Effects of temperature on the properties of primary auditory fibres of the spectacled caiman,Caiman crocodilus (L.)

Summary The effect of temperature on the response properties of primary auditory fibres in caiman was studied. The head temperature was varied over the range of 10–35 ° C while the body was kept at a standard temperature of 27 °C (T_s). The temperature effects observed on auditory afferents were fully reversible. Below 11 °C the neural firing ceased.The mean spontaneous firing rate increased nearly linearly with temperature. The slopes in different fibres ranged from 0.2–3.5 imp s^–1 °C^–1. A bimodal distribution of mean spontaneous firing rate was found (<20 imp s^–1 and >20 imp s^–1 at T_s) at all temperatures.The frequency-intensity response area of the primary fibres shifted uniformly with temperature. The characteristic frequency (CF) increased nearly linearly with temperature. The slopes in different fibres ranged from 3–90 Hz °C^–1. Expressed in octaves the CF-change varied in each fibre from about O.14oct °C^–1 at 15 °C to about 0.06 oct °C^–1 at 30 °C, irrespective of the fibre's CF at T_s. Thresholds were lowest near T_s. Below T_s the thresholds decreased on average by 2dB°C^–1, above T_s the thresholds rose rapidly with temperature. The sharpness of tuning (Q_10db) showed no major change in the temperature range tested.Comparison of these findings with those from other lower vertebrates and from mammals shows that only mammalian auditory afferents do not shift their CF with temperature, suggesting that a fundamental difference in mammalian and submammalian tuning mechanisms exists. This does not necessarily imply that there is a single unifying tuning mechanism for all mammals and another one for non-mammals.Abbreviations BF best frequency: frequency of maximal response at an intensity 10 dB above the CF-threshold - CF characteristic frequency - FTC frequency threshold curve, tuning curve - T _s standard temperature of 27 °C     

Specific heats of lipid dispersions in single phase regions

Differential scanning calorimetry has been used for the first time to measure the specific heat, C_p, as a function of temperature in the single phase regions above and below the main phase transition temperature, T_m, for dispersions of saturated phosphatidylcholines and phosphatidylethanolamines. Within error limits C_p, when expressed per gram, does not vary in any systematic way with chain length or headgroup. Its temperature dependence in both single phase regions qualitatively resembles that of n-alkanes. Contributions to C_p from intrachain vibrations and interchain van der Waals' interactions have been calculated and account for nearly all the measured C_p at temperatures above T_m. However, these contributions do not yield the observed temperature dependence below T_m. It is conjectured that such a temperature dependence arises from the unhindering of chain vibrations as the lipids undergo thermal expansion, and the result of a preliminary calculation which supports this conjecture is presented.     

Thermotropic gelation of ovalbumin: 2. Boundary conditions on the formation and the concentration dependence of equilibrium elastic modulus for thermotropic ovalbumin gels

Studies were made on the boundary conditions for thermotropic ovalbumin gelation at pH within the range 2.5 to 10.0. The pH dependence of the gelation threshold, C₀, and denaturation temperature, T_d, were obtained. The dependence C₀(pH) has a sharp minimum close to the isoelectric point (pl). Over pH range 2.5 to 4.0 the dependence T_d(pH) is linear; although above pI it shows unusual behaviour. T_d increases smoothly, becoming a constant value (T_d=80°C) at pH 7. Analysis of the temperature dependence of Leu's line integral intensity in the p.m.r. spectrum of ovalbumin shows that the temperature threshold of thermotropic gelation closely approximates to T_d. A diagram for the state of an ovalbumin -water system was constructed in temperature-concentration-pH coordinates. The dependences of the initial shear modulus for thermotropic ovalbumin gels on the concentration (0.06≤C≤0.25g/cm³ were obtained at pH 4.0, 7.0, 8.5, 10.0. They are equivalent to the concentration dependence of the equilibrium elastic modulus E_e(C). The dependences obtained may be reduced to the theoretical master dependence of Hermans, $\text{E}{}_{\mathrm{e}}\text{(rm}\text{C}\text{?}\text{)}$, where $\text{C}\text{?}\text{=}\text{C}\text{/}\text{C}{}_0$ is the reduced concentration. Hermans' theory, based o the model for random cross-linking of linear identical macromolecules without cyclization, adequately describes the equilibrium elastic properties of thermotropic ovalbumin gels.     

A dilatometric investigation of the effects of general anaesthetics,alcohols and hydrostatic pressure on the phase transition in smectic mesophases of dipalmitoyl phosphatidylcholine

The effect of general anaesthetics, alcohols and hydrostatic pressure on the thermal transition in dipalmitoyl phosphatidylcholine multilayer liposomes has been measured using dilatometry. The volume increasse at the transition (ΔV_t) is 0.0350 ± 0.0003 ml/g. the transition temperature (T_t) 41.84 ± 0.09°C and the width of the transition 1.025 ± 0.18°C. ΔH calculated by the Clapeyron-Clausius equation is 8.4 kcal/mol. The n-alcohols C₃C₅ reduced the transition temperature without affecting the transition width which was however, increased by n-hexanol. Trichloroethylene, the fluorescent probe N-phenyl-1-naphthyl-amine, and methoxyflurane all increased the transition width (reduced the cooperativity of the transition) with a simultaneous depression of T_t. Methoxyflurane caused a two-stage transition expansion. Diethyl ether's effect has similarities with both the C₃ and C₆ alcohols. Generally ΔV_t was unaffected by the agents.Pressure increased T_t by 0.0238°C/atm linearly over the range 1–300 atm in both treated and untreated liposomes, and therefore cannot be said to antagonize anaesthetics. In both treated and untreated liposomes ΔV_t and the width of the transition were unaffected by pressure. Pressure thus reverses the effects of anaesthetics on T_t but not their spread of the transition width.     

Lateral organization of mixed, two-phosphatidylcholine liposomes as investigated by GPS, the slope of Laurdan generalized polarization spectra

The effect of the excitation or emission wavelengths on Laurdan generalized polarization (GP) can be evaluated by GPS, a quantitative, simplified determination of the GP spectrum slope, the thermotropic dependence of which allows the assessment of phospholipid lamellar membrane phase, as shown in a recent publication of our laboratory [J.B. Velázquez, M.S. Fernández, Arch. Biochem. Biophys. 455 (2006) 163-174]. In the present work, we applied Laurdan GPS to phase transition studies of mixed, two-phosphatidylcholine liposomes prepared from variable proportions of dimyristoyl- and dipalmitoylphosphatidylcholine (DMPC and DPPC, respectively). We have found that the GPS function reports a clear limit between the gel/liquid-crystalline phase coexistence region and the liquid-crystalline state, not only at a certain temperature T_c for liposomes of constant composition submitted to temperature scans, but also at a defined mole fraction X_c, for two-component liposomes of variable composition at constant temperature. The T_c or the X_c values obtained from GPS vs. temperature or GPS vs. composition plots, respectively, allow the construction of a partial phase diagram for the DMPC-DPPC mixtures, showing the boundary between the two-phase coexisting region and the liquid-crystalline state. Likewise, at the onset of the transition region, i.e., the two-phase coexisting region as detected by GPS, it is possible to determine, although with less precision, a temperature T_o or a mole fraction X_o defining a boundary located below but near the limit between the gel and ripple phase, reported in the literature. These GPS results are consistent with the proposal by several authors that a fraction of L_α phospholipids coexists with gel phospholipids in the rippled phase.     

Predict the glass transition temperature of glycerol-water binary cryoprotectant by molecular dynamic simulation

Vitrification is proposed to be the best way for the cryopreservation of organs. The glass transition temperature (T_g) of vitrification solutions is a critical parameter of fundamental importance for cryopreservation by vitrification. The instruments that can detect the thermodynamic, mechanical and dielectric changes of a substance may be used to determine the glass transition temperature. T_g is usually measured by using differential scanning calorimetry (DSC). In this study, the T_g of the glycerol-aqueous solution (60%, wt/%) was determined by isothermal-isobaric molecular dynamic simulation (NPT-MD). The software package Discover in Material Studio with the Polymer Consortium Force Field (PCFF) was used for the simulation. The state parameters of heat capacity at constant pressure (C_p), density (ρ), amorphous cell volume (V_cell) and specific volume (V_specific) and radial distribution function (rdf) were obtained by NPT-MD in the temperature range of 90–270 K. These parameters showed a discontinuity at a specific temperature in the plot of state parameter versus temperature. The temperature at the discontinuity is taken as the simulated T_g value for glycerol–water binary solution. The T_g values determined by simulation method were compared with the values in the literatures. The simulation values of T_g (160.06–167.51 K) agree well with the DSC results (163.60–167.10 K) and the DMA results (159.00 K). We drew the conclusion that molecular dynamic simulation (MDS) is a potential method for investigating the glass transition temperature (T_g) of glycerol–water binary cryoprotectants and may be used for other vitrification solutions.     

Superior Energy‐Storage Capacitors with Simultaneously Giant Energy Density and Efficiency Using Nanodomain Engineered BiFeO3‐BaTiO3‐NaNbO3 Lead‐Free Bulk Ferroelectrics

Dielectric capacitors are receiving a great deal of attention for advanced pulsed power owing to their high power density and quick charge/discharge rate. However, the energy density is limited and the efficiency and the thermal stability are also not ideal, which has been a longstanding obstacle to developing desirable dielectric materials. These concerns have are addressed herein by fabricating nanodomain‐engineered BiFeO₃‐BaTiO₃‐NaNbO₃ bulk ferroelectrics, integrating a high‐spontaneous‐polarization gene, wide band gaps, and a heterogeneous nanodomain structure, generating record‐excellent comprehensive performance of giant energy‐storage density W_rec ≈8.12 J cm^?3, high efficiency η ≈90% and excellent thermal stability (±10%, ?50 to 250 °C) and ultrafast discharge rate (t_0.9 < 100 ns). Significantly enhanced dielectric breakdown strength of BiFeO₃‐based solid solutions is mainly attributed to the substitution of NaNbO₃, which provides an increased band gap, refined grain size, and increased resistivity. The formation of nanoscale domains as evidenced by piezoresponse force microscopy and transmission electron microscopy enables nearly hysteresis‐free polarization‐field response and temperature‐insensitive dielectric response. In comparison with antiferroelectric capacitors, the current work provides a new solution to successfully design next‐generation pulsed power capacitors by fully utilizing relaxor ferroelectrics in energy‐storage efficiency and thermal stability.     

The effect of dielectric polarization of the electrode on anomalous temperature effects in the electrical double layer

Monte Carlo simulations and a modified Poisson–Boltzmann (MPB) theory are used to investigate the temperature dependence of the capacitance (around the potential of zero charge) of an electric double layer in the presence of surface polarization due to a dielectric boundary. Within the context of the restricted primitive model planar double layer, whose solvent dielectric constant is ε₂, the cases when the electrode is an insulator (ε₁ = 1), when the electrode and the electrolyte have the same permittivity (ε₁ = ε₂, no polarization), and when the electrode is a conductor (ε₁ → ∞) are studied for the case where the electrolyte concentration is 0.1 M. The simulations reveal a capacitance anomaly, that is, a positive temperature dependence of the capacitance at low temperatures for the former two situations. The MPB theory also shows this effect for these two situations and is in qualitative or better agreement with the simulation data. In these two cases, both the simulations and theory show a dramatic increase of the diffuse layer potential in the temperature regime where capacitance anomaly occurs. However, in the latter situation, where the electrode is metallic, the capacitance always has a negative temperature derivative for the MPB theory and probably also for the simulation data.     

Effect of salt on the stability of the pH 4.2 rA8 double helix in a series of organic/aqueous mixed solvents: A test of oligoelectrolyte theory

Temperature-dependent uv absorption spectroscopy has been used to investigate the salt dependence of the order–disorder transition for the pH 4.2 rA₈ double helix in 100% aqueous buffer and in a series of organic/aqueous mixed solvents. Melting temperature, T_m, data were obtained for the transitions in the different solvents by analysis of the uv melting curves. For the pure aqueous buffer solvent, the melting temperature was found to exhibit a reduced salt dependence (?t_m/? log Na⁺) when compared to the corresponding polymer. This reduction is explained in terms of end effects and is shown to be consistent with the theoretical treatments of oligoelectrolyte transitions developed by Record and Lohman [Biopolymers, 17 , 159–166 (1978)]. In the mixed solvents, the salt dependence of the melting temperature (?t_m/? log Na⁺) is shown to exhibit a linear dependence on the bulk dielectric constant of the medium for all of the hydroxyl-containing solvents studied. Significantly, N,N-dimethylformamide demonstrated different behavior.     

Structural transition temperature of hemoglobins correlates with species’ body temperature

Human red blood cells (RBCs) exhibit sudden changes in their biophysical properties at body temperature (T _B). RBCs were seen to undergo a spontaneous transition from blockage to passage at T _C = 36.4 ± 0.3°C, when the temperature dependency of RBC-passages through 1.3 μm narrow micropipettes was observed. Moreover, concentrated hemoglobin solutions (45 g/dl) showed a viscosity breakdown between 36 and 37°C. With human hemoglobin, a structural transition was observed at T _B as circular dichroism (CD) experiments revealed. This leads to the assumption that a species’ body temperature occupies a unique position on the temperature scale and may even be imprinted in the structure of certain proteins. In this study, it was investigated whether hemoglobins of species with a T _B different from those of human show temperature transitions and whether those were also linked to the species’ T _B. The main conclusion was drawn from dynamic light scattering (DLS) and CD experiments. It was observed that such structural temperature transitions did occur in hemoglobins from all studied species and were correlated linearly (slope 0.81, r = 0.95) with the species’ body temperature. We presumed that α-helices of hemoglobin were able to unfold more readily around T _B. α-helical unfolding would initiate molecular aggregation causing RBC passage and viscosity breakdown as mentioned above. Thus, structural molecular changes of hemoglobin could determine biophysical effects visible on a macroscopic scale. It is hypothesized that the species’ body temperature was imprinted into the structure of hemoglobins.     

Foam behavior of biological media

Summary The concentration dependence of the foaming of aqueous bovine serum albumin (BSA) solutions with and without salt additives and that of the turbidity temperature, T_T, of p-isononylphenol-10-glycolether in presence of KCl, MgSO₄, or K₄ [Fe(CN)₆] were determined. The differences between the turbidity temperatures of the solutions with and without salt additives were used to calculate the apparent concentration BSA in the salt solutions and to estimate their foaming. The measured and calculated foaminesses agree well.Symbols BSA Bovine Serum Albumin - C concentration - C_BSA concentration of BSA - C_salt salt concentration - C_O actual protein concentration in the absence of a salt - C₁ apparent protein concentration in the presence of a salt - C' NP-10 concentration - k constant in Eq. (4) - T_corr correction for T_TO of the salt-free NP-10 solution - T_T turbidity temperature - V_s equilibrium volume of the foam above the liquid layer - V_tg volumetric gas flow rate - foaminess - NP-10 p-isononylphenol-10-glycolether     

Thermodynamics of partitioning and efflux of phenothiazines from liposomes

Summary The partitioning of nine phenothiazines between dimyristoylphosphatidylcholine (DMPC) liposomes and 0.9% wt/vol saline at pH 6 has been studied both below and above the phase transition temperature (T _c ) of the phospholipid. Higher partitioning was observed aboveT _c . Both the entropy and enthalpy of partitioning were positive below and aboveT _c , and a linear relationship between the entropy and enthalpy has been derived. In general, the partitioning and transport of alkylaminophenothiazines in DMPC liposomes over the temperature range of 5 to 40°C is entropically controlled. The entropies and enthalpies of partitioning of various groups in the phenothiazine structure have been calculated.No relationship was found between particle size of the DMPC liposomes and the equilibrium partition coefficient at 25°C. However, the particle size of liposomes did increase with increasing acyl chain length of the phospholipid.Using differential scanning calorimetry, the enthalpy and entropy of transition of the DMPC liposomes in the absence and presence of phenothiazines has been calculated. The temperature dependence of the first-order rate constant of trimeprazine tartrate transport in DMPC liposomes was investigated and was found to be maximum at theT _c of the phospholipid.     

Effects of pressure and pentanol on the phase transition in the membrane of Acholeplasma laidlawii B

The phase transition of a complex, biological membrane was studied in relation to two variables: high pressure and the alcohols, pentanol and benzyl alcohol, in order to determine whether earlier studies of defined phospholipid bilayers may be applied to natural membranes. The isolated membrane of A choleplasma laidlawii was used and three independent methods were chosen to detect the transition; optical transmission (giving T_t, an index of the end-of-melting temperature); fluorescence polarisation (giving T_p, the temperature midway through the change in polarisation which characterises the transition) and differential thermal analysis giving a record of the temperature range occupied by the endothermic process. Pressure increased T_t by 0.017 K·atm⁻¹ and T_p by 0.016 K·atm⁻¹, consistent with dT/dP = T·ΔV/ΔH. Pentanol (and benzyl alcohol) lowered T_t, T_p and the temperature of the endotherm seen with differential thermal analysis. Thus the membrane transition responds to pressure and alcohols in agreement with thermodynamic theory.     

Analysis of the bilayer phase transition temperatures of phosphatidylcholines with mixed chains

The analysis of the chain-length dependence of the chain-melting transition temperatures of bilayers composed of lipids with identical chains (Marsh, D. 1991. Biochim. Biophys. Acta. 1062: 1-6) is extended to include lipids with chains of unequal length. The bilayer transition temperatures of saturated asymmetrical phosphatidylcholines are interpreted by assuming that the transition enthalpy and transition entropy are linearly related to the absolute value of the difference in chain length between the sn-1 and sn-2 chains, with constant end contributions. Such an assumption is supported by calorimetric data on phosphatidylcholines of constant mean chainlength and varying chain asymmetry. In particular, a symmetrical linear dependence is observed on the chain asymmetry, Δn, which is centered around a value Δn° that corresponds to the conformational inequivalence of the sn-1 and sn-2 chains. The transition temperature then takes the form: T_t = T_t^∞(n - n_H - h′ | Δn + Δn° |)/(n - n_s - s′ | Δn + Δn°) where n_H, n_s are the end contributions, and h′, s′ are fractional deficits in the incremental transition enthalpy and entropy, respectively, arising from the overlapping regions of the longer chains. Optimization on the transition temperature data for the dependence on chain asymmetry of three series of phosphatidylcholines with constant mean chainlength, n, yields parameters that are capable of predicting the dependence of the transition temperatures on chain asymmetry for other mean chainlengths. The dependence of the transition temperature on mean chainlength for phosphatidylcholines in which the chain asymmetry is maintained constant, as well as the dependence on both mean chain length and chain asymmetry for phosphatidylcholines in which one of the two chains is maintained of constant length, are also described with high accuracy by using the same parameters.     

Study of the phase transition in lysozyme crystals by Raman spectroscopy

BackgroundRecently, it has been revealed that tetragonal lysozyme crystals show a phase transition at 307 K upon heating. The underlying mechanisms of the phase transition are still not fully understood. Here we focus on the study of high-frequency vibrational modes arising from the protein and their temperature evolution in the vicinity of T_ph as well as on the detailed study of crystalline water dynamics near T_ph.MethodsRaman experiments have been performed at temperatures 295–323 K including T_ph. The low-frequency modes and the modes of fingerprint region, CH- and OH-stretching regions have been analyzed.Results and conclusionsIn spite of the absence of noticeable rearrangements in protein structure, the high-frequency vibrational modes of lysozyme located in the fingerprint region have been found to exhibit the features of critical dynamics near T_ph. Pronounced changes in the dynamics of α-helixes and Tyr residues exposed on the protein surface point to the important role of H-bond rearrangements at the phase transition. Additionally the study of temperature evolution of OH-stretching modes has shown an increase in distortions of tertahedral H-bond network of crystalline water above T_ph. These changes in water dynamics could play a crucial role in the mechanisms of the phase transition.General significanceThe present results shed light on the mechanisms of the phase transition in lysozyme crystals.     

The protein glass transition as measured by dielectric spectroscopy and differential scanning calorimetry

The glass transition and its related dynamics of myoglobin in water and in a water–glycerol mixture have been investigated by dielectric spectroscopy and differential scanning calorimetry (DSC). For all samples, the DSC measurements display a glass transition that extends over a large temperature range. Both the temperature of the transition and its broadness decrease rapidly with increasing amount of solvent in the system. The dielectric measurements show several dynamical processes, due to both protein and solvent relaxations, and in the case of pure water as solvent the main protein process (which most likely is due to conformational changes of the protein structure) exhibits a dynamic glass transition (i.e. reaches a relaxation time of 100 s) at about the same temperature as the calorimetric glass transition temperature T_g is found. This glass transition is most likely caused by the dynamic crossover and the associated vanishing of the α-relaxation of the main water relaxation, although it does not contribute to the calorimetric T_g. This is in contrast to myoglobin in water–glycerol, where the main solvent relaxation makes the strongest contribution to the calorimetric glass transition. For all samples it is clear that several proteins processes are involved in the calorimetric glass transition and the broadness of the transition depends on how much these different relaxations are separated in time.     

The mobility of PSI and PQ molecules in <Emphasis Type="Italic">Spirulina platensis</Emphasis> cells during state transition

Monomerization and trimerization of photosystem I (PSI) in cyanobacteria are reversible to response to light switched off and on, which leads to “energy spillover” to regulate excitation of the two photosystems in balance. Considering that PSI is a trans-membrane protein embedded in thylakoid membranes, the monomerization or trimerization must involve a movement of PSI in the membranes. In this work, the mobility of PSI was demonstrated by dependence of the monomerization and trimerization on temperature for intact Spirulina platensis cells undergoing a light-to-dark or a dark-to-light transition. Based on the characteristic absorbance of monomers and trimmers, it confirms that both monomerization and trimerization are temperature-sensitive. The relative populations of the monomers and trimmers are invariable above the phase transition temperature (T _PT) while directly proportional to temperature below T _PT. On the other hand, the rate to reach the equilibrium population is proportional to temperature above T _PT but invariable below T _PT. The PSI mobility and the temperature-dependent population are contrary to those of plastoquinone (PQ) molecules because PSI is a trans-membrane protein while PQ molecules are small diffusive electron carriers in thylakoid membranes as well as their distinctive sizes and environments. The less monomerization of PSI but the invariable time constant at lower temperature below T _PT may be due to that accumulation of the reduced PQ molecules results in decrease of the stromal-side H⁺ concentration which is a driving force of PSI monomerization.     

Phase transitions and ion currents in a model ferroelectric channel unit

The hypothesis of ferroelectric electrodiffusion is examined mathematically. A thermodynamic potential, the elastic Gibbs function, written in polynomial form, provides the dielectric equation of state for the model. The other equations of electrodiffusion theory complete the model. This system reduces to a second-order partial differential equation, which is formally solved by the phase-plane method. This solution, applied to the Na channel, leads to a propagating phase-transition wave accompanied by movement of ionic charge. This may be readily interpreted as a transmembrane wave traveling along a ferroelectric unit within, and transporting ions through, the channel. Comparison of the temperature dependence of axonal conduction velocity with that of the spontaneous polarization of Rochelle salt suggests that the Na channel of squid axon contains a ferroelectric unit having a lower Curie point, but decomposing before reaching its upper Curie point. Comparison with data from reconstitution experiments suggests that the ferroelectric unit is a carbohydrate enclosed in an intrinsic protein structure to form a glycoprotein channel. The value experimentally estimated for the surface charge of the Na channel is within the range of spontaneous polarizations of typical ferroelectric crystals. It is argued that the ferroelectric probably is a single crystal of the order-disorder type, which undergoes a first-order transition between a ferroelectric and a paraelectric state during excitational activity. The hypothesis of ferroelectric channel units is consistent with the existence and directionality of the observed "gating" currents.     

High‐Performance Relaxor Ferroelectric Materials for Energy Storage Applications

Relaxor ferroelectrics usually possess low remnant polarizations and slim hystereses, which can provide high saturated polarizations and superior energy conversion efficiencies, thus receiving increasing interest as energy storage materials with high discharge energy densities and fast discharge ability. In this study, a relaxor ferroelectric multilayer energy storage ceramic capacitor (MLESCC) based on 0.87BaTiO₃‐0.13Bi(Zn_2/3(Nb_0.85Ta_0.15)_1/3)O₃ (BT‐BZNT) with inexpensive Ag/Pd inner electrodes is prepared by the tape casting method. The MLESCC with two dielectric layers (layer thicknesses of 5 µm) sintered by a two‐step sintering method exhibits excellent energy storage properties with a record‐high discharge energy density of 10.12 J cm^?3, a high energy efficiency of 89.4% achieved at an electric field of 104.7 MV m^?1, a high temperature stability of the energy storage density (with minimal variation of <±5%), and energy efficiency (>90%) over a range of ?75 to 150 °C at 40 MV m^?1. These results suggest that the BT‐BZNT relaxor ferroelectric ceramic material can provide realistic solutions for high‐power energy storage capacitors.     

Temperature dependence of the dynamic light scattering of linear phi 29 DNA: implications for spontaneous opening of the double-helix

The apparent diffusion coefficient D_app(K) of a single sample of linear ?29 DNA (M_r = 11.5 × 10⁶) has been measured as a function of K² from 0.21 × 10¹⁰ to 20 × 10¹⁰ cm^?2 at a variety of temperatures from ?0.5 to +70°C. D_app(K) scales closely as T/η at every value of K². All of these data are simulated by a particular Rouse-Zimm model comprised of a constant number of subchains with constant rms subchain extension b = 1057 Å and an apparent subchain diffusion coefficient D_plat that scales at T/η from ?0.5 to +70°C. It is inferred from these results that any temperature dependence of the flexural and torsional rigidities of DNA must be rather weak. A less firm inference is that these rigidities actually increase slightly with temperature, possibly in proportion to T, which is weak T dependence in this context. These findings eliminate the possibility that spontaneous transient opening of the DNA structure has any significant effect on the flexural and torsional rigidities of the DNA filament. A review of the most pertinent available data from other experiments concerning spontaneous transient opening of the DNA is presented. The formaldehyde kinetics data do not unequivocally implicate an open base-pair intermediate and provide only an upper limit to the fraction of open base pairs. An alternative nonopening model with a protonated doorway state is proposed to accommodate the hydrogen-exchange data. It is concluded that there is presently no incontrovertible evidence for a fraction of unstacked open base pairs greater than about 10^?4.