6 Give three examples of condensation reactions that occur i

6. Give three examples of condensation reactions that occur in the cell to form polymeric macromolecules from monomeric precursors. For one of these examples, draw the structure of the reactants (before the reaction) and the product (after the reaction).

10. What are the main noncovalent interactions that hold molecules together? Explain how these noncovalent interactions are important to allow molecules to bind to each other.

12. You are working in a lab and studying the characteristics of a particular protein. You have paid for a commercial company to synthesize the protein for you. Somehow, they have mistaken your instructions and synthesized it with the amino acids in the reverse order. A friend of yours says that since the same amino acids were used, it should be fine to characterize this protein. What do you think?

Solution

10.There are four main types of noncovalent bonds in biological systems: hydrogen bonds, ionic bonds, van der Waals interactions, and hydrophobic bonds.

A Hydrogen bond is a noncovalent bond between an electronegative atom (commonly oxygen or nitrogen) and a hydrogen atom covalently bonded to another electronegative atom. They are Particularly important in stabilizing the three-dimensional structure of proteins and formation of base pairs in nucleic acids.Hydrogen bonds among water molecules are largely responsible for the properties of both liquid water and the crystalline solid form (ice). An important feature of all hydrogen bonds is directionality. In the strongest hydrogen bonds, the donor atom, the hydrogen atom, and the acceptor atom all lie in a straight line. Nonlinear hydrogen bonds are weaker than linear ones; still, multiple nonlinear hydrogen bonds help to stabilize the three-dimensional structures of many proteins. It is only because of the aggregate strength of multiple hydrogen bonds that they play a central role in the architecture of large biological molecules in aqueous solutions

Ionic bonds result from the electrostatic attraction between the positive and negative charges of ions. In aqueous solutions, simple ions of biological significance, such as Na+, K+, Ca2+, Mg2+, and Cl, do not exist as free, isolated entities. Instead, each is surrounded by a stable, tightly held shell of water molecules. An ionic interaction occurs between the ion and the oppositely charged end of the water dipole.

Ions play an important biological role when they pass through narrow, protein-lined pores, or channels, in membranes. For example, ionic movements through membranes are essential for the conduction of nerve impulses and for the stimulation of muscle contraction.

Vander wauls interaction is A weak noncovalent attraction due to small, transient asymmetric electron distributions around atoms (dipoles).

Van der Waals interactions, involving either transient induced or permanent electric dipoles, occur in all types of molecules, both polar and nonpolar. In particular, van der Waals interactions are responsible for the cohesion between molecules of nonpolar liquids and solids, such as heptane, CH3—(CH2)5—CH3, that cannot form hydrogen bonds or ionic interactions with other molecules. When these stronger interactions are present, they override most of the influence of van der Waals interactions. Heptane, however, would be a gas if van der Waals interactions could not form.

Hydrophobic bonds occur between nonpolar molecules, such as hydrocarbons, in an aqueous environment. Hydrophobic bonds result mainly because aggregation of the hydrophobic molecules necessitates less organization of water into “cages” (and, hence, less reduction in entropy) than if many cages of water molecules had to surround individual hydrophobic molecules.