A note on the quantitative properties of McGhee-von Hippel model

Effect of pressure on the solubilization of a fluorescent merocyanine dye by a nonionic surfactant

The target dye, which is a derivative of Merocyanine 540 bearing a naphthoxazole headgroup, persists as a monomer in ethanol solution but dimerizes in water under ambient conditions. Analysis of the absorption spectrum indicates that the dimer has an oblique geometry with the two molecules being held at an angle of ca. 55°. Applying high pressure to the system forces the two molecules into closer contact, resulting in a decreased partial molar volume of 3.1 cm(3). One molecule of the monomeric dye enters a neutral micelle formed from Triton X-100, where it is highly fluorescent and free of exciton coupling. The result of applied pressure on these latter systems depends on the concentration of surfactant. Above the critical micelle concentration (CMC), applied pressure has little effect other than to increase the viscosity inside the micelle. At very low surfactant concentration, applied pressure forces monomeric dye into the dimeric form, as observed in the absence of Triton X-100. It is notable, however, that the pressure effect on the dimerization constant is exaggerated in the presence of surfactant. At intermediate surfactant concentrations, applied pressure leads to a marked change in the CMC. In particular, applied pressure reduces the partial molar volume of the micelle by ca. 7.9 cm(3) and induces micelle formation at relatively low concentration of surfactant. For example, the CMC falls from ca. 250 μM at atmospheric pressure to only 50 μM at 460 MPa.

Recognition and hydrolysis of noncrystalline cellulose

Cellulase Cel5A from alkalophilic Bacillus sp. 1139 contains a family 17 carbohydrate-binding module (BspCBM17) and a family 28 CBM (BspCBM28) in tandem. The two modules have significantly similar amino acid sequences, but amino acid residues essential for binding are not conserved. BspCBM28 was obtained as a discrete polypeptide by engineering the cel5A gene. BspCBM17 could not be obtained as a discrete polypeptide, so a family 17 CBM from endoglucanase Cel5A of Clostridium cellulovorans, CcCBM17, was used to compare the binding characteristics of the two families of CBM. Both CcCBM17 and BspCBM28 recognized two classes of binding sites on amorphous cellulose: a high affinity site (K(a) approximately 1 x 10(6) M(-1)) and a low affinity site (K(a) approximately 2 x 10(4) M(-1)). They did not compete for binding to the high affinity sites, suggesting that they bound at different sites on the cellulose. A polypeptide, BspCBM17/CBM28, comprising the tandem CBMs from Cel5A, bound to amorphous cellulose with a significantly higher affinity than the sum of the affinities of CcCBM17 and BspCBM28, indicating cooperativity between the linked CBMs. Cel5A mutants were constructed that were defective in one or both of the CBMs. The mutants differed from the wild-type enzyme in the amounts and sizes of the soluble products produced from amorphous cellulose. This suggests that either the CBMs can modify the action of the catalytic module of Cel5A or that they target the enzyme to areas of the cellulose that differ in susceptibility to hydrolysis.