Protein bonds, intermolecular

Organic solvents such as ethyl alcohol and acetone are capable of forming intermolecular hydrogen bonds with protein disrupting the intramolecular hydrogen bonding. This causes precipitation of protein. 

The reactive site of the cysteinyl residue is the thiol group, which is deprotonated at alkaline pH (pXa around 8.5). The residue under oxidizing conditions (and neutral to alkaline pH) is able to react with a similar residue under formation of a disulfide bond. Many proteins are stabilized by intramolecular disulfide bonds (e.g., insulin, growth hormone, lGF-1), but intermolecular bonds may also result from the reaction under formation of aggregates. In order to avoid unintended disulfide bond formation/cleavage, the redox potential of the solution must be monitored and controlled. In practice, aqueous buffers contain micromolar amounts of dissolved oxygen assuring a redox potential of 200-600 mV, which is sufficient to maintain the intramolecular disulfide bonds. Proteins with free cysteines may... 

Proteins consist of 20, in some bacteria 22, different amino acids covalently linked with each other by peptide bonds. The amino acid side chains form many hydrophobic bonds, intermolecular hydrogen bridge bonds, and salt bridges within the protein chains or with another partner molecule. 

The increase in apparent viscosity with time points to slow rearrangements of protein structure and possibly to the formation of intermolecular bonds. A protein like /Mactoglobulin, which contains an —SH group, is known to be subject to —S—S— bond reshuffling, leading to bonds between molecules if these are close to each other. In an adsorbed layer, keeps increasing for days, leading to values well over 1 N s m 1. 

This aggregation may involve any of the known kinds of intermolecular protein bonds H—, hydrophobic, electrostatic, or covalent (S—S). Of these four, by far the strongest and the least likely to be reversed simply by raising the temperature is the S—S bond. 

Wavefunction s Spartan Student Edition (6) is used extensively in this course for building molecules and visualizing molecular shape, polarity, intermolecular interactions, hydrogen bonding, solubility, protein and nucleic acid structure. Spartan exercises are used to investigate the connection between the structure and shape of water and its polarity, intermolecular interactions, and... 

Quaternary Bonds These are weak intermolecular bonds (those between two different polymers). They stabilize the three-dimensional stmcture of a polymeric material, linking the different chains without forming covalent bonds (which would cause a network to form). This breakdown of bonding in proteins is essentially the same for all polymers, although commercial homopolymers do not display the same structural complexity of proteins. 

When thinking about chemical reactivity, chemists usually focus their attention on bonds, the covalent interactions between atoms within individual molecules. Also important, hotvever, particularly in large biomolecules like proteins and nucleic acids, are a variety of interactions between molecules that strongly affect molecular properties. Collectively called either intermolecular forces, van der Waals forces, or noncovalent interactions, they are of several different types dipole-dipole forces, dispersion forces, and hydrogen bonds. 

Figure 26.9 X-ray crystal structure of citrate synthase. Part (a) is a space-filling model and part (b) is a ribbon model, which emphasizes the a-helical segments of the protein chain and indicates that the enzyme is dimeric that is, it consists of two identical chains held together by hydrogen bonds and other intermolecular attractions. Part (cl is a close-up of the active site in which oxaloacetate and an unreactive acetyl CoA mimic are bound.

When modified fibres of type 5 are treated with hydroxylamine, oxime groups are also easily formed. The interaction with a protein affords a sandwich polymer22. Fibres modified in this way have enhances dyeability. When copolymer fibres are treated with diamine solutions or in acid medium with Fe+3 salts, intermolecular chemical bonds are formed, which results in a considerable increase of the temperature of zero strength and of the heat resistance of fibres. These conversions are shown in Scheme 2. 

The secondary structure of a protein is the shape adopted by the polypeptide chain—in particular, how it coils or forms sheets. The order of the amino acids in the chain controls the secondary structure, because their intermolecular forces hold the chains together. The most common secondary structure in animal proteins is the a helix, a helical conformation of a polypeptide chain held in place by hydrogen bonds between residues (Fig. 19.19). One alternative secondary structure is the P sheet, which is characteristic of the protein that we know as silk. In silk, protein... 

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