The circular dichroism of aromatic polypeptides: Theoretical studies of poly-L-phenylalanine and some para-substituted derivatives

We have calculated rotational strengths and circular dichroism (CD) curves for sidechain and backbone transitions in poly-L -Phenylalanine (PLP), POLY-p-amino-L -phenylalanine (PPALP), poly-p-chloro-L -phenylalanine (PPCLP), poly-o-acetyl-L -tyrosine (POALT), and poly-p-nitro-L -phenylanine (PPNLP), using methods applied previously to poly-L -tyrosine (PLT). Comparison of the theoretical CD curves with available experimental data for PLP and PPALP indicate that these polypeptides form right-handed helices with side-chain conformations similar to that of PLT. For PPNLP, where experimental data are also available, no conformational assignment could be made, as none of the calculated curves gave good agreement with experiment. Possible reasons for this lack of agreement are discussed. For the other two polypeptides, PPCLP and POALT, although no experimental data are yet available, the calculated curves indicate that an unambiguous assignment should be possible. For the conformations (RA and LA) in which the side chains are packed more loosely, there are strong similarities in the calculated CD curves of a particular conformation, regardless of the para substituent. In the tighter R1 and L1 conformations, few generalizations can be drawn, each derivative having a distinctive pattern. In PLP, PPCLP, and POALT, where the side-chain L^a band is in the 200–210 nm region, the L1 conformation exhibits a negative nπ* rotational strength, opposite to that expected for a left-handed helix. One must therefore be cautious about assigning the helix sense of aromatic polypeptides on the basis of the sign of the nπ* CD band. Side-chain nπ* transitions present in POALT and PPNLP were found to have small rotational strength.