Viscosometric investigation of the ordinary–extraordinary phase transition in dilute polylysine solutions

Low shear viscosities have been determined for a 1 mg/ml poly(L -lysine) solution as a function of added salt concentration in the region of the previously reported ordinary–extraordinary phase transition. The measured viscosities indicate that the polyions are far from completely extended at the transition. Estimates of the longest internal relaxation time for an equivalent free-draining Rouse-Zimm chain give τ ? 10^?5 sec, similar to that of the rapid, angle-independent component previously observed in the dynamic light-scattering correlation function at the transition. An unusual peak and valley are observed in the curve of [η]₀ versus [NaBr] in the transition region. Possible interpretations of these features, and their bearing on the nature of the extraordinary phase, are discussed.     

Ionic strength effects on macroion diffusion and excess light-scattering intensities of short DNA rods

Quasielastic and static light-scattering measurements were made on DNA isolated from chicken erythrocyte mononucleosomes as a function of ionic strength between 6 × 10^?4 and 1.0M. A transition from single-exponential autocorrelation functions to markedly non-single-exponential decays was observed around 10^?2M ionic strength and was accompanied by a large decrease in the excess light-scattering intensity. Autocorrelation functions recorded below 10^?2M salt were well fit by the sum of two exponential relaxation which differed by as much as 100-fold in time constants. Apparent diffusion coefficients for the fast and slow processes plateaued around 10^?3M with numerical values approximately 10-fold and 1/10, respectively, of the translational diffusion coefficient for mononucleosome DNA at high ionic strength. This behavior is similar to that observed with poly(L -lysine), for which the slow decay has been associated with a transition to an extraordinary phase. The strong and complex salt dependence observed here illustrates potential difficulties in deriving structural information from scattering by polyions at low ionic strength.     

Kinetic properties and the electric field effect of the helix-coil transition of poly(gamma-benzyl L-glutamate) determined from dielectric relaxation measurements

Dielectric relaxation of poly(γ-benzyl L -glutamate) in solution has been studied in the 5 kcps-10 Mcps range for various values of the helix content. The results give first experimental evidence for three effects of major significance. (1) The system exhibits dielectric relaxation due to a chemical rate process (namely helix formation). This confirms recent theoretical predictions. (2) The mean relaxation time τ* of the helix–coil transition could be evaluated as a function of the degree of transition. The results are in excellent agreement with a previously developed theory. At the midpoint of transition it is found τ*_max = 5 × 10^?7 sec. The elementary process of helical growth turns out to be practically diffusion-controlled (with a rate constant of hydrogen bond formation of 1.3 × 10¹⁰ sec^?1). (3) There is a considerable electric field effect of the helix–coil transition. This indicates that conformation changes in biological systems could be potentially caused by direct action of an electric field.     

Effect of salt on sodium polystyrene sulfonate measured by light scattering

The quasielastic light scattering method was used to study the ionic strength dependence of the mutual diffusion coefficient of sodium polystyrene sulfonate (NaPSS) as a function of NaCl and CaCl₂ concentrations. The results indicate a splitting in the relaxation times that depends on the ratio C_p/C_s, where C_p and C_s are the polyion and added salt concentrations. A universal relationship taking into account Manning's theory of condensation and the Debye screening due to the added salt is proposed to characterize the fast–slow relaxation time transition.     

On the possibility of true phase transition in heterogeneous DNA during thermal denaturation under conditions close to equal stability of A+T and G+C pairs

The DNA helix–coil transition has been studied in the presence of high concentrations of manganese ions (about 10^?3M), which corresponds to the conditions close to equal stability of the A+T and G+C pairs, at the ionic strengths of 10^?1, 10^?2, and 1.6 × 10^?3M Na⁺. With the Mn²⁺ ion effect, the transition range is significantly reduced to not more than 0.2°C at 1.2 × 10^?3M Mn²⁺ and 1.6 × 10^?3M Na⁺. The melting curves display a sharp kink at the end of the helix–coil transition, which is interpreted as an indication of the second-order phase transition. It is shown that the melting curves obtained can be approximated by a simple analytical expression 1 – θ = exp[–a(t_c - t)], where θ is the DNA helix fraction, t_c is the phase transition temperature, and a is an empirical parameter characterizing the breadth of the melting range and responsible for the magnitude of a jump of the helicity derivative with respect to the temperature at the phase transition point.     

Monomolecular condensation of lambda-DNA induced by cobalt hexamine

Measurements of static and dynamic light scattering have been used to distinguish between monomolecular DNA condensation and aggregation of condensed molecules. In low salt, using Co³⁺(NH₃)₆ as the condensing agent, and at λ-DNA concentrations below 0.2 μg/mL, the transition curves for monomolecular condensation and aggregation are well separated for times of 16 h. In these conditions, the intensity of scattered light (90°) and also the diffusion coefficient of the condensed DNA show reasonable values for monomolecular condensation that are independent of DNA concentration and also of Na⁺ Co³⁺(NH₃)₆ concentrations for which monomolecular condensation is complete. At higher Co³⁺(NH₃)₆ concentrations, which produce aggregation (as judged by the intensity of scattered light), the diffusion coefficient decreases sharply. The transition curve for monomolecular condensation is independent of DNA concentration but shows a hysteresis loop. The kinetics of condensation are slow in the forward direction and fast in the reverse direction, indicating that the actual transition curve is measured closely by reversal experiments. Aggregation is blocked kinetically in both the forward and reverse directions when Co³⁺(NH₃)₆ is the condensing agent at low Na⁺ concentrations. When spermine or spermidine is the condensing agent and observations are made at 16 h, it is not possible to separate the transition curves for monomolecular condensation and for aggregation in conditions that are successful with Co³⁺(NH₃)₆. Some interesting properties of monomolecular condensation are noted. (1) The transition is not a two-state reaction, as judged by measurements of the diffusion coefficient through the transition zone. (2) The transition for monomolecular condensation is diffuse. (3) The dimensions of the monomolecular condensates have been calculated from the translational diffusion coefficient for an assumed toroidal shape by the formula derived by Allison and coworkers [(1981) Biopolymers 20 , 469–488]. These dimensions are in reasonable agreement with ones deduced from electron microscopy by Chattoraj and coworkers [(1978) J. Mol. Biol. 121 , 327–337]. (4) The phase diagram relating the Na⁺ to the Co³⁺(NH₃)₆ concentrations needed for condensation has a slope of 0.6 in a log–log plot. According to numerical solutions of Manning's theory for the atmospheric binding of competing cations to DNA, this means that condensation occurs at a late stage in the replacement of Na⁺ by Co³⁺(NH₃)₆ around the DNA. The fraction of DNA phosphate charge neutralized at condensation is computed to be in the neighborhood of 0.9, as found by Wilson and Bloomfield [(1979) Biochemistry 18 , 2192–2196], but to vary with the Na⁺ concentration.     

Dielectric relaxation of DNA in aqueous solutions.

Using a four-electrode cell and a new electronic system for direct detection of the frequency differences specturm of solution impedance, the complex dielectric constant of calf thymus DNA (M_r = 4 × 10⁶) in aqueous NaCl at 10°C is measured at frequencies ranging from 0.2 Hz to 30 kHz. The DNA concentrations are C_p = 0.01% and 0.05%, and the NaCl concentrations are varied from C_s = 10^?4 M to 10^?3 M. A single relaxation regions is found in this frequency range, the relaxation frequency being 10 Hz at C_p = 0.01% and C_s = 10^?3 M. At C_p = 0.05% it is evidenced that the DNA chains have appreciable intermolecular interactions. The dielectric relaxaton time τ_d at C_p = 0.01% agrees well with the rotational relaxation time estimated from the reduced visocisty on the assumption that the DNA is not representable as a rigid rod but a coiled chain. It is concluded that the dielectric relaxiatioinis ascribed to the rotation of the molecule. Observed values of dielectric increment and other experimental findings are reasonably explained by assuming that the dipole moment of DNA results from the slow counterion fluctuation which has a longer relaxation time than τ_d.     

Effects of divalent cations and pH on phosphatidylserine model membranes: A 31P NMR study

The influence of divalent cations, and pH on the behaviour of phosphatidylserine, derived from egg phosphatidylcholine, has been examined employing ³¹P-NMR techniques. The addition of Ca²⁺ results in the observation of a “rigid lattice” ³¹P-NMR spectra and more than an order of magnitude increase in the spin-lattice relaxation time T₁. This corresponds to a strong and specific headgroup immobilization by Ca²⁺, similar to that observed for anhydrous phosphatidylserine. At pH 7.4 the hydrated sodium salt of (egg) phosphatidylserine adopts the bilayer phase, whereas when the pH is decreased through 3.5 a bilayer to hexagonal (H_II) polymorphic phase transition is observed at 50°C, which is unaffected by equimolar cholesterol. The same transition is shown to occur at 37°C for phosphatidylserine isolated from human erythrocytes.     

Thermodynamics of conformational transition and chain association of ι-carrageenan in aqueous solution: Calorimetric and chirooptical data

Isothermal microcalorimetry, differential scanning calorimetry (DSC), and chirooptical data obtained for ι-carrageenan in NaCl, LiCl, and NaI aqueous solutions are presented. The experiments have been performed as a function of concentration both for the polymer and for the simple salt as a cosolute. The experimental findings consistently show the occurrence of a salt-induced disorder-to-order transition. From microcalorimetric experiments the exothermic enthalpy of transition ΔH_tr is obtained as the difference between the theoretical, purely electrostatic ΔH_el enthalpy change and the actual mixing enthalpy ΔH_mix, measured when a ι-carrageenan salt-free solution at constant polymer concentration is mixed with a 1:1 electrolyte solution of variable concentration. In the case of added NaCl, the absolute values of enthalpy changes |ΔH_tr| are in good agreement with those obtained for the opposite process, at comparable polymer and salt concentrations, from DSC melting curves. The microcalorimetric results show that the negative maximum value of ΔH_tr corresponding to the interaction of Li⁺ counterion with ι-carrageenan polyion results to be significantly lower than the corresponding values obtained for Na⁺ counterion. At variance with the microcalorimetric data, chirooptical results show that the salt-induced disorder-to-order transition, occurring in the 0.02–0.2M salt concentration range, appears to be complete at a concentration of about 0.08–0.1M of the simple ion, irrespective of the polymer concentration and of the nature of added electrolyte. © 1998 John Wiley & Sons, Inc. Biopoly 45: 105–117, 1998     

Proton and phosphorus NMR studies of d-CpG(pCpG)n duplexes in solution. Helix-coil transition and complex formation with actinomycin-D.

The Watson–Crick imino and amino exchangeable protons, the nonexchangeable base and sugar protons, and the backbone phosphates for d-CpG(pCpG)_n, n = 1 and 2, have been monitored by high-resolution nmr spectroscopy in aqueous solution over the temperature range 0°–90°C. The temperature dependence of the chemical shifts of the tetramer and hexamer resonances is consistent with the formation of stable duplexes at low temperature in solution. Comparison of the spectral characteristics of the tetranucleotide with those of the hexanucleotide with temperature permits the differentiation and assignment of the cytosine proton resonances on base pairs located at the end of the helix from those in an interior position. There is fraying at the terminal base pairs in the tetranucleotide and hexanucleotide duplexes. The Watson–Crick ring imino protons exchange at a faster rate than the Watson–Crick side-chain amino protons, with exchange occurring by transient opening of the double helix. The structure of the d-CpG(pCpG)_n double helices has been probed by proton relaxation time measurements, sugar proton coupling constants, and the proton chemical shift changes associated with the helix–coil transition. The experimental data support a structural model in solution, which incorporates an anti conformation about the glycosyl bonds, C(3) exo sugar ring pucker, and base overlap geometries similar to the B-DNA helix. Rotational correlation times of 1.7 and 0.9 × 10^?9 sec have been computed for the hexanucleotide and tetranucleotide duplexes in 0.1 M salt, D₂O, pH 6.25 at 27°C. The well-resolved ³¹P resonances for the internucleotide phosphates of the tetramer and hexamer sequences at superconducting fields shift upfield by 0.2–0.5 ppm on helix formation. These shifts reflect a conformational change about the ω,ω′ phosphodiester bonds from gauche-gauche in the duplex structure to a distribution of gauche-trans states in the coil structure. Significant differences are observed in the transition width and midpoint of the chemical shift versus temperature profiles plotted in differentiated form for the various base and sugar proton and internucleotide phosphorous resonances monitoring the d-CpG(pCpG)_n helix–coil transition. The twofold symmetry of the d-CpGpCpG duplex is removed on complex formation with the antibiotic actinomycin-D. Two phosphorous resonances are shifted downfield by ～2.6 ppm and ～1.6 ppm on formation of the 1:2 Act-D:d-CpGpCpG complex in solution. Model studies on binding of the antibiotic to dinucleotides of varying sequence indicate that intercalation of the actinomycin-D occurs at the GpC site in the d-CpGpCpG duplex and that the magnitude of the downfield shifts reflects strain at the O-P-O backbone angles and hydrogen bonding between the phenoxazone and the phosphate oxygens. Actinomycin-D is known to bind to nucleic acids that exhibit a B-DNA conformation; this suggests that the d-CpG(pCpG)_n duplexes exhibit a B-DNA conformation in solution.     

Dynamic light scattering studies of internal motions in DNA. II. Clean viral DNAs

Internal Brownian motions of clean ?29 and λ-DNAs have been studied using photon-correlation techniques at both visible (λ₀ = 632.8 nm) and uv (λ₀ = 363.8 nm) wavelengths. The present dynamic light scattering data, which extend to K² = 19 × 10¹⁰ cm^?2, can in every case be satisfactorily simulated by a Rouse-Zimm model polymer with an appropriate choice of the three model parameters. The effects of pH, salt concentration, single-strand breaks, and molecular weight on those model parameters have also been investigated. Intact clean DNAs exhibit surprisingly little variation with pH from 7.85 to 10.25, with salt concentration from 0.01 NaCl to 5.4M NH₄Cl, or with molecular weight or GC content. The single-strand breaks have no effect at pH 9.46, but produce dramatic changes in the model parameters at pH 10.0 and 10.25, indicating the introduction of titratable joints at those pHs. The failure of either single-strand breaks or a large change in GC content to alter the model parameters in the neutral pH range is a strong indication that local denaturation is not required for those flexions and torsions that dominate the relaxation of fluctuations in the scattered light. The Langevin relaxation time for the slowest internal mode of a particular Rouse-Zimm model derived from the dynamic light scattering data is compared with pertinent literature data extrapolated to the same molecular weight. The present algorithm for determining model parameters from the light-scattering D_app vs K² curve actually yields a Langevin time in fairly good agreement with the literature value. For unknown reasons the light-scattering D₀ values generally exceed those obtained from the molecular weight and sedimentation coefficient by about 20%.     

13C Nuclear magnetic resonance study of salt induced conformational change of basic polypeptides: Poly (L-lysine), poly (L-arginine), and poly (L-ornithine)

A ¹³C-nmr study of the salt-induced helix–coil transition of the basic polypeptides poly(L -lysine) [(Lys)_n], poly(L -arginine) [(Arg)_n], and poly (L -ornithine) [(Orn)_n] was performed to serve as a reference of the helical portion of histones and other proteins. As is the case with pH-induced helix–coil transition, the downfield displacement of the C^α and carbonyl carbon signals are observed in the helical state. The upfield shift of the C^β signals, on the other hand, is noted in the salt-induced transition. Regardless of the differences in the side chains and also the salts used, very similar helix-induced chemical shifts are obtained for (Lys)_n and (Arg)_n. However, the displacement of the C^α, C^β, and carbonyl carbons of (Orn)n in the presence of 4M NaClO₄ is found to be almost 50% of that of (Lys)_n and (Arg)_n. This is explained by the fact that the maximum helical content is about 50%, consistent with the ORD result. Further, the motion of the backbone and side chains of the helical from was estimated by measuring the spin-lattice relaxation time (T₁), nuclear Overhauser enhancement (NOE), and line width. In the case of (Lys)_n, the motion of the side chains is charged very little in comparison with that of the random coil. Indicating that the aggregation of the salt-induced helix is small in contrast to that of the pH-induced helix. For (Arg)_n, however, the precipitate of the helical polymers is mainly due to aggregation.     

Comparison of oligoribonucleotides and oligodesoxyribonucleotides. I. Formation of double helices

The ability of oligodesoxyribonucleotides of various chain lengths to form complexes has been compared with that of oligoribonucleotides. Four series of oligonucleotidcs were prepared and investigated, i.e., dC_n at acid pH versus rC_n, dA_n and dT_n versus. rA_n and rU_n at neutral pH. The results indicate that in dilute solution, the formation of complexes is greatly facilitated in the case of desoxyoligomers and occurs for shorter oligomere than in the corresponding ribooligomers. The spectrophotometric titration of deoxyribooligo C indicates the appearance of two pK values in the 4–5 pH region characteristic of the double-stranded form, which occurs for much shorter dC_n than rC_n. The circular dichroism (CD.) spectra of deoxycytidylies in dilute solution starting from the trimer are conservative, characteristic of the double-stranded helical form of poly C at acid pH. In contrast, the CD spectra of a series of corresponding ribo C_n, under identical conditions is of nonconservative character similar to that of the single-stranded form of poly C at neutral pH, but differs in the band position. This spectrum is called intermediate. Only at higher concentrations of oligonucleotidcs (i.e., 10^?3Minstead of 10^?4M) does the circular dichroism spectrum of longer ribocytidylics assume conservative character. Thermal denaturation of deoxycytidylces at acid pH are strongly dependent on chain length and concentration, its one would expect for a cooperative helix-coil transition. The circular dichroism spectra measured at different temperatures shows one isosbestic point. In dilute solution, the standard-state enthalpy change found was 5–6 kcal/mole for higher oligomers (dC₇). These properties are all in agreement with a structural transition from the d-C_n double-stranded form to a coil for n > 3. Studies of dA_n and dT_n in solutions of high ionic strength at low temperature indicate that complex formation occurs already at the level of trimer and for high oligomers. Under identical conditions a complex between rA_n and rU_n is detected only for oligomers longer than the hexamer. The nature of the “intermediate” form of oligoribo C at acid pH and low temperature was investigated by sedimentation and circular dichroism. A model of rC_n is proposed of linear molecules which are partially double-stranded and partially single-stranded, which probably are slowly rearranged by “slippage” into a regular-double-stranded helical form.     

Brownian Dynamics Simulations of Colloidal Liquids: Hydrodynamics and Stress Relaxation

Abstract

We describe the statistical mechanics background and additional algorithmic features of a recently proposed simple mean-field Brownian Dynamics algorithm formulated to include many-body hydrodynamics, using a local density approximation for the friction coefficient. We show that the equations of motion satisfy the incompressibility of phase space. We make further developments to the model, computing the hydrodynamic effects on the shear stress relaxation function. We show that stress relaxation takes place over two well-defined regimes, in both cases with and without mean field hydrodynamics, MFH. At short times ta ²/D ₀ < 10^?3, where a is the radius of the colloidal particle and D ₀ is the self-diffusion coefficient at infinite dilution, decay of the stress autocorrelation function, C_s(t) is essentially independent of volume fraction and does not fit to a simple analytic form. At longer times than ta ²/D ₀ < 10^?2 the decay has the fractional exponential form ～exp(-t ^β) with β ? 1. The transition between these two regimes coincides with a rapid fall in the time-dependent diffusion coefficient from the so-called short-time to long-time values. We do not find any evidence for power law decay in the C_s(t) as predicted by recent mode-coupling based analytical expansions.     

COPPER TOXICITY TO SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1,2

Copper toxicity to Skeletonema costatum (Grev.) Cleve has been studied in batch cultures of chemically defined culture media. The alga is relatively insensitive to cupric ion activity, demonstrating no effect on growth up to (Cu²⁺) = 10^?8.5 M. Cultures inoculated from stationary phase stocks exhibit a prolongation of the lag phase with increasing copper concentrations near and above the point of precipitation of the copper. The toxicity of copper is a function of the silicic acid concentration in the medium. This effect is observed in a range of Si(OH)₄ concentrations (10^?5 M to 10^?4 M) above known values for the saturation of silicon uptake kinetics, thus suggesting an influence of copper on silicate metabolism.     

Na−K−2Cl cotransport in winter flounder intestine and bovine kidney outer medulla: [3H] bumetanide binding and effects of furosemide analogues

Summary The effects of several sulfamoyl benzoic acid derivatives on Na–K–Cl cotransport were investigated in winter flounder intestine. The relative efficacy (IC₅₀ values) and order of potency of these derivatives were benzmetanide, 5×10^–8 m> bumetanide 3×10^–7 m>piretanide 3×10^–6 m>furosemide 7×10^–6 m> amino piretanide 1×10^–5 3-amino-4-penoxy-5-sulfamoyl benzoic acid. Binding of [³H] bumetanide was studied in microsomal membranes from winter flounder intestine and compared to that in bovine kidney outer medulla. Binding was also studied in brush-border membranes from winter flounder intestine. The estimated values forK _d and number of binding sites (n) were: bovine kidney,K _d =1.6×10^–7,n=10.5 pmol/mg protein; winter flounder intestine,K _d 1.2×10^–7,n=7.3 pmol/mg protein, and brush-border membranes from winter flounder,K _d =5.3×10^–7,n=20.4 pmol/mg protein. The estimatedK _d for bumetamide binding to winter flounder brush-border membranes derived from association and dissociation kinetics was 6.8×10^–7 m. The similarity in magnitudes of IC₅₀ andK _d for bumetanide suggests that the brush-border cotransporter is ordinarily rate-limiting for transmural salt absorption and that bumetanide specifically binds to the cotransporter. Measurement of bumetanide binding at various concentrations of Na, K and Cl showed that optimal binding required all three ions to be present at about 5mm concentrations. Higher Na and K concentrations did not diminish binding but higher Cl concentrations (up to 100mm Cl) inhibited bumetanide binding by as much as 50%. Still higher Cl concentrations (500 and 900mm) did not further inhibit bumetanide binding. Scatchard analysis of bumetanide binding at 5 and 100mm Cl concentrations showed that bothK _d andn were lower at the higher Cl concentration (5mm Cl:K _d =5.29×10^–7 m,n=20.4 pmol/mg protein; 100mm Cl:K _d =2.3×10^–7 m,n=8.8 pmol/mg protein). These data suggest two possibilities: that bumetanide and Cl binding are not mutually exclusive (in contrast to pure competitive inhibition) and that they each bind to separate sites or that two distinct bumetanide binding sites exist, only one of which exhibits Cl inhibition of binding. This inhibition would then be consistent with a competitive interaction with Cl.     

A Brillouin scattering study of the hydration of Li- and Na-DNA films

We have used Brillouin spectroscopy to study the velocities and attenuation of acoustic phonons in wet-spun films of Na-DNA and Li-DNA as a function of the degree of hydration at room temperature. Our data for the longitudinal acoustic (LA) phonon velocity vs water content display several interesting features and reveal effects that we can model at the atomic level as interhelical bond softening and relaxation of the hydration shell. The model for interhelical softening makes use of other physical parameters of these films, which we have determined by gravimetric, x-ray, and optical microscopy studies. We extract intrinsic elastic constants for hydrated Na-DNA molecules of c₁₁ ? 8.0 × 10¹⁰ dynes/cm² and c₃₃ ? 5.7 × 10¹⁰ dynes/cm², which corresponds to a Young's modulus, E ? 1.1 × 10¹⁰ dynes/cm² (with Poisson's ratio, σ = 0.44). The negative velocity anisotropy of the LA phonons indicates that neighboring DNA molecules are held together by strong interhelical bonds in the solid state. The LA phonon attenuation data can be understood by the relaxational model in which the acoustic phonon is coupled to a relaxation mode of the water molecules. Na-DNA undergoes the A to B phase transition at a relative humidity (rh) of 92% while Li-DNA (which remains in the B form in this range) decrystallizes at an rh of 84%. We find that our Brillouin results for Na- and Li-DNA are remarkably similar, indicating that the A to B phase transition does not play an important role in determining the acoustic properties of these two types of DNA.     

Solvent effects on the dynamics of (dG-dC)3

The kinetics of the coil-to-helix transition of (dG-dC)₃ in M NaCl, 45 mM sodium cacodylate, pH 7, were measured in H₂O, D₂O, 10 mol % ethanol, 10 mol % urea, and 10 mol % glycerol. At 43°C in H₂O the recombination rate is 1.3 ± 0.2 × 10⁷ M^?1 s^?1; the dissociation rate is 68 ± 10 s^?1. The destabilization of the helix in 10 mol % ethanol and 10 mol % urea relative to water is primarily due to a large increase in the helix-dissociation rate. In 10 mol % glycerol, the destabilization of the helix is due to a decrease in the recombination rate and an increase in the dissociation rate. Above 20°C, two exponential decays longer than 1 μs are observed after a temperature jump. The slower relaxation time is 4–10 times faster than the bimolecular component and is independent of oligomer concentration. We attribute this relaxation to a rapid equilibrium between two helical states. At low temperatures and oligomer concentrations of 1 mM or greater, the helices aggregate in 1M NaCl. Experimental data are presented under conditions where aggregation is unimportant and evidence is given that the ΔH-determined spectroscopically is unaffected by aggregation.     

Condensed states of nucleic acids. II. Effects of molecular size, base composition, and presence of intercalating agents on the psi transition of DNA.

Circular dichroism spectroscopy has been used to investigate the influence of DNA molecular size, base composition, and the presence of intercalating agents upon the Ψ transition of DNA brought about by high concentrations of poly(ethylene oxide) and salt (Lerman (1971) Proc. Natl. Acad. Sci. (U.S.) 68 , 1886–1890). A molecular weight of 0.15–3.0 × 10⁶ daltons yields maximum formation of Ψ-DNA. Both the amplitude of the large negative CD band at 265 nm—a chief characteristic of the Ψ state—and the thermal stability of Ψ-DNA increase linearly with increasing mole fraction of guanine plus cytosine in the DNA sample. Either ethidium or proflavine, at concentrations where approximately one dye is bound per 5–10 nucleotide residues, can prevent the transition completely. Striking similarities between the Ψ-DNA produced by poly(ethylene oxide) + salt and the complexes formed between DNA and lysine-rich histone f1 suggest the presence of similar nucleic acid–nucleic acid interactions in both types of condensed phase.     

New developments in first-principles excited-state dynamics simulations: unveiling the solvent specificity of excited anionic cluster relaxation and electron solvation

Charge-transfer-to-solvent excited iodide–polar solvent molecule clusters, [I^? (Solv)_n]^*, have attracted substantial interest over the past 20 years as they can undergo intriguing relaxation processes leading ultimately to the formation of gas-phase molecular analogues of the solvated electron. In this review article, we present a comprehensive overview of the development and application of state-of-the-art first-principles molecular dynamics simulation approaches to understand and interpret the results of femtosecond photoelectron spectroscopy experiments on [I^? (Solv)_n]^* relaxation, which point to a high degree of solvent specificity in the electron solvation dynamics. The intricate molecular details of the [I^? (Solv)_n]^* relaxation process are presented, and by contrasting the relaxation mechanisms of clusters with several different solvents (water, methanol and acetonitrile), the molecular basis of the solvent specificity of electron solvation in [I^? (Solv)_n]^* is uncovered, leading to a more refined view of the manifestation of electron solvation in small gas-phase clusters.