Carboxylate ions, orientation

Two other noteworthy conclusions can be drawn from the results shown by Osawa First, when a monolayer of PNBA is adsorbed on the surface of silver or gold, the acid is deprotonated so that the C=0 stretching vibration of the PNBA is replaced by the vibrational bands of the corresponding carboxylate ion. Since the carboxylate group is bound to the surface of the metal by both oxygen atoms, the symmetric -C02 stretch is intensified while the intensity of the symmetric -C02 stretch is greatly reduced in comparison to the same band in, say, the spectrum of a KBr disk of PNBA. Second, when multiple layers are cast from solution onto the surface of the silver film, the surface selection rule only applies to the first layer of PNBA as subsequent layers are randomly oriented. Finally, the enhancement is largely confined to molecules that are within about 5nm of the silver surface. 

From the pH-dependence data (10), the pKg values of the cata-lytically essential carboxyl groups of Asp-32 and Asp-215 are about 1.5 and 4.5, respectively. This difference can be attributed (13) to the existence of a hydrogen-bonded system between these two carboxylic groups. Once a proton has been eliminated from the system, the orientation of the carboxyl groups may change, and as a consequence, the catalytic activity decreases (Fig.5). Such an interpretation is consistent with the pH-dependence of pepsin catalysis, provided the carboxylate ion of Asp-32 is involved in the catalysis at the rate-limiting step of the process. 

Figure 12.11 Schematic diagram of the ion pore of the K+ channel. From the cytosolic side the pore begins as a water-filled channel that opens up into a water-filled cavity near the middle of the membrane. A narrow passage, the selectivity filter, links this cavity to the external solution. Three potassium ions (purple spheres) bind in the pore. The pore helices (red) are oriented such that their carboxyl end (with a negative dipole moment) is oriented towards the center of the cavity to provide a compensating dipole charge to the K ions. (Adapted from D.A. Doyle et al.. Science 280 69-77, 1998.)...

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

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