Amphipols are a new class of surfactants that make it possible to handle membrane proteins in detergent-free aqueous solution as though they were soluble proteins. The strongly hydrophilic backbone of these polymers is grafted with hydrophobic chains, making them amphiphilic. Amphipols are able to stabilize in aqueous solution under their native state four well-characterized integral membrane proteins: (i) bacteriorhodopsin, (ii) a bacterial photosynthetic reaction center, (iii) cytochrome b 6 f, and (iv) matrix porin.Integral membrane proteins usually are extracted from membranes and are kept soluble in aqueous solutions using detergents (1, 2). Detergent molecules equilibrate between a monolayer covering the transmembrane surface of the protein (3, 4), free monomers, and protein-free micelles. The presence of free micelles is a source of difficulty in membrane protein biochemistry and biophysics. It can induce, for instance, protein inactivation caused by the dissociation of subunits, lipids, or hydrophobic cofactors, phase separation during crystallization attempts, and an increased viscosity of the solutions in NMR experiments. We have endeavored to develop a new class of polymers, ''amphipols,'' that are designed to keep membrane proteins soluble in water in the absence of free surfactant. High molecular weight (MW) congeners of amphipols are known to exhibit a high affinity for hydrophobic particles (5-9). We show here that low MW amphipols can keep soluble under their native state four integral membrane proteins: (i) bacteriorhodopsin (BR), (ii) the photosynthetic reaction center from Rhodobacter sphaeroides R-26 (RC), (iii) the cytochrome b 6 f complex from Chlamydomonas reinhardtii, and (iv) the matrix porin (OmpF) from Escherichia coli.

pH-responsive gels of hydrophobically modified (HM) weak polyacid were prepared from acrylic acid and n-alkyl acrylates (n = 8, 12, 18). The HM gels obtained bear up to 20 mol % of n-alkyl acrylate units randomly distributed along the network chains. The pH-driven swelling of these gels upon ionization in an aqueous medium was studied. The effect of the fraction and of the side chain length of n-alkyl acrylate groups on the equilibrium degree of swelling was examined. It was shown that the swelling transition shifts to alkaline pH with increasing hydrophobicity of the gel. This was explained by the stabilization of the collapsed state of the gel by hydrophobic aggregation of n-alkyl side chains. The formation of such aggregates, which break down in the course of gel ionization, was confirmed by the fluorescent probe method with pyrene as a probe and by NMR spectroscopy. Potentiometric titration data of HM poly(acrylic acid) (PAA) gels and of the corresponding linear copolymers evidence that the introduction of hydrophobic repeat units only slightly affects the apparent dissociation constant of PAA, except for the most hydrophobic gels.

No abstract

No abstract

No abstract

No abstract

No abstract

SynopsisStarch graft superabsorbents were prepared by graft copolymerization either with polyacrylonitrile (PAN) and saponification of the resulting copolymer or with trimethyl-aminoethylacrylate chloride (CMA) and methylene bisacrylamide as crosslinking agent. The influence of several structural parameters on the swelling properties of these absorbents have been studied: crosslink density, ionic content, and composition of the starch. As expected, the absorbency decreases with an increase of the crosslink density. It increases with the ionic content of the graft copolymer up to a maximum, an excess of charges leading t o a swelling decrease. The PAN branch length depends on the origin of the starch. High molar weight PAN was grafted onto waxy corn, a starch with high amylopectin content. The swelling increases with increasing molar weight of PAN up to 5 x lo5. In saline solution the absorbency of both ionic gels decreases significantly. However, in the presence of multivalent ions their behavior is different. For cationic absorbents the swelling depends on the ionic strength but not on the ion valency. On the other hand, anionic absorbents are significantly affected by multivalent cations.