Hydrogen bonds, contact with polar surfaces

Almost everywhere a preference for orientations in which the water dipole moment is more or less parallel to the interface has been observed. The driving force for the avoidance of orientations that can lead to surface electrostatic polarization are, according to Lee et ai, [46], the balance of (i) the packing forces which tend to produce a dense layer in contact with the surface and (ii) the tendency of molecules to maintain a maximal number of hydrogen bonds. 

Using the Hansen equation for the surface tension (Chapter 3), provide an estimation of the dispersion, polar and hydrogen bonding contributions of the surface tension of PA-6 (yd, yp, yfi). Estimate, based on the above, the contact angle of formamide with PA-6. 

It should be noted that in forming this dimeric channel structure all the hydrogen bonds are parallel to the channel axis and that the inner surface is lined with the polar polypeptide groups. In addition the various lipophilic side chains coat the outer wall of the structure and are thus in contact with the lipid hydrocarbon chains. The resulting gramicidin A channel is a most efficient means of ion transport with approximately 107 sodium ions traversing the channel per second, under conditions of 1 M NaCl, 100 mV applied potential and a temperature of 25 °C 225). The detailed mechanism by which this can be achieved is under active study 226). 

A comparative analysis of the dimer interface between NOS isoforms is important since there are questions on significant variation in dimer stability between isoforms (84, 85). The dimer interface is extensive with approximately 2700 A of surface area buried per monomer. The interface contacts involve a mix of nonpolar and polar interactions, including hydrogen bonding. Approximately 60% of the interface in both iNOS and eNOS is hydrophobic, although the higher resolution eNOS... 

We have now collected almost all the pieces required for a first version of COSMO-RS, which starts from the QM/COSMO calculations for the components and ends with thermodynamic properties in the fluid phase. Although some refinements and generalizations to the theory will be added later, it is worthwhile to consider such a basic version of COSMO-RS because it is simpler to describe and to understand than the more elaborate complete version covered in chapter 7. In this model we make an assumption that all relevant interactions of the perfectly screened COSMO molecules can be expressed as local contact energies, and quantified by the local COSMO polarization charge densities a and a of the contacting surfaces. These have electrostatic misfit and hydrogen bond contributions as described in Eqs. (4.31) and (4.32) by a function for the surface-interaction energy density... 

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