Terminology

Amphipathic - For a molecule, the property of having both hydrophobic and hydrophilic portions. Usually one end or side of the molecule is hydrophilic and the other end or side is hydrophobic.

Ampholyte - A substance containing both acidic and basic groups (Figure 2.19).

Buffer - A mixture of an acid and its conjugate base at a pH near their pKa to minimize pH changes caused by an influx of acid or base.

Clathrate structures - Hydrophobic molecules that dissolve in aqueous solutions form regular icelike structures called clathrate structures rather than the hydration shells formed by hydrophilic molecules (Figure 2.13).

Coulomb's Law - Describes the force between two charges in a vacuum. The force, F, is defined as F = k* (q 1 q 2)/r2, where q 1 and q 2 are the charges and r is the distance between the charges.

Covalent bonds - The chemical bonds between atoms in an organic molecule that hold it together are referred to as covalent bonds.

Debye-Huckel radius - A quantitative expression of the screening effect of counterions on spherical macroions. r = K / I0.5

Dielectric constant - A constant, called , that modifies Coulomb's law to account for shielding of molecules between the charges in a non-vacuum. With this modification, the force, F, between charges in a dielectric medium (non-vacuum) is F = k* (q 1q 2)/(*r2), where q 1 and q 2 are the charges and r is the distance between the charges.

Dipole Moment - Molecules which have an asymmetric distribution of charge are dipoles. The magnitude of the asymmetry is called the dipole moment of the molecule (Figure 2.4).

Energy of Interaction - The energy of interaction, U, for two charged particles is the energy required to separate them from a distance, r, to an infinite distance. It is a measure of the energy required to overcome the electrostatic forces between them. U = k*(q 1 q 2)/(*r), where k is a constant, and q 1 and q 2 are the charges.

Henderson-Hasselbalch equation - pH = pK a + log ( [A-]/[HA] )

Hydration shell - The interactions of dipoles with cations and anions in aqueous solution causes the ions to become hydrated - surrounded by shells of water molecules called a hydration shell.

Hydrogen bond - An attractive interaction between the hydrogen atom of a donor group, such as OH or =NH, and a pair of nonbonding electrons on an acceptor group, such as O=C. The donor group atom that carries the hydrogen must be fairly electronegative for the attraction to be significant (Figure 2.7).

Hydrophilic - Refers to the ability of an atom or a molecule to engage in attractive interactions with water molecules. Substances that are ionic or can engage in hydrogen bonding are hydrophilic. Hydrophilic substances are either soluble in water or, at least, wettable.

Hydrophobic - The molecular property of being unable to engage in attractive interactions with water molecules. Hydrophobic substances are nonionic and nonpolar; they are nonwettable and do not readily dissolve in water.

Induced Dipole - A molecule, such as benzene, which has a symmetric shape and no dipole moment in the absence of external interactions, can have a slight redistribution of electronic charge due to interaction with an electric field. This causes formation of an induced dipole in the symmetric molecule.

Ionic Strength - Counterions provide shielding for macroions. For the purposes of the Debye-Huckel theory, the ionic strength, I, is defined as Equation 2.23.

Isoelectric focusing - A technique for separating ampholytes and polyampholytes based on their pI. Also used to determine pI.

Isoelectric point - The pH at which an ampholyte or polyampholyte has a net charge of zero. Same as pI.

Macroions - A collection of polyelectrolytes and/or polyampholytes. Nucleic acids and proteins are macroions.

Noncovalent interactions - Attractive or repulsive forces, such as hydrogen bonds or charge-charge interactions, which are non-covalent in nature, are called noncovalent interactions.

Permanent Dipole - Molecules, such as water, which have an asymmetric distribution of electronic charge due to molecular geometry and electronegativity differences between atoms, are permanent dipoles.

pH - The negative logarithm of the hydrogen ion concentration in an aqueous solution (Figure 2.16).

pI - The pH at which an ampholyte or polyampholyte has a net charge of zero. Same as isoelectric point.

pK a - The negative logarithm of the dissociation constant for an acid or base in aqueous solution.

Polyampholyte - Molecules which contain multiple acidic and basic groups.

Polyelectrolyte - Macromolecules, such as polylysine, which carry multiples of only positive or negative charge.

Polyprotic acids - Acids which are capable of donating more than one proton.

Salting out - The effect of an extreme excess of ions which makes proteins precipitate from solution.

Salting in - The effect of moderate amounts of ions, which increases the solubility of proteins in solution.

van der Waals Radius - The effective radius of an atom or a molecule that defines how close other atoms or molecules can approach to it; it is thus the effective radius for closest molecular packing (Figure 2.2).