Alcohol effects on peptide conformations

The solvent is often treated as a continuum with a particular dielectric constant. This continuum model is, however, not capable of describing the solvent effects. For example, acetonitril and methanol share about the same dielectric constant, but they exhibit fairly different solvation properties for many solutes. When the continuum model is adopted, the solvent has no effects except that the Coulombic interaction between atoms in a solute molecule is simply divided by the dielectric constant. Notice, however, that the division is valid only when the distance between the atoms is far larger than the atomic scale. The solvent effects are sensitively dependent on the conformation of the solvent molecule as well as that of the solute molecule and on the density and ori- 

2 Conformational stability of Met-enkephalin in water, methanol, and ethanol [44] 

The two conformations shown in Fig. 3-8, conformations 1 and 2, are chosen for C-peptide. Conformation 1 has the ct-helix structure, while conformation 2 is almost fully extended and has no intramolecular hydrogen bonds. Just for C-peptide in water, however, the temperature was set at 273 K and pyd is 0.7338. The temperature difference is minor and not likely to alter our conclusions. 

As mentioned above, the total energy is an index of the conformational stability of peptides in solvent. Table 3-10 gives Eg, A/is, and A t for conformations 1 and 4 of Met-enkephalin. Conformation 1 has intramolecular hydrogen bonds. Conformation 4 is almost fully extended and has no intramolecular hydrogen bonds. Aixs,w, AiJ,s,u, and A s.e denote the solvation free energies in water, methanol, and ethanol, respectively. fi x.W) t,m t,e denote the total energies in water. 

2 of C-peptide. Conformation 1 has a-helix structure, but conformation 2 is almost fully extended and has no intramolecular hydrogen bonds. The values in water and methanol are given. In terms of Er, conformation 2 is more stable by 87kcal/mol in water, but only by 15kcal/mol in methanol. Thus, the conformations with intramolecular hydrogen bonds are much more stabilized in alcohol than in water. 

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