Mauro Missori

Mechanical properties of zona pellucida hardening

We have investigated the changes in the mechanical properties of the zona pellucida (ZP), a multilayer glycoprotein coat that surrounds mammalian eggs, that occur after the maturation and fertilization process of the bovine oocyte by using atomic force spectroscopy. The response of the ZP to mechanical stress has been recovered according to a modified Hertz model. ZP of immature oocytes shows a pure elastic behavior. However, for ZPs of matured and fertilized oocyte, a transition from a purely elastic behavior, which occurs when low stress forces are applied, towards a plastic behavior has been observed. The high critical force necessary to induce deformations, which supports the noncovalent long interaction lifetimes of polymers, increases after the cortical reaction. Atomic force microscopy (AFM) images show that oocyte ZP surface appears to be composed mainly of a dense, random meshwork of nonuniformly arranged fibril bundles. More wrinkled surface characterizes matured oocytes compared with immature and fertilized oocytes. From a mechanical point of view, the transition of the matured ZP membrane toward fertilized ZP, through the hardening process, consists of the recovery of the elasticity of the immature ZP while maintaining a plastic transition that, however, occurs with a much higher force compared with that required in matured ZP.

The Measurement of the Polaron Size in the Metallic Phase of Cuprate Superconductors

The statistical distribution of the Cu site conformations in the CuO2 plane of Bi2Sr2CaCu2O8+y (Bi 2212) has been measured by a fast and local probe Cu K-edge extended x-ray absorption fine structure (EXAFS) as a function of temperature. The Cu site conformations are frozen at T < T* = 120 K ~ 1.4 Tc in two well-defined conformations characterized by two Cu-O(apical) bond lengths separated by 0.17 ± 0.02 A. The Cu site conformations with short bond 2.38 ± 0.02 A have a probability of 41 ± 2% at T < T*. The diffraction experiments show that the Cu site conformations exhibit a one-dimensional anharmonic modulation, with period λp = 25 A, that has been measured by electron diffraction. These data show that the modulation is anharmonic and the plane is decorated with stripes of distorted lattice characterized by the short Cu-O(apical) bonds of width W = 11 ± 1 A at T < T* that form domain walls for the stripes of width L ~ 14 ± 1 A, characterized by the long Cu-O(apical) bond. The stripes of distorted lattice run below the lines of interstitial oxygen ions in the BiO plane, playing the role of acceptors. The lattice modulation is therefore assigned to polaronic impurity states ordered in linear arrays. The measure of the width W of the stripes provides the measure of the area of the polarons Sp ~ 4a2 (a = 5.4 A) where each hole dressed by the lattice distortion spans a domain of 8 Cu sites.

The instability of a 2D electron gas near the critical density for a Wigner polaron crystal giving the quantum state of cuprate superconductors

Abstract Experimental evidence that the metallic phase in the Bi2212 high Tc cuprate superconductors results from an instability of a 2D-electron gas with density lower or larger than the critical density ϱc for the formation of a generalized Wigner polaronic crystal is reported. The critical density ϱc is obtained from ϱ c = 1 S p where Sp is the measured area of condensed polarons. We have found that the 2D-electron gas at charge densities ϱ≠ϱc in the range ϱ c 2 c and ϱc T T ∗ and 2) a Fermi liquid with local density ϱi≠ϱc. The polaron pair filling factor ν=2ϱSp is shown to be the good variable to describe the phase diagram of the 2D-electron gas in the CuO2 plane as function of doping in several families of cuprate superconductors. At integer values of ν we find the insulating phases of a 2D-polaron gas: the generalized Wigner CDW at ν=1, and the generalized Wigner crystal at ν=2. At non integer values 1 δ i > l > near 1 (near 2), where is the average hole doping for the Fermi liquid and for the polaron gas. The sharp drop of the isotope exponent from ∼0.5 for ν 2 supports the proposed model for the pairing mechanism.

Evidence of elastic to plastic transition in the zona pellucida of oocytes using atomic force spectroscopy

We have investigated the mechanical properties of the zona pellucida ZP, a multilayer glycoprotein coat that surrounds mammalian eggs, using atomic force spectroscopy. The response of the membrane to mechanical stress has been reconstructed using a modified Hertz model. The transition from elastic behavior, which occurs when low stress forces are applied characterized by a Young’s modulus E=225 kPa, toward plastic behavior is observed. The critical indentation necessary to induce plastic deformations occurs at yield=55050 nm. This high critical value, corresponding to two ZP layers, well supports the noncovalent long lifetimes of interactions that take place between constituent glycoproteins.

Ristocetin-induced self-aggregation of von Willebrand factor.

Von Willebrand factor (VWF) is a large multimeric adhesive glycoprotein, with complex roles in thrombosis and hemostasis, present in circulating blood and in secretory granules of endothelial cells and platelets. High shear stress triggers conformational changes responsible for both binding to the platelet receptor glycoprotein GpIb and its self-association, thus supporting the formation of platelet plug under flow. Ristocetin also promotes the interaction of VWF with GpIb and is able to induce platelet aggregation, and thus is largely used to mimic this effect in vitro. In this research paper, we followed the time course of VWF self-association in solution induced by ristocetin binding by light scattering and at the same time we collected atomic force microscopy images to clarify the nature of the assembly that is formed. In fact, this process evolves initially through the formation of fibrils that subsequently interact to form supramolecular structures whose dimensions would be capable of trapping platelets even in the absence of any degree of shear stress or interaction with external surfaces. This intrinsic property, that is the ability to self-aggregate, may be involved in some pathological settings that have been revealed in clinical practice.

Docosahexaenoic acid reverts resistance to UV-induced apoptosis in human keratinocytes: involvement of COX-2 and HuR

The dramatic increase in the incidence of nonmelanoma skin cancer over the last decades has been related to the augmented exposure to ultraviolet (UV) radiation (UVR). It is known that apoptosis is induced as a protective mechanism after the acute irradiation of keratinocytes, whereas apoptotic resistance and carcinogenesis may follow the chronic exposure to UVR. We found that not all the human keratinocytes lines studied underwent apoptosis following acute exposure to UVR (10-60 mJ/cm(2)). Whereas UVR induced apoptosis in the HaCaT cells, NCTC 2544 and nr-HaCaT cells showed apoptosis resistance. The cytokeratin pattern of the apoptosis-resistant cells indicated that they possessed a degree of differentiation lower than that of HaCaT cells. They also showed an enhanced expression of cyclooxygenase-2 (COX-2), an early marker of carcinogenesis in various tissues, including skin. n-3 polyunsaturated fatty acids have drawn increasing interest as nutritional factors with the potential to reduce UVR carcinogenesis, and since they are apoptosis inducers and COX-2 inhibitors in cancer cells, we investigated the ability of n-3 polyunsaturated fatty acids to influence the resistance to UVR-induced apoptosis in keratinocytes. We observed that docosahexaenoic acid (DHA) reverted the resistance of nr-HaCaT cells to UVR-induced apoptosis, increasing the Bax/Bcl-2 ratio and caspase-3 activity, and reduced COX-2 levels by inhibiting the expression of the human antigen R (HuR), a known COX-2 mRNA stabilizer in keratinocytes. The transfection of nr-HaCaT cells with HuR siRNA mimicked the proapoptotic effect of DHA. Overall, our findings further support the role of DHA as a suitable anticarcinogenic factor against nonmelanoma skin cancers.

High critical temperature in a superlattice of quantum wires

Here we report experimental evidence that the high Tc superconductivity in a cuprate perovskite occurs in a superlattice of Josephson coupled quantum wires. We show that this particular heterostructure provides the physical mechanism raising Tc from the low temperature range Tc<23K to the high temperature range 30K<Tc<150K by amplification of the critical temperature of the homogeneous CuO2 plane by a factor ∼10. The structure of the high Tc superconducting CuO2 plane in Bi2Sr2CaCu2O8+y (Bi2212) at the mesoscopic level (10–100 Å) has been determined. The superconducting plane is decorated by a plurality of parallel superconducting stripes of width L defined by the domain walls formed by stripes of width W characterized by a short Cu-O(apical) distance and large tilting ∼ 110° of the distorted square pyramids. We have measured the width of the domain walls W=11±1 Å and of the stripes L=14±1 Å. The critical parameters raising Tc are: 1) the Fermi level is near the bottom of the second subband of the stripes, with k2y=2π/L, formed by the quantum size effect and 2) W is of the order of the superconducting coherence length ξ0.

Structure of the Different Cu Sites in the Corrugated CuO2 Plane in High Tc Superconductors

By using x-ray absorption spectroscopy different configurations of the Cu site structure in the CuO2 plane of the Bi 2:2:1:2 high Tc superconductor are found. The different Cu sites are characterized by short 2.3 A and long 2.45 A Cu-O(apical) distances. The linear arrays of different Cu sites forming domains with a corrugated-iron-foil shape is proposed to be a key feature of superconducting domains in the CuO2 plane. The wavelength λ of the modulation is close to the superconducting coherence length ξ and kFλ~2π. The ordering of the distorted Cu sites is suggested to be evidence for ordering of polarons driven by the pseudo Jahn Teller electron lattice interaction.

Local lattice instability of CuO2 plane in La1.85Sr0.15CuO4 by polarized Cu K edge absorption

Abstract The temperature dependence of the local structure of the CuO 2 plane in La 1.85 Sr 0.15 CuO 4 has been studied by in-plane polarized Cu K edge extended X-ray absorption fine structure (EXAFS). The high signal-to-noise ratio in the E ‖ ab EXAFS allows us to separate the contributions of CuO(planar), CuOCu and CuLa/Sr shells and to study their temperature dependence in the low-temperature orthorhombic (LTO) phase. We have observed an anomalous behavior of the local structure below 100 K with clear anomalies at ∼ T c (35 K ) and T ∗ (∼1.6T c ) that are not expected on the basis of the average structure. These results suggest that the structure of the CuO 2 plane is non-homogeneous and it shows anomalous changes at T ∗ and T c .

Optical properties of Pb2+ ions in KMgF3 crystals

Abstract Changes induced by Pb 2+ impurities in the optical behaviour of crystals of the perovskitelike KMgF 3 have been studied. Absorption and emission spectra of samples, containing dopant concentrations ranging from 0.1 to 10 mole%, show that isolated Pb 2+ ions, replacing substitutionally Mg 2+ ions in the cubic lattice of KMgF 3 , assume a configuration very similar to that of (ns) 2 univalent metal impurities in crystals of alkali halides. Aggregates of Pb 2+ ions in KMgF 3 are also found, even at low doping levels.