Disulfide hydrogen bonds

D-ribofuranosyl)-uracil-5-yl] disulfide Hydrogen-bonded complex of 2 -deoxyguanosine and 5-bromo-2 -deoxycytidine C-3 C-2 endo C-2 endo C-4 0.454 0.540 102... 

Hydrogen bonding stabilizes some protein molecules in helical forms, and disulfide cross-links stabilize some protein molecules in globular forms. We shall consider helical structures in Sec. 1.11 and shall learn more about ellipsoidal globular proteins in the chapters concerned with the solution properties of polymers, especially Chap. 9. Both secondary and tertiary levels of structure are also influenced by the distribution of polar and nonpolar amino acid molecules relative to the aqueous environment of the protein molecules. Nonpolar amino acids are designated in Table 1.3. 

Equation (8.97) shows that the second virial coefficient is a measure of the excluded volume of the solute according to the model we have considered. From the assumption that solute molecules come into surface contact in defining the excluded volume, it is apparent that this concept is easier to apply to, say, compact protein molecules in which hydrogen bonding and disulfide bridges maintain the tertiary structure (see Sec. 1.4) than to random coils. We shall return to the latter presently, but for now let us consider the application of Eq. (8.97) to a globular protein. This is the objective of the following example. 

Figure 17.11 Structure of EMPl dimer from x-ray crystallography. In the presence of EBP, the EMPl peptide forms a dimer. Each monomer (shown in red and blue) forms a p hairpin structure stabilized by hydrogen bonds (red dashes) and a disulfide bond (yellow).

Also important for stabilizing a protein s tertiary stmcture are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the presence of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein. 

If the principal cohesive forces between solute molecules are London forces, then the best solvent is likely to be one that can mimic those forces. For example, a good solvent for nonpolar substances is the nonpolar liquid carbon disulfide, CS2-It is a far better solvent than water for sulfur because solid sulfur is a molecular solid of S8 molecules held together by London forces (Fig. 8.19). The sulfur molecules cannot penetrate into the strongly hydrogen-bonded structure of water, because they cannot replace those bonds with interactions of similar strength. 

The data suggest that the hydroxyl groups of 1 are involved in hydrogen bonds to carbonyl oxygens of 32 a, as is shown in Scheme 1. Moreover, although 32 is easily oxidized to disulfide 34 in solution and in the solid state, 32a in 33 is stable. 

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