Hydrogen disulfide structure

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. 

H2S2 (hydrogenpersulfide), the smallest member of the polysulfane series [15], has been studied extensively by molecular spectroscopy and theoretical calculations [16] (and references therein). By now, accurate knowledge of its structure, torsional potential and vibrational modes has been established. Ab initio calculations readily reproduce these properties [17]. The value of the SSH angle in hydrogen disulfide was a subject of controversies for some time. However, recent experiments led to a different value which is in favour of the ab initio calculated value [17]. 

The hydrogen-bonded structural units, plus the disulfide bonds, constitute the secondary structure of the protein. 

Assuming covalent bonds, write electronic structures for the molecules GIF (chlorine fluoride), BrFg (bromine trifluofide), SbCl5 (antimony penta chloHde), HgSg (hydrogen disulfide). In which of these molecules are there atoms with electron configurations that are not noble-gas configurations ... 

In the case of hydrogen disulfide, the S—S bond length as determined by Stevenson and Beach (211) from electron diffraction indicates that branching does not occur. Electron diffraction studies by Palmer (184) and Guthrie (188) on disulfur dichloride show an unbranched, nonplanar structure. Smyth (206) has discussed the dipole moments of hydrogen disulfide, disulfur dichloride, and diselenium dichloride on the same basis and Hooge and Ketelaar (145), the vibrational spectra of hydrogen disulfide and disulfur difluoride, dichloride, and dibromide. 

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).

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