Optical activity of polypeptides in the infrared. Predicted CD of the amide I and amide II bands

Publisher Website
Google Scholar
Abstract
The Miyazawa‐Blout‐Krimm (M‐B‐K) treatment of polypeptide absorption in the infrared is extended to the calculation of circular dichroism (CD), linear dichroism, and oriented CD for the amide I and amide II transitions. Matrix methods are applied to the α helix and β structures using measured values for the strengths and directions of the transition dipole moments and empirical values from M‐B‐K for the coupling constants.Relatively small aggregates, a 36‐residue helix, and 8‐chain × 4‐residue β sheets, are large enough to show calculated absorption agreeing with M‐B‐K results, which are based on infinite lattices.In all cases the predicted CD is an approximately conservative couple. The strongest CD should appear in the α helix, Δε/ε ≃± 10−3 for both transitions. The amide II transition should show moderate CD couples in both β structures, Δε/ε ≃ (+2 to −1) × 10−4. The amide I transitions in β structures should show weak CD couples, Δε/ε = (+3 to −2) × 10−5, except that the negative branch in the antiparallel structure may be detectable (Δε/ε ≃ −2 × 10−4) because absorption is very low at its wavelength peak.CD on oriented samples should be enhanced over the unoriented cases, giving values as large as Δε/ε = 3 × 10−3 because particular directions of observation allow the light to avoid much of the absorption in the sample.If all three structures are considered as helices, then the larger distance of the transition dipoles from the axis in the α helix, and the orientations of the transitions in the different structures, are the factors that, in terms of our previous theoretical work [Snir and Schellman (1973) J. Phys. Chem. 77, 1653] satisfactorily explain the calculated results. Simple dipole–dipole interaction is calculated to make a substantial contribution to the coupling between groups.

This publication has 26 references indexed in Scilit: