Polar molecules, interaction with water

Closely related to the London interaction is the dipole-induced-dipole interaction, in which a polar molecule interacts with a nonpolar molecule (for example, when oxygen dissolves in water). Like the London interaction, the dipole—induced-dipole interaction arises from the ability of one molecule to induce a dipole moment in the other. However, in this case, the molecule that induces the dipole moment has a permanent dipole moment. The potential energy of the interaction is... 

Nonpolar (hydrophobic) compounds dissolve poorly in water they cannot hydrogen-bond with the solvent, and their presence forces an energetically unfavorable ordering of water molecules at their hydrophobic surfaces. To minimize the surface exposed to water, nonpolar compounds such as lipids form aggregates (micelles) in which the hydrophobic moieties are sequestered in the interior, associating through hydrophobic interactions, and only the more polar moieties interact with water. 

Most electrically charged groups are on the surface of the molecule, interacting with water. Exceptions to this rule are often catalytically important residues in enzymes, which may well be partially stabilized by specific polar interactions within a hydrophobic portion of the molecule. 

Also obviously true, this historical interpretation of lipid-lipid interactions did not take into accotmt the hydrophobic effect. Conversely, the hydrophobic effect by itself would be ineffective if London forces did not stabilize the interactions between aliphatic chains. A wise position is to consider both. Lipids are brought together first because of the hydrophobic effect, which induces a selective orientation of lipid molecules with their polar part interacting with water and their apolar domain rejected on the opposite side. Then London forces stabilize the... 

Water is highly polar, but it is not ionic. How, then, can water act as a solvent for ionic solids A salt dissolves only if the interactions between the ions and the solvent are strong enough to overcome the attractive forces that hold ions in the ciystal lattice. When an ionic solid forms an aqueous solution, the cations and anions are solvated by strong ion-dipole interactions with water molecules. 

Among the common amino acids, eleven have side chains that contain polar functional groups that can form hydrogen bonds, such as —OH, —NH2, and — CO2 H. These hydrophilic amino acids are commonly found on the outside of a protein, where their interactions with water molecules increase the solubility of the protein. The other nine amino acids have nonpolar hydrophobic side chains containing mostly carbon and hydrogen atoms. These amino acids are often tucked into the inside of a protein, away from the aqueous environment of the cell. 

A luminescent unit extensively used to functionalize dendrimers is the so-called dansyl (5-dimethylamino-l-naphthalenesulphonamido group). Dendrimers (up to the third generation, compound 9) containing a single dansyl unit attached off center [39] show that this fluorescent unit, which is very sensitive to environment polarity, is progressively shielded from interaction with water molecules as the dendrimer generation increases. 

The intracellular and plasma membranes have a complex structure. The main components of a membrane are lipids (or phospholipids) and different proteins. Lipids are fatlike substances representing the esters of one di- or trivalent alcohol and two aliphatic fatty acid molecules (with 14 to 24 carbon atoms). In phospholipids, phosphoric acid residues, -0-P0(0 )-O-, are located close to the ester links, -C0-0-. The lipid or phospholipid molecules have the form of a compact polar head (the ester and phosphate groups) and two parallel, long nonpolar tails (the hydrocarbon chains of the fatty acids). The polar head is hydrophihc and readily interacts with water the hydrocarbon tails to the... 

Y. Ferro, A. Allouche, and V. Kempter, Electron solvation by highly polar molecules Density functional theory study of atomic sodium interaction with water, ammonia, and methanol. J. Chem. Phys. 120, 8683 8691 (2004). 

In this section, we explore how water molecules in the liquid phase interact with one another via cohesive forces, which are forces of attraction between molecules of a single substance. For water, the cohesive forces are hydrogen bonds. We also explore how water molecules interact with other polar materials, such as glass, through adhesive forces, forces of attraction between molecules of two different substances. 

Fig. 90. (A) Glycine-tyrosine bound to carboxypeptidase (443). Indirect attack of Glu-270 promotes the attack of a water molecule on the amido carbonyl group polarized by interaction with zinc. (B) Direct attack of Glu-270 on the amido carbonyl with formation of an anhydride.

Molecules may interact with water in at least four main ways—hydrogen bonding, ionic bonding, hydrophobic association (nonpolar molecules placed in the polar environment of water), and l.ondon dispersion or van der Waals forces. 

Carboxylic acids have an important practical use in the form of their metal salts as soaps. We have mentioned how fats, which are 1,2,3-propanetriol (glyceryl) esters of long-chain acids, can be hydrolyzed with alkali to give the corresponding carboxylate salts. It has been known as far back as Roman times (Pliny) that such substances have value for cleaning purposes.8 These salts have a complicated interaction with water because they are very polar at the salt end of the molecule and very nonpolar at the long-chain hydrocarbon end of the molecule. These hydrocarbon ends are not compatible with a polar solvent such as water.4... 

If a material of polar molecules, such as water, is exposed to a fixed or static electric field, the molecules will all rotate in an attempt to orient themselves in the direction of the field. The magnitude of separated charges of a polar molecule is defined as the dipole moment, and determines the strength of interaction with the field. The dipole moment is also a measure of the dielectric constant e. A symmetrical molecule, with no dipole moment, is said to be non-polar and does not react with an electric field. If an electric field impinging upon a polar molecule is alternating, the molecules will rotate, following reversals of field. 

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