Cyclic Peptide with Ionic Denaturant

Interactions of Cyclic Peptide with Ionic Denaturant 

The structure of Cyclo-(Qy-X-Gly)2, where X = Qu, Glu(OMe), Gtu(OBzl), Asp, and Asp(OBzl) in DMSO or water, has been discussed in Section 3.8.1. It was wn that th take the C2-symmetric conformation consisting of two -turns with the X residues residing at the comer position and a pair of glycyi residues preceding the intramolecularly l drogen-bonded X residues. 

Since the anion of the added salt is not well solvated in DMSO, it will tend to associate with the peptide proton. This may also induce a change in conformational distribution. 

The peptide NH protons which became more solvent-exposed as a result of conformational change are subjected to the polar effect of solvent. The downfield shift due to the polar effect is proportional to the square of the electric field 134). The Gly-NH proton of Cyclo-(Sar-Gly) is etqiosed to solvent and the downfield shift 

IS ppm (see Fig. 28) could be conadered as the polar effect vdiich the present cydk hexapeptides suffer. As a consequence, the sum of the m etic anisotropic effect and the polar effect amoimts to 0.37 ppm. 

The rest of the downfield shift (ca. 0.3 ppm) could be explained in terms of the increasing extent of the hydrogen bonding of the peptide NH protons with solvent. The exposed NH will tend to associate also with the anion of the added salts which is not well solvated in DMSO. In so far as the immediate interaction of NH with anion resembled hydrogen bonding, it will give rise to a downfield shift. Fig. 29 shows that the temperature coefficient of the chemical shift for the intramolecularly 

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It has been own that the effects of salts on the cyclic peptides ate quite different in water and in DMSO. This information may be useful when one deals with the interactions of ionic denaturant and linear polypeptides. 

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