Critical slowing down in periodically perturbed chemical oscillations

Abstract

The differences in the spectra are most likely due to differences in the secondary structures of the molecules. Poly(dA).poly(dT) belongs to the B-form family but shows some distinct features, e.g., a hydration spine and a much narrower minor groove compared with “normal” B-DNA.26,27 Indeed, the low-frequency Raman spectrum of poly(dA).poly(dT) is anomalous compared with the other B-form spectra. This is not due to the high AT content since the spectrum of poly(dA-dT).poly(dA-dT) is like that of any other B-form double helix. The 63-cm-] mode in the B-form polynucleotides shifts to about 70 cm-’ and becomes considerably more intense whereas the other modes do not shift considerably compared to spectra of “normal” B-DNA. This effect cannot be explained with the narrowing of the minor groove alone since C-DNA spectra are nearly identical with those of B-DNA,29 although the minor groove has narr~wed.~’ Another example for the influence of the secondary DNA structure on the low-frequency vibrations is poly(rC). At pH 7 poly(rC) forms a single helix with 6 bases per turn, whereas at pH 4 it forms a parallel double helix (probably with - 12 base pairs per turn). The most prominent mode in the double-helical form is at 147 cm-I, whereas in the single-helical form it is well below 100 cm-I. These differences are not due to the formation of hydrogen bonds (see above) and must therefore represent the changes in RNA conformation. These observations can, so far, not be related to specific interactions within the DNA, or to “simple” motions of the single or the double helix. Torsional modes, for example, should be (30) Arnott, S.; Bond, P. J.; Selsing, E.; Smitch, P. J. C. Nucleic Acids