Micellization-Induced Conformational Change of a Chiral Proline Surfactant

A proline surfactant including two chiral carbons, sodium N-dodecanoyl-(4R)-hydroxy-L-prolinate (SDHP), has been synthesized, and its micellization behavior in aqueous solution has been investigated by 1H NMR spectroscopy. Two conformational isomers of SDHP, namely, Z and E, are discriminated in the NMR time scale, and critical micelle concentration is derived for each isomer separately. The transformation from E to Z is observed upon micellization, and the amount of Z isomer is approximately three times that of E isomer in the equilibrated system. Moreover, the variation in chemical shifts with the surfactant concentration reveals the shielding effect of the carboxyl group on the syn-side protons of the pyrrolidine ring, which implies that the pyrrolidine rings arrange in a side-to-side manner and lie parallel to the plane of the carboxyl bonds in the neighboring molecules. The difference in the directions of the carbonyl group between Z and E isomers essentially determines their different micellization abilities and molecular arrangements in the micellization process

Molecular Dynamics Simulations of Ammonium Surfactant Monolayers at the Heptane/Water Interface

Molecular dynamics simulations have been performed on the monolayers of dodecyltrimethylammonium bromide and gemini surfactants 12−S−12 with S = 3, 6, and 12 at the n-heptane/water interfaces. The normal density profiles of the interface show that the distributions of surfactants at the liquid/liquid interface are significantly broader than those at air/water interfaces from comparisons with neutron reflection experiments and previous simulations. The spacers of 12−3−12 and 12−6−12 do not migrate much from the interface, while that of 12−12−12 tends to bend into the oil phase. The conformation of the surfactants shows that the spacers are more flexible than the tails. The characteristic angles of the surfactant well depict the geometry of the surfactants at the interface. The connected N+s of 12−3−12 and 12−6−12 have a prominent peak in the radial distribution functions, while those of 12−12−12 have nearly the same peak with those not connected. It is also found by three-dimensional spatial distribution functions that water molecules and bromide ions prefer to be shared between the positively charged methyl or methylene groups

Atomistic Molecular Insight into the Time Dependence of Polymer Glass Transition

The most atomistic molecular details of polymer glass transition were analyzed through the frozen torsions in our molecular dynamics simulations. Different observation times were used to determine the frozen fractions and frozen chain lengths. The glass transition temperature was found to coincide well with the temperature at which the frozen fractions were reduced to 1/e. The frozen chain segments grow as the temperature decreases in a similar way with that of linear polymerization, and the inverse number-average frozen chain length leads to the formulation of configuration entropy during glass transition. The ideal glass transition temperature extrapolated to zero configuration entropy corresponds well with those reported in the literature, and the relation between the relaxation time and the configuration entropy shows perfect agreement with the Adam–Gibbs theory around the glass transition temperature. Volume spanning clusters are formed at the low temperature end, which might serve as a premature prototype for the formation of the “ideal glassy state” with limited accessible configurations

Conformational Transition Behavior of Amorphous Polyethylene across the Glass Transition Temperature

Molecular dynamics simulations have been used to investigate the conformational transition behavior in amorphous polyethylene with different chain lengths across the glass transition temperature (Tg). In the present study, we examined the barrier height of conformational transition rates in different states. It was found that two lines of the logarithmic rates versus inverse temperature in the melt state and in the glass state are evidently different. The two lines have an intersection, which indicates Tg well. The barrier height in the glass state was unexpectedly observed lower than that in the melt state. For gaining better understanding of the transition barrier reduction, we analyzed motion heterogeneity of the systems and found the torsional transition rate distribution becoming gradually heterogeneous when the temperature went down to the glass state. The result indicates that the motion heterogeneity was caused by the torsion transition being “frozen”. The frozen torsions made the system into a nonequilibrium state and possess a novel transition behavior, which accounted for most of the transitions that started at a location close to top of barrier, and also the enhancement of a small magnitude of transition jumps

Water molecules at the intracellular side.

<p>(A, A′) Number of water molecules within 4 Å of the NPxxY motif at TM7. Apo S1P₁ in black, complex with antagonist in green, and complex with agonist in red. (B) The final structures including water molecules near NPxxY motif in Apo (on left) and agonist-bound receptor (on right). Antagonist-bound structure is similar to the Apo S1P₁.</p

Water molecules in vicinity of residue D91^2.50 in agonist-bound receptor during MD simulation.

<p>(A) 0 ns; (B) 100 ns; (C) 700 ns. Only water molecules within 4 Å of residue D91^2.50 are shown.</p

Movements of transmembrane helices in S1P₁ receptor.

<p>(A) RMSD of S1P₁ TM regions during MD simulations. Apo S1P₁ in black, ML056/S1P₁ in green and cyan, and S1P/S1P₁ in red and blue. (B) Different states of agonist-bound receptor structure during MD simulation. The 3D plot shows distances between cytoplasmic ends of TM helices: TM7-TM3, TM3-TM6 and TM6-TM7. The central structures from each cluster are shown. The “intermediate” and “active” conformations are superimposed on the “inactive” one (in grey).</p

Binding of ligands in S1P₁ extracellular pocket.

<p>(A) Ligand structures after equilibration: antagonist (yellow) and agonist (purple). Helices represent the crystal structure; (B) The structures of ligand-receptor complexes after 700 ns MD simulations. The antagonist-receptor structure colored in blue, while agonist-receptor structure in yellow.</p

Movement of intracellular part of TM7 in agonist-bound receptor structure.

<p>(A) The superimposed initial (grey) and final (yellow) agonist-bound structures. (B) Plot of the kink angle in TM7 with a pivot point at P308^7.50 for both simulations with agonist. During the simulation TM7 is gradually bending and the kink angle is changing from 155° to 130°.</p

Proposition of activation mechanism of S1P₁.

<p>Binding of agonist (S1P) can lead to conformational changes of highly conserved residues W269^6.48 and F265^6.44 (step 1 and 2) forming a core of a transmission switch. Afterwards, rearrangement of centrally located residues facilitate the redirected flow of water molecules inside a receptor (step 3) which is a prerequisite for a larger motion of cytoplasmic parts of transmembrane helices (step 4).</p