M N Tamashiro | PubMed Health

PubMed2001

Helix-coil transition in homopolypeptides under stretching.

We consider the effect of an external applied force on the alpha-helix-coil transition of a single-stranded homopolypeptide chain. An annealed scenario is assumed, where the building amino acid monomers may interconvert between random-coiled and ordered alpha-helical configurations. By exact evaluation of the partition function of the freely jointed chain with helix-coil internal degrees of freedom in the thermodynamic limit, we obtain the result that the stress-strain characteristic has an asymmetrical sigmoid shape with a prominent pseudoplateau. Because of the one-dimensional nature of this system, fluctuations dominate over the mean-field approximation, which incorrectly predicts a second-order phase transition.

Amino Acids↗

PubMed1999

Electrolytic depletion interactions.

We consider the interactions between two uncharged planar macroscopic surfaces, immersed in an electrolyte solution, which are induced by interfacial selectivity. These forces are taken into account by introducing a depletion free-energy density functional, in addition to the usual mean-field Poisson-Boltzmann functional. The minimization of the total free-energy functional yields the density profiles of the microions and the electrostatic potential. The disjoining pressure is obtained by differentiation of the total free energy with respect to the separation of the surfaces, holding the range and strength of the depletion forces constant. We find that the induced interaction between the two surfaces is always repulsive for sufficiently large separations, and becomes attractive at shorter separations. The nature of the induced interactions changes from attractive to repulsive at a distance corresponding to the range of the depletion forces.

Journal Article↗