₅-I)₂]²⁻ (1), [{(AsI₂)(μ₂-I)(μ₃-I)}₆As]³⁻ (2) and [(SbI₃)₂(SbI₂)₂(SbI)₂(μ₂-I)₆(μ₃-I)₄]⁴⁻ (3). Each As atom in 2 and each Sb atom in 3 is the centre of a six-coordinate distorted octahedron, while the arsenic atoms in 1 have either four- or six-coordinate environments.     

Thallium(III) chloride in organic solvents: Synthesis, solutions and solvates. The crystal structures of trichlorobis(dimethylsulfoxide)thallium(III) and tribromobis(dimethylsulfoxide)thallium(III)

The synthesis of thallium(III) chloride and bromide was performed in solution by chlorination and bromination, respectively, of the suspensions of the corresponding thallium(I) halides in acetonitrile. Crystalline compounds TlX₃(CH₃CN)₂ (X = Cl⁻, Br⁻) were prepared from the acetonitrile solutions. Thallium(III) chloride and bromide in dimethylsulfoxide solution were obtained by dissolving the corresponding solid compounds TlX₃(CH₃CN)₂ (Cl⁻, Br⁻) in DMSO. Both acetonitrile and dimethylsulfoxide solutions of thallium(III) chloride were studied by UV-Vis and ²⁰⁵Tl NMR spectroscopy. The UV-Vis study of the TlCl₃-CH₃CN system showed presence of at least two thallium(III) chloride species. Only one signal arising from the thallium(III) species was, however, detected by the ²⁰⁵Tl NMR in the solution because of the fast chemical exchange. The ²⁰⁵Tl NMR study of thallium(III) chloride in dimethylsulfoxide showed three separate signals assigned to the solvated , TlCl₃ and species. The crystalline compounds of trichlorobis(dimethylsulfoxide)thallium(III) and tribromobis(dimethylsulfoxide)thallium(III) were prepared and their crystal structures were solved by single-crystal X-ray analysis. The thallium atom in the complexes has a trigonal bipyramidal environment built by three halide ions occupying equatorial positions of the polyhedron and two oxygen atoms of the DMSO molecules in the apical positions.     

Interactions of amiloride and small monovalent cations with the epithelial sodium channel. Inferences about the nature of the channel pore.

The voltage dependence of amiloride-induced inhibition of current flow through apical membrane sodium channels in toad urinary bladder was studied at different ionic conditions. The "inert" salt N-methyl-D-glucamine HCl (NMDG HCl) affected neither the apparent inhibition constant (Kl) for the amiloride-induced current inhibition nor the apparent fraction of the transmembrane voltage that falls between the mucosal solution and the amiloride-binding site (delta). When NMDG+ was replaced with Na+, Kl increased, reflecting amiloride-Na+ competition, whereas delta was unchanged. Similar results were obtained with another permeant cation, Li+. When NMDG+ was replaced by K+, an impermeant but channel-blocking cation, Kl increased whereas delta decreased. Similar results were obtained using another impermeant, channel-blocking cation guanidinium. The results are interpreted on the premise that Na+ and K+ compete with amiloride by binding to cation binding sites within the channel lumen such that ion occupancy of these sites vary with voltage. Occupancy by K+, which cannot traverse the channel, will increase as the mucosal solution becomes positive, relative to the serosal solution. Occupancy by Na+, which can traverse the channel, is comparatively voltage independent. Ion movement through the channels was simulated using discrete-state kinetic models. Two types of models could describe the shape of the current-voltage relationship and the voltage dependence of the amiloride-induced channel block. One model had a single ion-binding site with a broad energy barrier at the inner (cytoplasmic) side of the site. The other model had two binding sites separated from each other and from the aqueous solutions by sharp energy barriers.     

Self-assembled diorganotin(IV) moieties with 2,3,4,5-tetrafluorobenzoic acid: Syntheses, characterizations and crystal structures

A series of new organotin(IV) derivatives with 2,3,4,5-tetrafluorobenzoic acid: {[(2,3,4,5-F₄C₆HCO₂)R₂Sn]₂O}₂ (R = Et 1, n-Bu 2, Ph 3), [R₂Sn(O₂CC₆F₄H)₂]_n (R = n-Bu 4, Et 5, Ph 6), and Sn₂R₄(O₂CC₆F₄H)₃(OH) (R = Et 7, n-Bu 8, Ph 9), were synthesized by the reaction of diorganotin oxide and 2,3,4,5-tetrafluorobenzoic acid. All the complexes 1-9 have been characterized by elemental analysis, IR, ¹H, ¹³C, ¹¹⁹Sn NMR spectra. Among them complexes 2, 4, 8 were also characterized by X-ray crystallography diffraction analyses. The crystal structure of complex 2 exhibited a tetra-nuclear geometry with the Sn₂O₂ symmetry core. Complex 4 formed a 1D helical double-chain structure through intermolecular O→Sn coordinating and completed a DNA-like assembly. Complex 8 revealed that the both Sn atoms were held together by hydroxide and acetate bridges, forming a chair-like six-membered ring. Moreover, the supramolecular structures of dimer, 1D chain or 2D network have been found in complexes 4 and 8 by intermolecular C-H?F weak hydrogen bond and non-bonded F?F or F?Sn interaction, which were highly effective in the assembly of supramolecular structures and could lead to the formation of complexes with fascinating topologies properties.     

Five-, seven-, and eight-coordinate Cd(II) coordination polymers built by anthranilic acid derivatives: Synthesis, structures and photoluminescence

Two novel Cd(II) coordination polymers, [(CH₃)₂NH₂]₂[Cd(cma)₂](H₂O) (1) and [Cd₃(bcma)₂(H₂O)](H₂O) (2) (H₂cma = N-(carboxymethyl)-anthranilic acid, H₃bcma = N,N′-bis-(carboxymethyl)-anthranilic acid), have been synthesized under hydrothermal conditions and characterized by X-ray single crystal analysis, IR spectra and TGA. Compound 1 possesses 1D double-stranded chain, which further packs into square channels. Compound 2 consists of a novel 3D framework, which not only possesses unique meniscus-like channels but also contains infinite helical chains. Compound 2 is the first example of Cd(II)-aminopolycarboxylate coordination polymers containing three crystallographically independent Cd(II) centres, in which Cd(1), Cd(2), and Cd(3) present distorted pentagonal bipyramidal, tetragonal antiprismatic, and trigonal bipyramidal coordination geometry, respectively. Both compounds display intense room temperature photoluminescence in the solid state.     

Two cadmium-thiocyanate coordination polymers with organic cations: Synthesis, crystal structures and luminescent properties

Two new inorganic-organic hybrid polymers [ClBzQl]₂[Cd(SCN)_3.5Br_0.5]·0.25H₂O (1) and [ClBzMePy][Cd(SCN)₃] (2) (ClBzQl = 1-(4′-Cl-benzyl)quinolinium cation and ClBzMePy = 1-(4′-Cl-benzyl)-2-methylpyridinium cation) have been synthesized and characterized by IR, UV, elemental analysis and X-ray crystallography. Crystal structure analyses show that two polymers belong to the monoclinic space group P2/n (1) and P2₁/c (2) with a = 18.548(2) Å, b = 9.526(1) Å, c = 20.689(2) Å, β = 94.008(1)°, V = 3646.6(5) Å³ for 1, and a = 11.195(2) Å, b = 16.415(3) Å, c = 10.751(2) Å, β = 102.930(3)°, V = 1925.7(7) Å³ for 2. The Cd atom exhibits a distorted octahedral coordination geometry for 1 and 2. For 1, a pair of 1,1-μ-SCN⁻ anions and a pair of 1,3-μ-SCN⁻ anions are alternately bridge adjacent Cd centers to form infinite polymeric chains. For 2, adjacent Cd atoms are linked by three 1,3-μ-SCN⁻ anions to form infinite [Cd(SCN)₃⁻]_∞ polymeric chains. The luminescent properties of the two polymers in the solid state at room temperature were investigated.     

Comparative genotoxicity of 3-hydroxyanthranilic acid and anthranilic acid in the presence of a metal cofactor Cu (II) in vitro

Several clinical studies have reported that an increase in excretion of tryptophan metabolites 3-hydroxyanthranilic acid (3-OHAA), anthranilic acid (AA) and other metabolites in the urine of bladder cancer patients are implicated to play a role in the etiology of bladder cancer; however the mechanisms involved are unknown. The present study compares the genotoxicity of tryptophan metabolites AA and 3-OHAA to cause mutagenesis in vitro. The DNA damage effects of tryptophan metabolites were analyzed using plasmid relaxation assay performed with AA and 3-OHAA at various concentrations between 50μM and 400μM in the presence of plasmid DNA pSP-72. Both AA and 3-OHAA did not show any plasmid relaxation activity when tested alone. However, 3-OHAA in the presence of metal cofactor Cu (II) induced plasmid relaxation by causing nicks in the plasmid. This effect was not observed in the presence of other metal cofactors Fe (II) and Mn (III). Cu (II) at increasing concentrations between 5μM and 20μM and in the presence of 100μM 3-OHAA showed an apparent dose-response in causing DNA strand breaks. The Cu (II) mediated mutagenic activation of 3-OHAA was further investigated using Ames Salmonella/microsome mutagenicity assay with reactive oxygen species (ROS) sensitive tester strain Salmonella TA102. When 100μg of 3-OHAA per plate was incubated with Cu (II) a significant increase in TA102 revertants was observed with an increase in the concentration of Cu (II) from 2.5μg to 50μg. In contrast, AA with Cu (II) at such low concentration was unable to cause any significant increase in number of the TA102 revertants. This evidence for mutagenicity with only 3-OHAA and Cu (II) but not AA suggests the presence of hydroxyl group at ortho position to amino group in 3-OHAA structurally, is critical in reacting with Cu (II) to generate genotoxicity.     

Nonmutagenicity of maleic acid and its sodium salts in salmonella assays

Maleic acid (cis-butenedioic acid) and its mono- and disodium salts are shown to be non-mutagenic in the standard Ames Salmonella/mammalian microsome assay in the absence and presence of Aroclor-1254-induced rat liver S9. This lack of activity occurred despite depression of top agar pH in accordance with the degree of protonation of this polybasic acid. These results indicate that when chemical compounds which are maleate salts show activity in this assay, the effect is attributable to the base moiety rather than maleate or pH depression per se.     

Syntheses and crystal structures of dimethyltin (IV) derivatives with 2,6-pyridinedicarboxylic acid

A novel cyclic dimethyltin complex [Me₂Sn(2,6-pdc)]₃ (1) (2,6-pdc = 2,6-pyridinedicarboxylate) was synthesized by the reaction of dimethyltin (IV) dichloride and 2,6-pyridinedicarboxylate acid in methanol under solvothermal conditions (150 °C). However, under room temperature (25 °C), we obtained a ladder complex [Me₂Sn(2,6-pdc)]₂(MeOH)₂ (2). Characterization of complexes 1 and 2 was achieved using elemental analysis, IR, ¹H, ¹³C and ¹¹⁹Sn NMR spectra and X-ray diffraction. X-ray data of 1 revealed that it was an unusual cyclic complex with a discrete cyclotrinuclear unit, in which the 12-membered cyclic cavity is almost completely planar. X-ray data of 2 showed that it was a ladder complex, in which a crystallizing methanol molecule is found in each formula unit.     

Syntheses and crystal structures of di- and trimethyltin (IV) derivatives with phenylphosphonic acid

Three types of methyltin phosphonates, {[(CH₃)₃Sn]₄(O₃PPh)₂}_n (1), {[(CH₃)₃Sn]₂(O₃PPh) · CH₃OH}_n (2) and {[(CH₃)₂SnO₃PPh]₄}_n (3) were synthesized by the reaction of phenylphonic acid with trimethyltin (IV) chloride and dimethyltin (IV) dichloride, respectively. Complexes 1, 2 and 3 were characterized by elemental analysis, IR, NMR (¹H, ¹³C, ³¹P and ¹¹⁹Sn) spectroscopy, TGA and X-ray crystallography diffraction analysis. The X-ray analysis of complex 1 shows that the structure is a polymeric infinite 1D zigzag chain. In complex 2, the oxygen atom of methanol molecule is coordinated to the tin atoms, and a 2D network is generated via O–H?O hydrogen bonds. In complex 3, a novel 2D network containing 12-membered (Sn₃O₆P₃) rings is formed.     

Synthesis, crystal structures and spectroscopy of meclofenamic acid and its metal complexes with manganese(II), copper(II), zinc(II) and cadmium(II). Antiproliferative and superoxide dismutase activity

Some new complexes of meclofenamic acid (N-(2,6-dichloro-m-tolyl)anthranilic acid), Hmeclo (1), with potentially interesting biological activities are described. Complexes [Mn(meclo)₂] (2), [Cu(meclo)₂(H₂O)₂] (3), [Zn(meclo)₂(H₂O)₂] (4) and [Cd(meclo)₂(H₂O)₂] (5) were prepared and structurally characterized by means of vibrational, electronic and ¹H and ¹³C NMR spectroscopies. The crystal structure of complexes [Cu₄(meclo)₆(OH)₂(DMSO)₂]2DMSO (3a) and [Cd(meclo)₂(DMSO)₃] (5a) have been determined by X-ray crystallography. Complex (3a) is a centrosymmetric tetramer built up around the planar cyclic Cu₂(OH)₂ unit. Complex 5a is mononuclear seven-coordinated complex with the meclofenamato ligand behaving as a bidentate deprotonated chelating ligand. Intra and intermolecular hydrogen bonds stabilize these two structures, while the crystal packing is determined by π-π and C−H−-π interactions. Meclofenamic acid and its metal complexes have been evaluated for antiproliferative activity in vitro against the cells of three human cancer cell lines, MCF-7 (breast cancer cell line), T24 (bladder cancer cell line), and A-549 (non-small cell lung carcinoma), and a mouse fibroblast L-929 cell line. Complex 5 exhibits the highest selectivity against MCF-7 and 4 shows the highest selectivity against T-24. Complexes 2-5 were found to be more potent cytotoxic agents against T-24 and complex 5 against MCF-7 cancer cell lines than the prevalent benchmark metallodrug, cis-platin. The superoxide dismutase activity was measured by the Fridovich test which showed that complex [Cu(meclo)₂(H₂O)₂] is a good superoxide scavenger.     

Synthesis, crystal structures, and molecular hyperpolarizabilities of a new Schiff base ligand, and its copper(II), nickel(II), and cobalt(II) metal complexes

A new ligand (HL) obtained from the Schiff base condensation of 4-(diethylamino)salicylaldehyde with 4-nitroaniline is reported, with its nickel(II), copper(II), and cobalt(II) complexes. The crystal structures are reported for the four derivatives. While, Ni^IIL₂ and Cu^IIL₂ are centrosymmetric molecules, Co^IIL₂ exhibits a pseudo-tetrahedral molecular structure. The quadratic hyperpolarizabilities (β) of HL and Co^IIL₂, measured by electric field induced second harmonic (EFISH) technique, are equal to 66 and 110 × 10⁻³⁰ cm⁵ esu⁻¹, respectively. Beside a geometric effect (pseudo-T_d symmetry), the coordination of the metal center provides an intrinsic enhancement of the NLO response. In addition, an enhancement of the thermal stability of about 60° is found upon metal complexation.     

Two salts containing bis(maleonitriledithiolate)nickel(III)/copper(II) anion and substituted benzyltriphenylphosphinium: Syntheses, crystal structures and magnetic properties

Syntheses, crystal structures and magnetic properties of two salts, [FBzTPP][Ni(mnt)₂](1) and [FBzTPP]₂[Cu(mnt)₂](2) ([FBzTPP]⁺ = 1-(4′-fluorobenzyl)triphenylphosphinium, mnt²⁻ = maleonitriledithiolate), are investigated. In 1, the anions and cations stack into well-segregated columns, and the Ni(III) ions form a 1D alternating chain in a [Ni(mnt)₂]⁻ column through intermolecular Ni?S and π?π interactions with the Ni?Ni distances of 3.939, 4.057 and 4.101 Å, and the C-H?N hydrogen bonds are found between the [Ni(mnt)₂]⁻ anion and the neighboring [FBzTPP]⁺ cation. The [Cu(mnt)₂]²⁻ anions in 2 do not form a column and no weak interactions exist between the anions duo to isolation of the [FBzTPP]⁺ cations, while C-H?F and C-H?S hydrogen bonds were found in 2. Magnetic susceptibility measurements in the temperature range 2-300 K show that 1 exhibits a spin-gap transition around 46 K, and antiferromagnetic interaction (θ = −49.0 K) in the high-temperature phase (HT) and spin gap (Δ/k_b = 88.2 K) in the low-temperature phase (LT), while 2 shows a very weak antiferromagnetic coupling behavior with θ = −1.33 × 10⁻² K.     

Syntheses and crystal structures of the metal complexes based on pyrazolecarboxylic acid ligands

This paper aims at introducing five new metal complexes, Co(HL₁)₂(H₂O)₂ (1), {[Cu(HL₁)₂] · (H₂O)}_n (2), {[Zn(HL₂)(H₂O)₃] · 2H₂O}_n (3), {[Co(HL₂)(H₂O)₃] · 2H₂O}_n (4) and [K(H₂L₂)]_n (5), (in which H₂L₁ is 3-methyl-5-pyrazolecarboxylic acid and H₃L₂ is 3,5-pyrazoledicarboxylic acid, respectively), which have been synthesized under hydrothermal conditions or by using diffusion methods. Their molecular structures have been characterized by elemental analyses, IR spectroscopy and single-crystal X-ray diffraction analyses. The X-ray analyses reveal that complex 1 has a mononuclear structure, which is further assembled to a three-dimensional (3-D) framework through intermolecular hydrogen bonds; complex 2 is a 1-D coordinated polymer, which is further assembled to a 2-D layer through intermolecular hydrogen bonds and π–π interactions; both complexes 3 and 4 possess similar 1-D chain structures, and the adjacent chains are further linked by hydrogen bonds to form 2-D supramolecular networks; complex 5 exhibits a 3-D structure, in which, the metal–metal weak interaction, K–K, plays an important role.     

Interactions of a 5-nitroxide-labeled poly(uridylic acid) with poly(adenylic acid)

The physicochemical properties of a high-molecular-weight spin-labeled nucleic acid, (RUGT,U)_n, synthesized by enzymatic copolymerization, were evaluated by uv and ESR spectroscopy. It was shown earlier that spin labeling of nucleic acids by chemical modification to an extent which gives a nitroxide-to-nucleotide ratio greater than 0.002 can cause noticeable lattice perturbations (A. M. Bobst, A. Hakam, P. W. Langemeier, and S. Kouidou (1979), Arch. Biochem. Biophys. 187, 339–345). The presence of RUGT, a 5-nitroxide-labeled uridine residue, in a (U)_n lattice at a $\text{RUGT}\text{U}$ ratio of 0.01 is shown here not to affect the complexation with (A)_n, since the uv melting temperature (T₀^OD) of the 2 → 1 transition and the hypochromicity changes were the same for (RUGT,U)_n· (A)_n and (U)_n·(A)_n. ESR measurements indicated that the nitroxide radical reflects the transition accurately within the error limit, although a slight destabilization of the spinlabeled segment could not be excluded. Computer simulations showed conclusively that the spin melting temperature (T_m^sp) corresponds to the temperature at which half of the spin-labeled segments are no longer complexed, for the ESR spectrum at T_m^spcan be simulated with equal contributions from the line shapes of ESR spectra taken before and after the transition. Arrhenius plots obtained by using two different approaches for computing correlation times were qualitatively the same. Computer analysis also revealed that the formation of a (RUGT,U)_n·(A)_n complex can be described by a two-state model, in contrast to results obtained with chemically spin-labeled (U)_n. Thus, using (RUGT,U)_n over chemically spin-labeled (U)_n can offer distinct advantages.