The optical properties of tyrosine-containing cyclic dipeptides

The chiroptical properties of the cyclic dipeptides cyclo (L -alanyl-L -tyrosine) and cyclo(L -tyrosyl-L -tyrosine) have been investigated as a function of molecular conformation. Theoretical optical calculations and conformational energy calculations have been carried out as a function of the side-chain dihedral angles χ¹ and χ², and as a function of the angle of fold of the cyclic dipeptide backbone. The results of these theoretical calculations have been compared with experimental circular dichroism (CD) data. Theoretical predictions are in very good agreement with experiment for c(L -Tyr-L -Tyr). Agreement was not quite as good for c(L -Ala-L -Tyr), although the signs of all of the Cotton effects were apparently predicted correctly except for that associated with the lowest energy tyrosine absorption band.