Circular dichroism of oriented α helices. I. Proof of the exciton theory

Abstract

Moffitt’s exciton theory for α helices, an important cornerstone of the circular dichroism (CD) theory for biopolymers, was recently cast in doubt by a linear dichroism measurement of electric field oriented polypeptides [Yamaoka et al., J. Am Chem. Soc. 108, 4619 (1986)]. In particular the prediction that the polarization of an exciton split component at 208 nm should be parallel to the α-helical axis was not borne out. This revealed the inadequacy of previous experiments which used long polypeptides to prove the theory. We performed two experiments to measure the effect of orientation of α helices on CD, one with a short membrane peptide, alamethicin, oriented in defect-free multibilayers and another with long polypeptides oriented with electric field. We found the result of the experiment with alamethicin to be consistent with the exciton theory. An elaborate procedure was established to measure the CD of protein molecules embedded in lipid multibilayers with light incident on bilayers at various angles (the helical sections of alamethicin are perpendicularly embedded in bilayers). Thus we are able to measure the polarization of the 208 nm band and prove that it is indeed parallel to the α-helical axis. The problem of electric field oriented polypeptides is discussed in paper II.

Structure-Activity Analysis of the Dermcidin-derived Peptide DCD-1L, an Anionic Antimicrobial Peptide Present in Human Sweat

“…Our data imply that self-assembly of DCD-1L can lead to the formation of ion channels, with a diameter similar to or even smaller than the barrel-stave model of CAMPs (43). Several studies have previously shown that peptide self-assembly in the membrane-bound state correlates with antimicrobial activity (30), whereas complex formation in an aqueous environment had no effect on antimicrobial activity (44).…”

Interaction of Antimicrobial Peptides with Lipopolysaccharides

“…OCD has been successfully used to study the physical state of the peptides embedded in phospholipid bilayers. OCD determines not only the secondary structure of a peptide but also its orientation in membranes (15,16). The ease of OCD measurement makes it a very effective method for probing the peptide orientation in membranes, particularly if the orientation changes with membrane conditions.…”

“…For most experiments as in this one, normal (rather than oblique) incidence OCD is sufficient for the orientation analysis (16). OCD has been used to determine the orientation of various helical peptides including alamethicin (15)(16)(17)27), magainin (28), melittin (29) and synthetic helices (30), -sheet peptides protegrins (31), and cyclic peptides θ-defensins (32). LPS bilayers have a strong OCD.…”

“…Oriented circular dichroism (OCD) is a simple method for detecting the orientation of peptides embedded in lipid bilayers using a conventional CD machine (15,16). A Jasco J-500A spectropolarimeter was used for this experiment.…”

“…We then used the method of oriented circular dichroism (OCD) (15,16) to study the orientation of antimicrobial peptides in LPS bilayers. This was the method that discovered peptides changing orientation as a function of peptide concentration in phospholipid bilayers (17).…”