Conformation of Asn-Ala-Gly-Ala-Asn and its derivatives in solution--a CD spectra analysis.

The CD method has been used to study the conformation of the pentapeptide Asn-Ala-Gly-Ala-Asn (NAGAN) and the tetrapeptides Asn-Ala-Gly-Ala (NAGA) and Asn-D-Ala-Gly-Ala-NH2 (NDAGA) in solution. At low pH the far UV CD spectra of aqueous solutions of NAGAN show a positive extremum at 215 nm and a negative extremum at 232.5 nm. Under this condition, the molecule of NAGAN may assume a beta-turn structure. In the solution of 90% ethanol or tri-fluoroethanol at pH 5.8, NAGAN reveals CD spectra with a negative 218 nm extremum. The[theta]218 values are independent of the concentration of NAGAN. Au intramolecular beta-pleated sheet-like structure is suggested to explain the negative peak. The CD spectra of NAGA show a single positive band at around 215 nm. The possibility of forming a beta-turn by NAGA has been discussed. Only the 196 nm negative extremum can be seen in the spectra of NDAGA. It seems as if NDAGA is in the form of a random coil. According to the report made by Pan et al. all these peptides possess analgesic activity. Therefore, it seems that the analgesic activity is not related to the special conformation of the peptides.