Salt-concentration dependence of melting profiles of lambda phage DNAs: evidence for long-range interactions and pronounced end effects.

Melting profiles of DNAs from wild-type λ phage and a deletion mutant phage λb2 were examined in a wide range of salt concentration. The fine structure of the melting profiles changed sharply with salt concentration, especially in the range [Na⁺] ? 10 mM. A comparison of the melting profiles between the wild-type and the deletion mutant DNAs provided good evidence for extremely high melting cooperativity under low salt conditions, which is clearly manifested as the long-range interactions and the pronounced end effects; a large melting peak appeared as a result of the b2 deletion without any inserted sequence in the salt range [Na⁺] ? 2.8 mM. It was also suggested that in the further reduced salt range [Na⁺] ? 2.0 mM, melting of a λ DNA molecule starts from its right end rather than the most (A + T)-rich central region. The molecular basis of the high melting cooperativity at low salt concentrations can be explained in terms of the increased free energy associated with loop formation in the double-helical structure of DNA.     

Mesurement of longitudinal ion profiles in single roots of Hordeum and Atriplex by use of flameless atomic absorption spectroscopy

Summary A method is described by which the Na⁺ and K⁺ content in 0.5 mm sections of single roots of Hordeum distichon L. and Atriplex hortensis L. can be determined by use of flameless atomic absorption spectroscopy. By this method the longitudinal profiles of K⁺ and Na⁺ along low salt roots and roots which had been equilibrated with or grown in K⁺-free 1 mM Na⁺-solution were determined. The profiles reveal that high K⁺/Na⁺ ratios in the cytoplasm are maintained also in K⁺-free solutions. In solutions containing 1 mM Na⁺ a high K⁺/Na⁺ selectivity was found to be dependent on sufficient aeration. From the ion profiles the cytoplasmic (110 mM) and vacuolar (20 mM) K⁺ concentration in low salt barley roots—values which are unobtainable by compartmental analysis—could be estimated.     

Scanning ion‐selective electrode technique and X‐ray microanalysis provide direct evidence of contrasting Na+ transport ability from root to shoot in salt‐sensitive cucumber and salt‐tolerant pumpkin under NaCl stress

Grafting onto salt‐tolerant pumpkin rootstock can increase cucumber salt tolerance. Previous studies have suggested that this can be attributed to pumpkin roots with higher capacity to limit the transport of Na⁺ to the shoot than cucumber roots. However, the mechanism remains unclear. This study investigated the transport of Na⁺ in salt‐tolerant pumpkin and salt‐sensitive cucumber plants under high (200 mM) or moderate (90 mM) NaCl stress. Scanning ion‐selective electrode technique showed that pumpkin roots exhibited a higher capacity to extrude Na⁺, and a correspondingly increased H⁺ influx under 200 or 90 mM NaCl stress. The 200 mM NaCl induced Na⁺/H⁺ exchange in the root was inhibited by amiloride (a Na⁺/H⁺ antiporter inhibitor) or vanadate [a plasma membrane (PM) H⁺‐ATPase inhibitor], indicating that Na⁺ exclusion in salt stressed pumpkin and cucumber roots was the result of an active Na⁺/H⁺ antiporter across the PM, and the Na⁺/H⁺ antiporter system in salt stressed pumpkin roots was sufficient to exclude Na⁺. X‐ray microanalysis showed higher Na⁺ in the cortex, but lower Na⁺ in the stele of pumpkin roots than that in cucumber roots under 90 mM NaCl stress, suggesting that the highly vacuolated root cortical cells of pumpkin roots could sequester more Na⁺, limit the radial transport of Na⁺ to the stele and thus restrict the transport of Na⁺ to the shoot. These results provide direct evidence for pumpkin roots with higher capacity to limit the transport of Na⁺ to the shoot than cucumber roots.     

Na+/H+ antiport activity in tonoplast vesicles isolated from sunflower roots induced by NaCl stress

Na⁺ transport across the tonoplast and its accumulation in the vacuoles is of crucial importance for plant adaptation to salinity. Mild and severe salt stress increased both ATP- and PP_i-dependent H⁺ transport in tonoplast vesicles from sunflower seedling roots, suggesting the possibility that a Na⁺/H⁺ antiport system could be operating in such vesicles under salt conditions (E. Ballesteros et al. 1996. Physiol. Plant. 97: 259–268). During a mild salt stress, Na⁺ was mainly accumulated in the roots. Under a more severe salt treatment, Na⁺ was equally distributed in shoots and roots. In contrast to what was observed with Na⁺, all the salt treatments reduced the shoot K⁺ content. Dissipation by Na⁺ of the H⁺ gradient generated by the tonoplast H⁺-ATPase, monitored as fluorescence quenching of acridine orange, was used to measure Na⁺/H⁺ exchange across tonoplast-enriched vesicles isolated by sucrose gradient centrifugation from sunflower (Helianthus annuus L.) roots treated for 3 days with different NaCl regimes. Salt treatments induced a Na⁺/H⁺ exchange activity, which displayed saturation kinetics for Na⁺ added to the assay medium. This activity was partially inhibited by 125 μM amiloride, a competitive inhibitor of Na⁺/H⁺ antiports. No Na⁺/H⁺ exchange was detected in vesicles from control roots. The activity was specific for Na⁺. since K⁺ added to the assay medium slightly dissipated H⁺ gradients and displayed non-saturating kinetics for all salt treatments. Apparent K_m for Na⁺/H⁺ exchange in tonoplast vesicles from 150 mM NaCl-treated roots was lower than that of 75 mM NaCl-treated roots, V_max remaining unchanged. The results suggest that the existence of a specific Na⁺/H⁺ exchange activity in tonoplast-enriched vesicle fractions, induced by salt stress, could represent an adaptative response in sunflower plants, moderately tolerant to salinity.     

Calorimetric studies of oxyhemoglobin dissociation. I. Reaction of sodium dithionite with oxygen

Calorimetric studies of the reduction of free oxygen in solution by sodium dithionite are in agreement with a stoichiometry of 2 moles Na₂S₂O₄ per mole of oxygen. The reaction is biphasic with ΔH_t - 118±7 kcal mol^?1 (?494 ± 29 kJ mol^?1). The initial phase of the reaction proceeds with an enthalpy change of ca ?20 kcal (?84 kJ) and occurs when 0.5 moles of dithionite have been added per mole dioxygen present. This could be interpreted as the enthalpy change for the addition of a single electron to form the superoxide anion. Further reduction of the oxygen to water by one or more additional steps is accompanied by an enthalpy change of ca ?100 kcal (?418. 5 kJ). Neither of these reductive phases is consistent with the formation of hydrogen peroxide as an intermediate. The reduction of hydrogen peroxide by dithionite in 0.1 M phosphate buffer, pH 7.15, is a much slower process and with an enthalpy change of ca ? 74 kcal mol^?1 (?314 kJ mol^?1). Dissociation of oxyhemoglobin induced by the reduction of free oxygen tension with dithionite also shows a stoichiometry of 2 moles dithionite per mole oxygen present and an enthalpy change of ca. ?101 ±9 kcal mol^?1 (?423± 38 kJ mol^?1). The difference in the observed enthalpies (reduction of dioxygen vs. oxyhemoglobin) has been attributed to the dissociation of oxyhemoglobin, which is 17 kcal mol^?1 (71 kJ mol^?1).     

Thermodynamics of transfer ribonucleic acids: the effect of sodium on the thermal unfolding of yeast tRNAPhe.

The thermal unfolding of yeast phenylalanine-specific tRNA (tRNA^Phe) has been calorimetrically investigated at several salt concentrations in the absence of magnesium. Application of the deconvolution theory of macromolecular conformational transitions allows calculation of the thermodynamic parameters of unfolding. It is demonstrated that the unfolding of tRNA^Phe occurs in a sequential fashion and that four separate transitions or five macromolecular thermodynamic states exist in the temperature range 8–72°C under the experimental conditions of these studies (0.067–0.52M Na⁺). The enthalpy and entropy changes between states and the relative population of each state as a function of temperature and salt concentration have been obtained. Sodium stabilizes the low-temperature conformations of tRNA^Phe. The increase in the melting temperatures of each transition is shown to be linearly dependent on the logarithm of sodium concentration. These results allow calculation of the “phase” diagram for the transitions as a function of salt concentration.     

Thermodynamic parameters for the helix-coil thermal transition of ribonuclease-S-peptide and derivatives from 1H-NMR data

Values for the thermodynamic quantities, ΔH° = 11.8 ± 2.0 Kcal/mole and ΔS° = 43.6 ± 6.0 e.u., of the 3-13 helix–coil equilibrium of isolated S-peptide (19 residue N-terminal fragment of ribonuclease A) in aqueous solution (3 m M, 1M NaCl, pD 5.4) have been determined from a joint analysis of the Thr 3γ, Ala 6β, Phe 8meta, and Phe 8para ¹H chemical shift vs temperature curves (?7 to 80°C) in several aqueous–trifluorethanol mixtures. Chemical shifts in the coil and in the helix have been determined for up to 16 protons belonging to the 3-13 fragment. Thermodynamic parameters have also been determined for C-peptide (13 residue fragment) and a number of S-peptide derivatives. From the variation of the values of the thermodynamic parameters at pD 2.5, 5.4, and 8.0, a quantitation of the two helix-stabilizing side-chain interactions can be made: (1) Δ(ΔH°) ? 5 Kcal/mole and Δ(ΔS°) ? 18 e.u. for the salt bridge Glu 2^? … Arg 10⁺ and (2) Δ(ΔH°) ? 3 Kcal/mole and Δ(ΔS°) = 9 e.u. for the one in which the His 12⁺ imidazolium group is involved, presumably a partial stacking with the Phe 8 side chain.     

Regulation of the uptake and distribution of Na+ in shoots of rice (Oryza sativa) variety Pokkali: role of Ca2+ in salt tolerance response

Soil salinity is a major factor affecting crop productivity worldwide. This study explores mechanisms that contribute to salt tolerance in rice (Oryza sativa L.). Hydroponically grown, 2-week-old salt tolerant and sensitive indica rice varieties, Pokkali and Jaya, respectively, were exposed to a 48-h stress period with NaCl (0–250 mM). When exposed to 200 mM NaCl, micromolar levels of external Ca²⁺ elevated survival of both varieties. The Ca²⁺ levels required were lower for Pokkali than for Jaya, but resulted in significantly higher survival. Estimates of Na⁺ and K⁺ in root and shoot compartments were made by flame photometry, while X-ray microanalysis was used to localize Na⁺ in the extracellular matrix of the shoot. Transpirational bypass flow was estimated using the apoplastic tracer, 8-hydroxypyrene-1,3,6-trisulphonic acid, trisodium salt. Our data demonstrate a Ca²⁺-dependent reduction in Na⁺ transport to shoots, which correlated with a decline in bypass flow and of Na⁺ in the transpirational stream. In addition, the Na⁺ that enters the shoot is partitioned among several distinct compartments. Survival is inversely correlated with Na⁺ levels in the shoot apoplastic fluid, which surrounds the cell and influences cytosolic composition. Pokkali maintained lower Na⁺ in its apoplast compared with the salt sensitive Jaya at the same total shoot Na⁺. Na⁺ in the apoplast appears to be regulated by sequestration into intracellular compartments. This sink supplements the primary response of reducing Na⁺ influx into the shoot and effectively buffers the apoplastic fluid in Pokkali. All of these mechanisms are operational in Jaya as well but are deployed less effectively.     

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     

tRNA methylases from HeLa cells: purification and properties of an adenine-1-methylase and a guanine-N2-methylase

Two enzymes (methylases) that catalyze the transfer of methyl groups from S-adenosyl-l-methionine to tRNA (prepared from Escherichia coli) have been partially purified from extracts of HeLa cells. One catalyzes the methylation of adenine residues of the tRNA to give 1-methyladenine units and the other is responsible for the conversion of guanine residues to N²-methylguanine and N²,N²-dimethylguanine (and may be a mixture of two enzymes). Activities of these relatively unstable enzymes could be maintained by storage at ?20 °C in the presence of 50% glycerol. Substrate specificity studies have revealed that bacterial tRNA (E. coli, Bacillus subtilis) can be used as substrate, whereas tRNA of animal origin (HeLa cells, rat liver) cannot be used. Of the specific tRNA's tested, E. coli tRNA^fMet was used as substrate by both enzymes. E. coli tRNA^Tyr was used by the adenine-1-methylase but not by the guanine-N²-methylase. The adenine-1-methylase catalyzed the transfer of approximately one methyl group per mole of either tRNA^fMet or tRNA^Tyr offered as substrate; in the presence of the guanine-N²-methylase 1 mole of E. coli tRNA^fMet accepted 1 mole of methyl. Studies with the use of both enzymes established that enzymic methylation of the guanine site of E. coli tRNA^fMet did not interfere with subsequent methylation of an adenine residue and neither did prior methylation of adenine interfere with the subsequent methylation of a guanine residue. In the presence of both enzymes, approximately 2 moles of methyl groups were accepted by 1 mole of the E. coli tRNA^fMet.     

Titles of related papers in other sections

Arrhenius plots of rabbit skeletal muscle sarcolemmal Na⁺,K⁺-ATPase contain no temperature breaks. The apparent activation energy (22.8 kcal/mole in the presence of 1 mM MgCl₂ or 15.9 kcal/mole in the presence of 3 mM MgCl₂) does not depend on the Na⁺/K⁺ ratio in the incubation medium, but decreases in the presence of anserine (instead of Tris buffer).     

Salt-dependent conformational transitions in the self-complementary deoxydodecanucleotide d(CGCAATTCGCG): Evidence for hairpin formation

Differential scanning calorimetry (DSC), temperature-dependent uv-absorption spectroscopy, and temperature-dependent CD were used to monitor and characterize the salt-dependent, thermally induced structural transitions in the deoxydodecanucleotide d(CGCGAATTCGCG). At the high oligomer concentrations required for DSC, the calorimetric scans revealed a single, monophasic transition curve at all salt concentrations. Based on previous nmr melting studies under similar conditions, we conclude that these monophasic transitions correspond to the cooperative duplex-to-single-strand conversion of the dodecamer. By contrast, at the lower oligomer concentrations used for the spectroscopic studies, the shapes of the uv and CD melting curves were found to depend on the concentration of the added salt. At high salt (≥0.1M Na⁺), a single, monophasic transition curve was observed. At lower salt (?0.01M Na⁺), the CD and uv melting curves exhibit biphasic behavior. Based on the concentration dependence, the enthalpy, and the cooperativity of each transition in the biphasic curve, we conclude that at low salt and low oligomer concentrations, the dodecamer melts in a sequential manner involving initial disruption of a duplex structure and subsequent disruption of a hairpin structure.     

Helix-coil dynamics of a Z-helix hairpin

The helix–coil transition of a Z-helix hairpin formed from d(C-G)₅T₄(C-G)₅ has been characterized by equilibrium melting and temperature jump experiments in 5M NaClO₄ and 10 mM Na₂HPO₄, pH 7.0. The melting curve can be represented by a simple all-or-none transition with a midpoint at 81.6 ± 0.4°C and an enthalpy change of 287 ± 15 kJ/mole. The temperature jump relaxation can be described by single exponentials at a reasonable accuracy. Amplitudes measured as a function of temperature provide equilibrium parameters consistent with those derived from equilibrium melting curves. The rate constants of Z-helix formation are found in the range from 1800 s^?1 at 70°C to 800 s^?1 at 90°C and are associated with an activation enthalpy of ?(50 ± 10) kJ/mole, whereas the rate constants of helix dissociation are found in the range from 200 s^?1 at 70°C to 4500 s^?1 at 90°C with an activation enthalpy +235 kJ/mole. These parameters are consistent with a requirement of 3–4 base pairs for helix nucleation. Apparently nucleation occurs in the Z-helix conformation, because a separate slow step corresponding to a B to Z transition has not been observed. In summary, the dynamics of the Z-helix–coil transition is very similar to that of previously investigated right-handed double helices.     

Hairpin Formation In d-AAGCTTAAGCTT Under High Salt Conditions Shows Unusual Properties

Abstract

The hairpin-duplex equillibria of the dodecamer d-AAGCTTAAGCTT and interaction of the duplex form with a pentapeptide, KGWGK, has been studied. UV thermal transitions are monophasic at low salt but biphasic at higher salt concentrations. At 10^?5M or less oligomer concentration biphasic melting curves persist till 900 mM NaCl. The d(Tm)/d log(Na⁺) for the duplex form is 12 °C and for the hairpin is 18 °C. The ΔH and ΔS values for duplex formation are low(-25 Kcal/mole and—59 Cal/mole respectively). KGWGK binds to the duplex form with a binding constant K = 3.4×10⁵M^?1measured from fluorescence quenching of tryptophan. These unusual results are markedly different from that reported for d-AGATCT- AGATCT (Biochemistry 31, 6241–6245) and are discussed in ternis of sequence dependence of loop folding and cruciform extrusion pathway of hairpin formation.     

Kinetic studies of a (Na++ K++ Mg2+)ATPase in sugar beet roots. III. A proposed model for the (Na++ K+) activation and its significance for field properties

A microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase preparation from sugar beet roots was used. The activation by simultaneous addition of Na⁺ and K⁺ at different levels was examined in terms of steady state kinetics. The observed data can be summarized in the following way: 1. The apparent affinity between the enzyme and the substrate MgATP depends on the ratio between Na⁺ and K⁺. At low Na⁺ concentration (below 5 mM), the apparent K_m decreases with increasing concentrations of K⁺ (1–20 mM). At 5 mM Na⁺, the K⁺ level does not change the apparent K_m, while at Na⁺ levels above 10 mM, the apparent K_m between enzyme and substrate increases with increasing concentration of K⁺. 2. When the MgATP concentration is kept constant, homotropic cooperativity (concerning one type of ligand) and heterotropic cooperativity (concerning different types of ligands) exist in the activation by Na⁺ and K⁺. The Na⁺ binding is cooperative with different K_m values and Hill coefficients (n) in the presence of low and high concentration of K⁺. At low Na⁺ level (< 5 mM). a negative cooperativity exists for Na⁺ (n_Na < 1) which is more pronounced in the presence of high [K⁺]. When the concentration of Na⁺ is raised the negative cooperativity disappears and turns into a positive one (n_Na > 1). Only K⁺ binding in the presence of low [Na⁺] shows cooperativity with a Hill coefficient that reflects changes from negative to positive homotropic cooperativity with increasing concentrations of K⁺ (n_K < 1 → n_K > 1). In the presence of [Na⁺] > 10 mM, the changes in n_k are insignificant. 3. A model is proposed in which one or two different K sites and one or two Na sites control the catalytic activity, with multiple interactions between Na⁺, K⁺ and MgATP. 4. In the presence of Na⁺ (< 10 mM), K⁺ is probably bound to two K sites, one of which translocates K⁺ through the membrane by an antiport Na⁺/K⁺ mechanism. This could be connected with an elevated K⁺ uptake in the presence of Na⁺ and could therefore explain some field properties of sugar beets.     

Activity of Ionotropic Glutamate Receptors in Retinal Cells: Effect of Ascorbate/Fe2+-Induced Oxidative Stress

Abstract: The effect of oxidative stress induced by the oxidant pair ascorbate/Fe²⁺ on the activity of ionotropic glutamate receptors was studied in cultured chick retina cells. The release of [³H]GABA and the increase of the intracellular free Na⁺ concentration ([Na⁺]_i), evoked by glutamate receptor agonists, were used as functional assays for the activity of the receptors. The results show that the maximal release of [³H]GABA evoked by kainate (KA; ～20% of the total) or AMPA (～11% of the total) was not different in control and peroxidized cells, whereas the EC₅₀ values determined for peroxidized cells (33.6 ± 1.7 and 8.0 ± 2.0 µM for KA and AMPA, respectively) were significantly lower than those determined under control conditions (54.1 ± 6.6 and 13.0 ± 2.2 µM for KA and AMPA, respectively). The maximal release of [³H]GABA evoked by NMDA under K⁺ depolarization was significantly higher in peroxidized cells (7.5 ± 0.5% of the total) as compared with control cells (4.0 ± 0.2% of the total), and the effect of oxidative stress was significantly reduced by a phospholipase A₂ inhibitor or by fatty acid-free bovine serum albumin. The change in the intracellular [Na⁺]_i evoked by saturating concentrations of NMDA under depolarizing conditions was significantly higher in peroxidized cells (8.9 ± 0.6 mM) than in control cells (5.9 ± 1.0 mM). KA, used at a subsaturating concentration (35 µM), evoked significantly greater increases of the [Na⁺]_i in peroxidized cells (11.8 ± 1.7 mM) than in control cells (7.1 ± 0.8 mM). A saturating concentration (150 µM) of this agonist triggered similar increases of the [Na⁺]_i in control and peroxidized cells. Accordingly, the maximal number of binding sites for (+)-5-[³H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate ([³H]MK-801) was increased after peroxidation, whereas the maximal number of binding sites for [³H]KA was not affected by oxidative stress. These data suggest that under oxidative stress the activity of the ionotropic glutamate receptors is increased, with the NMDA receptor being the most affected by peroxidation.     

Rubidium and Potassium Absorption by Bean-leaf Slices Compared to Sodium Absorption

At salt concentrations of 0.1 mM as well as of 5.0 mM, the ²²Na⁺ absorption capacity of bean (Phaseolus vulgaris L. cv. ‘Brittle Wax’) leaf tissue increased during the period of leaf expansion and decreased rapidly after leaf maturation. The absorption capacity for ⁸⁶Rb⁺ and ⁴²K⁺ was highest in very young leaves and decreased continuously in expanding and in mature leaves. The ⁸⁶Rb⁺ absorption capacity of mature leaves was not increased by detopping the plants; this senescence-retarding treatment more than doubled ²Na⁺ absorption. The absorption of ²²Na⁺ by bean-leaf slices was not enhanced by light, whereas ⁸⁶Rb⁺ and ⁴²K⁺ absorption was much affected. Previously absorbed ⁸⁶Rb⁺ and ⁴²K⁺ were more available for exchange than ²²Na⁺.     

Accumulation of potassium and sodium by barley roots in a k-na replacement series

Excised roots of barley (Hordeum vulgare, var. Campana) were incubated for 24 hr in solutions containing constant total concentrations of KCl and NaCl but in which the mole fractions of K and Na were varied in replacement series. In solutions containing 1, 10, or 50 mm concentrations of K⁺ plus Na⁺, total cation accumulation was dependent upon the total salt concentration but was relatively independent of the mole fractions of K⁺ and Na⁺. These results imply that accumulation of K⁺ and Na⁺ was limited by a common factor. In solutions containing 0.01 mm K⁺ plus Na⁺ there was a strong preference for K⁺ over Na⁺ and the sum of K⁺ and Na⁺ accumulation increased with increasing K⁺ concentration.     

Thermodynamic parameters of helix-coil transition in polypeptide chains. I. Poly-(L-glutamic acid)

The helix-coil transitions for poly(L -glutamic acid) (PGA) in 0.2M NaCl and in its mixture with dioxane were studied by the methods of spectropolarimetry, viscometry, and potentiometric titration at different temperatures from 8 to 50°C. The enthalpy and entropy differences between the helical and coillike states of uncharged PGA molecules were determined from the curves of potentiometric titration. The temperature dependence of the cooperativity parameter σ was determined by two methods: from the sharpness of transition and from the dependence of the intrinsic viscosity on the helical content in the transition region. In 0.2MNaCl, σ= (2.5 ± 0.5) × 10^?3 and practically does not depend on temperature, i.e., the cooperativity of the helix-coil transition is connected mainly with the entropy decrease in initiating helical regions (ΔS_i ≈ ?12 is mole of helical regions). On the contrary, initiation of a helical region in the water-organic solvent mixture is accompanied by a considerable enthalpy increase.