Calculated Circular Dichroism of Acridine Orange Bound to Random-Coil Polypeptide

Abstract

ABSTRACT:A theory of circular dichroism for the amide transition of polypeptide in random-coil conformation has been developed on the basis of the exciton theory of optical activity and the Monte-Carlo approach to the treatment of statistical systems. This theory has further been extended in such a way as to be applicable for electronic transitions of chromophores bound to the side chains of the random-coil polypeptides. Circular dichroism is calculated for some binding models of acridine orange-randomcoil polypeptide complexes. The results are compared with those observed, and it is found that the longer axes of the bound dye molecules are almost parallel or perpendicular to a polypeptide chain, depending on whether ionization of the polypeptide is low or high.Recently we reported that various types of circular dichroism (CD) are induced on acridine orange bound to poly(S-carboxymethyl-L-cysteine) in solution. 1 These are observed at different pH and [P]/[D] (polymer residue/added dye) mixing ratios, and by polymers of different molecular weights. The observed CD is characteristic of the bonding species of the dye, dimeric dye or monomeric dye, as well as of the conformation of the polymer and can be classified into several types on the basis of the number and sign of the CD bands.Type I CD is induced on an aggregate of dimeric dyes bound to a polymer in the pconformation and has a pair of bands at 466 and 433 nm. A model for the binding mode has been presented and discussed, 2 ' 3 in which the dimeric dye molecules attach to ionized carboxyl groups and stack together to form linear arrays on both sides of an extended polypeptide chain. Types II and III CD have a pair of bands at 470 and 420 nm and additional bands at longer wavelength. These bands are opposite sign to each other for these two types. Types II' and III' CD have bands at 540 and 425 nm with opposite sign to each other. While Types II and 11 1 CD are exhibited by the solutions of random-coil polymers with low ionization, Types III and III' CD are characteristic of highly ionized random-coil polymers and of solutions of low [P]/[D] ratios.The acridine orange-poly(L-glutamic acid) mixture also induces Types III and III' CD at low [P]/[D] ratios and alkaline pH. 4 ' 5 The CD spectra similar to Types II and III were recently reported for an acridine orange-glycosaminoglycan mixture by Salter, et al.,6 whose designation accidentally coincided with our's except for Type I.Several theoretical investigations 7 -10 have attempted to evaluate the optical activity for the amide re-re* and n-re* transitions of polypeptide in random-coil conformation. However, these have relied on different approximations and methods and, consequently, have yielded inconsistent results. In the present work, we will apply a method of calculation to the optical 541

Calculation of induced circular dichroism of acridine orange-poly(riboadenylic acid) complexes and confirmation of their structure

“…The half-width values obtained differ from those adopted previously. 13,14,21,22 The numerical values of the calculated electronic and geometric parameters of a poly(rA) strand are summarized in Table .…”