Other Biologically Important Hydrogen Bonds

FIGURE 2.9 Hydrogen bonding between polar groups and water. 

Examples of Major Types of Hydrogen Bonds Found in Biologically Important Molecules 

How Basic Chemistry Affects Life The Importance of the Hydrogen Bond 

Many noted biochemists have speculated that the hydrogen bond 

If we look at the solubility of a simple ion like Na or Cl , we find that water is attracted to these ions by polarity. In addition, other water molecules form H bonds with those surrounding water molecules, typically 20 or more water molecules per dissolved ion. When we consider a simple biomolecule such as glyc-eraldehyde, the H bonds start at the molecule itself. At least eight water molecules bind directly to the glyceraldehyde molecule, and then more water molecules bind to those eight. 

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Limitations of MNDO. From its inception, some important limitations of MNDO were apparent. Sterically crowded molecules were calculated too unstable for example, the AHf of neopentane is predicted by MNDO to be —24.6 kcal/mol, compared with the observed -40.3 kcal/mol. On the other hand, four—membered rings were predicted to be too stable, this reaching a limit in cubane, which was predicted to be 49.6 kcal/mol too stable. Later on, other limitations were discovered, the most important from a biochemical standpoint being the virtually complete lack of a hydrogen bond. Other deficiencies included the extreme instability of hypervalent molecules. This effectivdy precluded the application of MNDO to organophosphorus compounds of biologic interest. Finally, activation barriers were predicted to be too high. 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

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