Evolution of Complexity. Molecular Aspects of Preassembly.

An extension of neo-Darwinism, termed preassembly, states that genetic material required for many complex traits, such as echolocation, was present long before emergence of the traits. Assembly of genes and gene segments had occurred over protracted time-periods within large libraries of non-coding genes. Epigenetic factors ultimately promoted transfers from noncoding to coding genes, leading to abrupt formation of the trait via de novo genes.

An Alternative Molecular View of Evolution: How DNA was Altered over Geological Time.

Four natural phenomena are cited for their defiance of conventional neo-Darwinian analysis: human intelligence; cat domesticity; the Cambrian explosion; and convergent evolution. 1. Humans are now far more intelligent than needed in their hunting-gathering days >10,000 years ago.

Coordination among Bond Formation/Cleavage in a Bifunctional-Catalyzed Fast Amide Hydrolysis: Evidence for an Optimized Intramolecular -Protonation Event.

A density functional theory (DFT) computational analysis, using the ωB97X-D functional, of a rapid amide cleavage in 2-carboxyphthalanilic acid (2CPA), where the amide group is flanked by two catalytic carboxyls, reveals key mechanistic information: (a) General base catalysis by a carboxylate coupled to general acid catalysis by a carboxyl is not operative. (b) Nucleophilic attack by a carboxylate on the amide carbonyl coupled to general acid catalysis at the amide oxygen can also be ruled out. (c) A mechanistic pathway that remains viable involves general acid proton delivery to the amide nitrogen by a carboxyl, while the other carboxylate engages in nucleophilic attack upon the amide carbonyl; a substantially unchanged amide carbonyl in the transition state; two concurrent bond-forming events; and a spatiotemporal-base rate acceleration.

Interaction vs Preorganization in Enzyme Catalysis. A Dispute That Calls for Resolution.

This essay focuses on the debate between Warshel et al. (proponents of preorganization) and Menger and Nome (proponents of spatiotemporal effects) over the source of fast enzyme catalysis. The Warshel model proposes that the main function of enzymes is to push the solvent coordinate toward the transition state.

Transforming a Stable Amide into a Highly Reactive One: Capturing the Essence of Enzymatic Catalysis.

Aspartic proteinases, which include HIV-1 proteinase, function with two aspartate carboxy groups at the active site. This relationship has been modeled in a system possessing an otherwise unactivated amide positioned between two carboxy groups. The model amide is cleaved at an enzyme-like rate that renders the amide nonisolable at 35 °C and pH 4 owing to the joint presence of carboxy and carboxylate groups.

Capacious and programmable multi-liposomal carriers.

Spherical polycationic brushes (SPBs) were synthesized by grafting polycationic chains onto 100 nm polystyrene particles. These particles were exposed to unilamellar egg-lecithin (EL) liposomes with a mean diameter of 40 nm that had been rendered anionic via the presence of 10 molar% of phosphatidylserine (PS(1-)). The liposomes also contained 30 mole% of a morpholinocyclohexanol-based lipid (MOCH) that undergoes a conformational flip when the pH is decreased from 7.

Lipid Segregation in Membranes of Anionic Liposomes Adsorbed onto Polycationic Brushes.

Two-phased: Complexation of liposomes to spherical polycationic brushes induces lipid segregation in the liposomal membrane. The greater the initial anionic lipid content in the membrane, the more the electroneutral lipid dilutes the induced anionic clusters.

Composition and properties of complexes between spherical polycationic brushes and anionic liposomes.

A spherical polycationic brush (SPB) is made by graft-polymerizing a cationic monomer onto the surface of a 100 nm polystyrene bead. It is possible to adsorb anionic liposomes (40-60 nm diameter) onto the SPBs while maintaining the liposome integrity. The liposomes were constructed with phosphatidyl choline (PC) admixed with 0.

Relationship between surface tension and surface coverage.

Surfactant action is caused in part by a dramatic reduction in surface tension. Using surface excess measurements from a radioactive surfactant, it was possible to show that (a) the surface tension declines only slightly when the occupancy of the air/water interface increases from 0 to 60% of the maximum and (b) the steep drop in surface tension in region B (Figure 1 ), frequently observed to be linear, begins at about 80% occupancy. Surfactant continues to enter the interface cooperatively up to and past the critical micelle concentration.

Complexation of anionic liposomes with spherical polycationic brushes.

Spherical polycationic brushes, consisting of polystyrene particles with linear cationic macromolecules grafted onto their surfaces, were electrostatically complexed with small unilamellar anionic liposomes. Complexation was monitored using a multimethod approach that included laser electrophoresis, dynamic light scattering, fluorescence, cryogenic transmission electron microscopy, and conductivity. Liposomes adsorb onto the outer edges of the brushes rather than penetrate into their dense polycationic layer.

Liposome fusion rates depend upon the conformation of polycation catalysts.

Cryo-TEM and NaCl-leakage experiments demonstrated that the cationic polymer polylysine induces fusion of anionic liposomes but that the cationic polymer poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) does not, although both polymers bind strongly to the liposomes. The difference was traced to the thickness of the coatings at constant charge coverage. Polylysine is believed to form planar β-sheets that are sufficiently thin to allow membrane fusion.

Remembrances of self-assemblies past.

Research on four types of self-assemblies (micelles, coacervates, gels, and vesicles) is discussed via a particular investigative methodology (in order of appearance): kinetics, dynamic NMR, PGSE-NMR, double-(13)C labeling, molecular dynamics computations, phase diagrams, cryo-HRSEM, rheology, light/electron microscopy, electrophoretic mobility, electroformation, confocal microscopy, and calorimetry. The emphasis here is on how a given method, each in its own special way, illuminates a complex system.

Uncertainty in chemistry.

It might come as a disappointment to some chemists, but just as there are uncertainties in physics and mathematics, there are some chemistry questions we may never know the answer to either, suggests Fredric M. Menger.

Liposomes remain intact when complexed with polycationic brushes.

Anionic liposomes adsorb onto the surface of spherical polymer particles bearing grafted linear cationic macromolecules. The size, shape, and encapsulation ability of the liposomes remain unchanged upon adsorption, thus providing immobilized self-organizing containers that have potential applications in the biomedical field.

Self-assembling systems: mining a rich vein.

This paper summarizes a few of the self-assembling systems investigated in the authors' laboratory over the years. These include systems that mimic an enzyme, solubilize drugs, release encapsulated guests, assemble via hydrophobic surfaces, exhibit hysteresis in films, link cancer cells to vesicles, and destroy toxic compounds. Although the amphiphilic molecules are all rather different, one overriding theme predominates: just as the properties of molecules are not simple extrapolations from atoms, properties of self-assemblies are not simple extrapolations from molecules.

A singularity model for chemical reactivity.

This article proposes a model for chemical reactivity that involves singularities ("catastrophes") in the timing of bond-making and bond-breaking events. The common stapler is a good mechanical analogy: As hand-pressure is increased on the machine, the staple hardly changes its configuration until the staple suddenly bends. This is viewed as a singularity or catastrophe, defined classically as an abrupt change resulting from a smooth increase or decrease in external conditions (pressure in the case of a stapler, distance in the case of reactivity).

Additional support for a revised Gibbs analysis.

Gibbs-determined areas of >60 A(2)/molecule for many common surfactants cause rather small surface tension reductions when measured on a Langmuir film balance. This is inconsistent with an air/water interface being saturated throughout the steep linear decline in plots of surface tension versus ln [surfactant]. In support for a gradually populating interface in the linear region, we have found that sodium docosanyl sulfate lowers the surface tension by only 7 mN/m when compressed to 50 A(2)/molecule.

Polymer migration among phospholipid liposomes.

Complexation of phospholipid lipsomes with a cationic polymer, poly(N-ethyl-4-vinylpyridinium bromide) (PEVP), and subsequent interliposomal migration of the adsorbed macromolecules, have been investigated. Liposomes of two different charge types were examined: (a) a liposomal system, with an overall charge near zero, consisting of zwitterionic phosphatidylcholine (egg lecithin, EL) with added doubly anionic phospholipid, cardiolipin (CL(2-)), and cationic dihexadecyldimethylammonium bromide (HMAB(+)), in a CL(2-)/HMAB(+) charge-to-charge ratio of 1:1; (b) an anionic liposomal system composed of an EL/CL(2-) mixture plus polyoxyethylene monocetyl ether (Brij 58). Both three-component systems were designed specifically to preclude liposomal aggregation upon electrostatic association with the PEVP, a phenomenon that had complicated analysis of data from several two-component liposomes.

Re-evaluating the Gibbs analysis of surface tension at the air/water interface.

Molecular areas of soluble films at the air/water interface have traditionally been calculated by applying the Gibbs equation to the steep linear decline in surface tension as the bulk concentration increases. This approach presupposes that the interface is saturated in the "Gibbs region," thereby allowing a single unique area to be calculated. We show that the areas derived by the Gibbs equation (typically 50-60 A(2)/molecule) are much too large to account for the abrupt surface tension decline.

Electrostatic binding among equilibrating 2-D and 3-D self-assemblies.

Six organic additives, each bearing a different number of anionic charges, were added to a large excess of cationic surfactant (dodecyltrimethylammonium bromide, DTAB). The surface-tension vs log [DTAB] plot for solutions containing DTAB/trianion = 15:1 showed an abrupt break (routinely taken as the critical micelle concentration, CMC) at 2.9 mM.

Biomembrane sensitivity to structural changes in bound polymers.

Anionic liposomes containing a 4:1 molar ratio of neutral to anionic phospholipids were treated with an excess of five zwitterionic polymers differing only in the spacer length separating their cationic and anionic moieties. Although the polymers do not disrupt the structural integrity of the liposomes, they can induce spacer-dependent molecular rearrangements within the liposomes. Thus, the following were observed: spacer length = 1, no binding to the liposomes; spacer length = 2, adsorption to the liposomes, but no molecular rearrangement; spacer length = 3, lateral lipid segregation but little or no flip-flop; spacer length = 4 or 5, lateral lipid segregation and flip-flop.

Hydrogen-bond-induced hysteresis in the compression/relaxation of monolayer films.

Two stereoisomers of surfactants were synthesized in which a diketopiperazine ring was inserted between a hydrocarbon chain (of variable length) and an anionic headgroup. It was found (by HPLC, conductivity, surface tension, and diffusion NMR) that these compounds have low solubilities in water, remarkably high Krafft temperatures, and low critical micelle concentrations. Of particular interest were the pressure/area isotherms of insoluble monolayers of nonionic analogues of the amphiphiles.

Polyelectrolyte-coated liposomes: stabilization of the interfacial complexes.

Anionic liposomes, composed of egg lecithin (EL) or dipalmitoylphosphatidylcholine (DPPC) with 20 mol% of cardiolipin (CL(2-)), were mixed with cationic polymers, poly(4-vinylpyridine) fully quaternized with ethyl bromide (P2) or poly-L-lysine (PL). Polymer/liposome binding studies were carried out using electrophoretic mobility (EPM), fluorescence, and conductometry as the main analytical tools. Binding was also examined in the presence of added salt and polyacrylic acid (PAA).

Self-assembly of cationic surfactants that contain thioether groups in the hydrophobic tails.

Self-assembly in aqueous solutions of cationic surfactants that carry thioether groups in their hydrophobic tails has been investigated. Of particular interest was the identification of possible changes in the aggregate structure due to the presence of sulfur atoms. Solutions of four different compounds [CH(3)CH(2)S(CH(2))(10)N(CH(3))(3)(+)Br(-) (2-10), CH(3)(CH(2))(5)S(CH(2))(6)N(CH(3))(3)(+)Br(-) (6-6), CH(3)(CH(2))(7)S(CH(2))(6)N(CH(3))(3)(+)Br(-) (8-6), and CH(3)(CH(2))(7)S(CH(2))(8)N(CH(3))(3)(+)Br(-) (8-8)] were characterized by (1)H NMR, (13)C NMR, NMR diffusometry, and conductivity measurements.

Conjugating a cyanine dye to a polymer surface. In search of a monomeric dye in apolar media.

Solution and solid-phase syntheses of a cyanine dye conjugated to polystyrene beads (desired for potentially interesting electronic properties) are described.