On the Apparent Attractive Interaction between Colloidal Particles of Like Charge

A new attractive force between colloidal particles of like charge has been discovered. This force is of entropic depletion origin, and is exhibited between colloidal particles in electrolyte solution. The attractive potential associated with this force is a function of the electrolyte concentration, and can have a minimum up to the order of -10 kT deep for certain parameters of the system. Implications of the force to various systems of interest will be discussed

On the reliability of mean-field methods in polymer statistical mechanics

The reliability of the mean-field approach to polymer statistical mechanics is investigated by comparing results from a recently developed lattice mean-field theory (LMFT) method to statistically exact results from two independent numerical Monte Carlo simulations for the problems of a polymer chain moving in a spherical cavity and a polymer chain partitioning between two confining spheres of different radii. It is shown that in some cases the agreement between the LMFT and the simulation results is excellent, while in others, such as the case of strongly fluctuating monomer repulsion fields, the LMFT results agree with the simulations only qualitatively. Various approximations of the LMFT method are systematically estimated, and the quantitative discrepancy between the two sets of results is explained with the diminished accuracy of the saddle-point approximation, implicit in the mean-field method, in the case of strongly fluctuating fields.Comment: 27 pages, 9 figure

Partitioning of a polymer chain between a confining cavity and a gel

A lattice field theory approach to the statistical mechanics of charged polymers in electrolyte solutions [S. Tsonchev, R. D. Coalson, and A. Duncan, Phys. Rev. E 60, 4257, (1999)] is applied to the study of a polymer chain contained in a spherical cavity but able to diffuse into a surrounding gel. The distribution of the polymer chain between the cavity and the gel is described by its partition coefficient, which is computed as a function of the number of monomers in the chain, the monomer charge, and the ion concentrations in the solution.Comment: 17 pages, 6 figure

Light-Induced Conformational Changes of S. aurantiaca: Bacteriophytochromes as Revealed by Atomic Force Microscopy

Bacteriophytochromes (BphPs) are red-light photoreceptors found in photosynthetic and non-photosynthetic bacteria. BphPs are composed of a photosensory core module (PCM) that consists of three domains PAS, GAF, and PHY along with an effector domain, usually a histidine kinase (HK). BphPs utilize a covalently attached biliverdin (BV), an open-chain tetrapyrrole, to photoconvert between red (Pr) and far-red (Pfr) light-absorbing states. Due to the lack of crystal structures of intact BphPs in their respective Pr and Pfr states, we have utilized Atomic Force Microscopy (AFM) to characterize the structure of intact BphPs in biologically relevant media. Specifically, we have focused on the AFM analysis of BphPs from myxobacterium Stigmatella aurantiaca, SaBphP1 and SaBphP2, that share 41% sequence identity, both bind BV and have different Pr/Pfr photoconversion. Unlike classical BphPs, wild-type SaBphP1 lacks a highly conserved His that stabilizes BV and undergoes limited Pr/Pfr photoconversion that can be restored by a single Thr (Thr289) to His mutation in PCM. Individual dimers of intact SaBphP2 have been observed on a mica surface in solution and compared to truncated PAS GAF variants. Quaternary structural changes in length and width of intact BphPs with respect to Pr and Pfr states have been revealed by AFM and compared to published electron microscopy (EM) data. The proteins appear larger by AFM than EM due to the hydration of the protein and the tip convolution. The ratio of length to width of these BphPs determined by AFM agrees within 10% with the EM data for related BphP from Deinococcus radiodurans in its Pfr state. Furthermore, the volume, orientation, and structure of these BphPs are in agreement with respective protein models generated using PyMOL software and X-ray crystallographic structures of similar BphPs