Ising Universality in Three Dimensions: A Monte Carlo Study

We investigate three Ising models on the simple cubic lattice by means of Monte Carlo methods and finite-size scaling. These models are the spin-1/2 Ising model with nearest-neighbor interactions, a spin-1/2 model with nearest-neighbor and third-neighbor interactions, and a spin-1 model with nearest-neighbor interactions. The results are in accurate agreement with the hypothesis of universality. Analysis of the finite-size scaling behavior reveals corrections beyond those caused by the leading irrelevant scaling field. We find that the correction-to-scaling amplitudes are strongly dependent on the introduction of further-neighbor interactions or a third spin state. In a spin-1 Ising model, these corrections appear to be very small. This is very helpful for the determination of the universal constants of the Ising model. The renormalization exponents of the Ising model are determined as y_t = 1.587 (2), y_h = 2.4815 (15) and y_i = -0.82 (6). The universal ratio Q = ^2/ is equal to 0.6233 (4) for periodic systems with cubic symmetry. The critical point of the nearest-neighbor spin-1/2 model is K_c=0.2216546 (10).Comment: 25 pages, uuencoded compressed PostScript file (to appear in Journal of Physics A

Thermal Denaturation and Aggregation of Myosin Subfragment 1 Isoforms with Different Essential Light Chains

We compared thermally induced denaturation and aggregation of two isoforms of the isolated myosin head (myosin subfragment 1, S1) containing different “essential” (or “alkali”) light chains, A1 or A2. We applied differential scanning calorimetry (DSC) to investigate the domain structure of these two S1 isoforms. For this purpose, a special calorimetric approach was developed to analyze the DSC profiles of irreversibly denaturing multidomain proteins. Using this approach, we revealed two calorimetric domains in the S1 molecule, the more thermostable domain denaturing in two steps. Comparing the DSC data with temperature dependences of intrinsic fluorescence parameters and S1 ATPase inactivation, we have identified these two calorimetric domains as motor domain and regulatory domain of the myosin head, the motor domain being more thermostable. Some difference between the two S1 isoforms was only revealed by DSC in thermal denaturation of the regulatory domain. We also applied dynamic light scattering (DLS) to analyze the aggregation of S1 isoforms induced by their thermal denaturation. We have found no appreciable difference between these S1 isoforms in their aggregation properties under ionic strength conditions close to those in the muscle fiber (in the presence of 100 mM KCl). Under these conditions kinetics of this process was independent of protein concentration, and the aggregation rate was limited by irreversible denaturation of the S1 motor domain

Swollen lamellar phases between two solid walls : undulation forces and generation of defects

Forces between curved mica surfaces across a brine swollen lamellar phase (CTAB/hexanol/Brine) have been measured. The bilayers oriented spontaneously parallel to the mica surfaces. The force-distance curves show regimes where brine is continuously expelled from between the lamellae, and regimes in which bilayers are ejected over a few seconds. We have investigated the effect on the forces of moving in the phase diagram at constant bulk repeat distance, as well as along the dilution line. We found that the force necessary to squeeze out bilayers becomes smaller as the system approaches the transition to the bicontinuous phase (Lα/L3), i.e. on increasing the hexanol content. Dynamic effects are observed and may be explained in terms of hydrodynamic coupling of the bilayers with the solvent. They may produce surprising behaviors : on decompressing the phase, lamellae are systematically ejected. The equilibrium state of the system is discussed. The data is consistent with the 1/d2 profile calculated by Helfrich for the undulation forces [1]. Least square fits of the data to 1/d2 gives bilayer curvature modulus values of the order of 5.5 ± 3 kB T.Nous avons mesuré les forces entre deux surfaces de mica immergées dans une phase lamellaire gonflée à l'eau (CTAB/hexanol/eau salée). Les bicouches s'orientent parallèlement aux surfaces de mica. Les courbes force-distance présentent des domaines où l'eau est continûment éjectée d'entre les lamelles, et des domaines où le système saute rapidement à une autre distance en éjectant des lamelles. Nous avons étudié l'effet sur les forces d'un déplacement dans le diagramme de phases à période spatiale constante et également le long de la ligne de dilution. Plus on se rapproche de la transition vers la phase bicontinue (Lα/L3), c'est-à-dire plus la concentration en hexanol est grande, plus la force nécessaire à l'éjection des lamelles devient faible. Certains effets dynamiques sont observés et peuvent s'expliquer par un couplage hydrodynamique entre les bicouches et le solvant. Ils peuvent produire des comportements surprenants : en décomprimant la phase lamellaire, des bicouches sont systématiquement éjectées. L'état d'équilibre du système est discuté. Les résultats sont en accord avec le profil en 1/d 2 calculé par Helfrich pour les forces d'ondulation. L'ajustement des courbes force-distance à une loi en 1/d2 nous donne une valeur de la rigidité de courbure des lamelles : kc = 5.5 ± 3 kB T

Light scattering study of a lower critical consolute point in a micellar system

We have studied aqueous micellar solutions of sodium dodecyl sulfate, butanol and sodium chloride. Due to the screening of electrostatic repulsive forces by the ionic atmosphere and to the existence of attractive forces (possibly related to hydration) between the micelles, lower critical consolute points can be found in these mixtures. Our data from surface and bulk light scattering are in perfect agreement with the renormalization group theories for critical phenomena. Moreover, the critical region happens to be wider than that predicted by the decorated Ising models used to describe lower critical consolute points.Nous avons étudié les solutions micellaires aqueuses de sulfate de dodécyle et de sodium, butanol et chlorure de sodium. En raison de l'écrantage des forces électrostatiques répulsives par les nuages ioniques et de l'existence de forces attractives entre les micelles (éventuellement reliées à l'hydratation), des points critiques inférieurs de démixtion peuvent être trouvés dans ces mélanges. Nos mesures par diffusion de lumière, de volume et de surface, sont en parfait accord avec les théories du groupe de renormalisation pour les phénomènes critiques. De plus, la région critique apparaît plus large que prévue par les modèles d'Ising décorés utilisés pour décrire les points critiques inférieurs