Protein water interface

Concerning proteins, the X value is strongly dependent on local polarity, which differs in different portions of such a mosaic structure as a protein globule. Positions of the donor and acceptor centers relative to the protein-water interface, chemical nature and mobility of adjacent groups can drastically affect X values. Thus, the precise calculation of real X in biological objects requires special theoretical approaches. 

Another reason for a deviation from relationships expected for a homogeneous nonconducting medium is the difference in reorganization energy for the same Ru-complexes located in the protein-water interface of different local dielectric constant and local electrostatic potentials. 

The second and third parts of this review develop, through correlation of the results described in the first part, a picture of the hydration process and the hydration shell (Section VI) and an assessment of how the hydration shell may modulate enzyme and other protein functions (Section VII). The literature on protein hydration is now rich enough to provide a comfortably detailed picture of the protein-water interface. The ways in which the interface enters into function are just beginning to emerge, and one purpose here is to point out directions in which one may move to understand better the relationship to function. Sections VI and VII are meant to stand alone as summary statements, and some overlap with the preceding sections describing results should be expected. 

Threads, clusters, or networks of water molecules have been detected by crystallographic, thermodynamic, and dynamic measurements. They appear to be a common feature of the protein—water interface, which should not be surprising, in view of the extensive hydrogen bonding of the bulk water. Moreover, the water of the interface differs from... 

In their native conformation, globular proteins have non-polar amino acid side chains oriented towards the interior of the protein and polar side chains oriented outwards, towards the solvent. The stability of the native conformation is determined by hydrophobic interactions within the interior of the molecule, and electrostatic interactions and hydrogen bond interactions at the protein-water interface. Disturbing these interactions can alter the balance between the intra- and intermole-cular interactions, which are responsible for maintaining the protein in soluble... 

Fig. 14.1 Scheme and plots illustrating the dependence of nanoscale-confinement parameters T and/on the radius 6 of the osculating sphere at the protein-water interface. First-order contacts with a polar or nonpolar patch on the protein surface are treated individually. Values were determined at equilibrium obtained by integrating Newton s equations of motion in an NPT ensemble with box size 103 nm3, starting with the PDB structure embedded in a pre-equilibrated cell of water molecules. The box size was calibrated so that the solvation shell extended at least 10 A from the protein surface at all times. Simulations were performed as described in Chap. 4... 

Thus, the overall free energy change AG associated with creating the protein-water interface by transferring the water molecules from the bulk to the interface is... 

To summarize, three conclusions transpire from the nanoscale thermodynamics results (a) The interfacial tension between protein and water is patchy and the result of both nanoscale confinement of interfacial water and local redshifts in dielectric relaxation (b) the poor hydration of polar groups (a curvature-dependent phenomenon) generates interfacial tension, a property previously attributed only to hydrophobic patches and (c) because of its higher occurrence at protein-water interfaces, the poorly hydrated dehydrons become collectively bigger contributors to the interfacial tension than the rarer nonpolar patches on the protein surface. 

Reliable circular dichroism spectra cannot be obtained from crystalline, oriented polypeptide material that is birefringent or scatters light. However, monolayers of protein do not appear to provide anomalous spectra (4,15), probably because of the relative uniformity of the film and the minimal refractive index increment at the protein/water interface. Nevertheless, previous results from this and other laboratories (3,16,17) have shown that albumin and fibrinogen molecules tend to lie flat, that is, with their long axes parallel to the surface, whereas globulins lie perpendicular to the surface. 

Approaches for modeUng intermolecular interactions. J Comput Chem 12 811-828 Gerstein M, Chothia C (1996) Packing at the protein-water interface. Proc Natl Acad Sci USA 93 10167-172... 

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