Folding of Pig Gastric Mucin Non-glycosylated Domains: A Discrete Molecular Dynamics Study

Mucin glycoprotein consist of tandem repeating glycosylated regions flanked by non-repetitive protein domains with little glycosylation. These non-repetitive domains are involved in the pH dependent gelation of gastric mucin, which is essential to protecting the stomach from autodigestion. We have examined the folding of the non-repetitive sequence of von Willebrand factor vWF-C1 domain (67 amino acids) and PGM 2X (242 amino acids) at neutral and low pH using Discrete Molecular Dynamics. A four-bead protein model with hydrogen bonding and amino acid-specific hydrophobic/hydrophilic and electrostatic interactions of side chains) was used. The simulations reveal that the distant N- and C-terminal regions form salt-bridges at neutral pH giving a relatively compact folded structure. At low pH, the salt bridges break giving a more open and extended structure. The calculated average value of the beta-strand increases from 0.23 at neutral pH to 0.36 at low pH in very good agreement with CD data. Simulations of vWF C1 show 4-6 beta strands separated by turns/loops and we found that pH did not affect significantly the folded structure. The average beta-strand structure of 0.32 was again in very good agreement with the CD results.

Kinetics of HEX-BCC Transition in a Triblock Copolymer in a Selective Solvent: Time Resolved Small Angle X-ray Scattering Measurements and Model Calculations

Time-resolved small angle x-ray scattering (SAXS) was used to examine the kinetics of the transition from HEX cylinders to BCC spheres at various temperatures in poly(styrene-b- ethylene-co-butylene-b-styrene) (SEBS) in mineral oil, a selective solvent for the middle EB block. Temperature-ramp SAXS and rheology measurements show the HEX to BCC order-order transition (OOT) at ~127 oC and order-disorder transition (ODT) at ~180 oC. We also observed the metastability limit of HEX in BCC with a spinodal temperature, Ts ~ 150 oC. The OOT exhibits 3 stages and occurs via a nucleation and growth mechanism when the final temperature Tf < Ts. Spinodal decomposition in a continuous ordering system was seen when Ts< Tf < TODT. We observed that HEX cylinders transform to disordered spheres via a transient BCC state. We develop a geometrical model of coupled anisotropic fluctuations and calculate the scattering which shows very good agreement with the SAXS data. The splitting of the primary peak into two peaks when the cylinder spacing and modulation wavelength are incommensurate predicted by the model is confirmed by analysis of the SAXS data.

Kinetics of phase transition from lamellar to hexagonally packed cylinders for a triblock copolymer in a selective solvent

We examined the kinetics of the transformation from the lamellar (LAM) to the hexagonally packed cylinder (HEX) phase for the triblock copolymer, polystyrene-b-poly (ethylene-co-butylene)-b-polystyrene (SEBS) in dibutyl phthalate (DBP), a selective solvent for polystyrene (PS), using time-resolved small angle x-ray scattering (SAXS). We observe the HEX phase with the EB block in the cores at a lower temperature than the LAM phase due to the solvent selectivity of DBP for the PS block. Analysis of the SAXS data for a deep temperature quench well below the LAM-HEX transition shows that the transformation occurs in a one-step process. We calculate the scattering using a geometric model of rippled layers with adjacent layers totally out of phase during the transformation. The agreement of the calculations with the data further supports the continuous transformation mechanism from the LAM to HEX for a deep quench. In contrast, for a shallow quench close to the OOT we find agreement with a two-step nucleation and growth mechanism.