Interstitial water molecules

Fig. 12-7.—The arrangement of water molecules in the chlorine hydrate crystal. Some of the water molecules are at the corners of pentagonal dodecahedra, as indicated. Some additional water molecules (circles) are needed to complete the structure. Hydrogen bonds are formed along the edges of the d decahedra, and also between adjacent dodecahedra and between the dodecahedra and the interstitial water molecules.

As a rule, the distortion of the water lattice that is found in water without a solute (1) can easily take place in cooperation with the accompanying cation except in the cases of potassium, rubidium, and cesium. These ions are large enough to fill the cavities of the water lattice and to attenuate the lattice vibrations, thus preventing a local collapse of the structure and an increase in the number of interstitial water molecules. The normal water structure is essentially retained, and the lattice, stabilized by cations of the proper size, rejects the complex nonfitting ion (2). 

More crystallographic variations of the water models are the vacant lattice, framework or cage models which include interstitial water molecules within a four-connected network. In one of these models, the void in the ice-like structure is occupied by a water molecule which at any particular instant may be nonbonded... 

A similar model [728, 729] was proposed with an interstitial water molecule in the clathrate hydrate water structure. Both these models are now not seriously considered by the specialists in the field [730] because they are too crystalline. ... 

By analyzing experimental data Akmal and Munoz [18] conclude that the top of the free energy barrier is reached when the protein in its search for the native state reaches a critical native density , i.e. is close enough to the native state to expel the interstitial water molecules and form a folding nucleus. At that point the stabilization energy starts to overcome the decrease of conformational entropy. In addition, the expelled interstitial solvent gains translational entropy as well [19]. 

Before considering the positions occupied by hydrogen atoms we can make one further general observation about the ice structure. From the fact that the co-ordination number is only 4 and from the picture given by fig. 2.1 it can be seen that the structure is a very open one with a good deal of empty space—almost enough to accommodate interstitial water molecules. It is this feature which leads to the relatively low density of ice and which accounts for some of the properties of liquid water and of the high-pressure ices, to all of which subjects we shall return later. 

Fig. 2.6 A schematic description of a two-dimensional interstitial model for water. The lattice molecule occupies the vertices of the hexagons. The interstitial water molecules occupy the holes in the lattice.

Despite the fact that the exact structure of biological water is unknown, some important generalizations can be made, which are relevant to the biological role of water. First, compared with the ice crystal, the ice-cluster of water is small. Second, although even the ice crystal presents protionic defects, these defects are much more common in the water cluster. And, finally, within the water cage, the cluster contains interstitial water molecules bound by hydrogen bonds to an icelike structure. 

Several Prussian blue derivatives with molecular formula (M3[M (CN)6]2i here denoted as [M3M 2]) have been obtained with the aforementioned procedure. The FTIR analysis was obtained with a Bruker Vertex 80 FTIR Spectrometer in the 400 - 4000 cm firequency range. Band stretches of samples as well as the starting materials have been tabulated in Table 1. The asymmetric cyanide stretching of [Fe3Co2] shifts from 2125 to 2168 cm as expected for Prussian blue derivatives (Fig. 1). Similarly, the cyanide band in the 2000 - 2200 cm range also shifts to higher frequencies for the other samples compared to that of [Fe(CN)6] The broad band at aroimd 3500 cm and a sharp small band at 1600 cm confirms the presence of interstitial water molecules inside the cavities. Moreover, the sharp band at 400 - 500 cm is attributed to metal to cyanide bond. 

It is also noteworthy that the entropic penalty for trapping an interstitial water molecule is significantly overcompensated by the formation of hydrogen bonds with the interstitial water and the protein, in particular main chain hydrogen bond donor and acceptor groups. 

Figure 13.14 functional group considerations for the displacement of conserved, weakly bound interstitial water molecules [68],... 

If it is assumed that this relative constancy with temperature may indicate that and Km are large compared with p the last two factors in the logarithm of (A16) are unity. With the further assumptions that the number of interstitial water molecules in phase I is negligible (i.e., that is large) and that A// i and are negligible, we arrive at... 

If we start from Eq. (A 12) and adapt it to our simpler model in which only water molecule lattice sites are taken into consideration and interstitial water molecule sites are ignored, we get... 

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