Oligomers rotational strength

Figure 5 shows schematically the distributions of rotational strengths predicted by Equation 2 for adenylate oligomers as single-chain helices with eight residues per turn (a = 45°). To estimate the band splittings, we have used the fact that the nearest-neighbor theory predicts that the absorption band of the infinite, stacked-base helix will be shifted from the monomer band by an amount (2 V cos a)/h. Since this shift is experimentally known, V can be eliminated from Equation 1. 

The predicted features of the dichroic spectra are in good agreement with observations for the adenylate oligomers in neutral solution. Taking the simplest case, the dimer, we see that there should be two bands of opposite sign, spaced above and below the position of the monomer band their rotational strength should be given by... 

Figure 5. Schematic distributions of rotational strength as predicted by Equation 3 for first several adenylate oligomers...

Band positions calculated as explained in text, a assumed to be 45°, Relative values of rotational strength given on basis of mole of oligomer... 

Figure 6. Rotational strength of positive dichroic hand at 0°C. as a function of chain length N for adenylate oligomers...

As expected, poly HEA cannot adopt this form under any conditions examined. It can be observed for poly A at pH values below 5 and at pH 4.5 all of the oligomers above the hexamer demonstrate circular dichroism characteristic of the two-chain helix. This is perhaps most clearly demonstrated in Figure 6, where the rotational strength increases suddenly at N = 7 in solutions at pH 4.5. On the other hand, at pH 7, the rotational strength increase smoothly with chain length from N = 2 to infinity. 

Figure 8. Change in rotational strength of positive dichroic hand with temperature for some adenylate oligomers at pH 7.4 Chain length indicated by legend...

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