Polarizability Theories of Polynucleotide Hypochromism

Abstract

WE have read with interest the preceding communication but we have the impression that DeVoe has misunderstood to some extent both Nesbet's and our work. In the first part of his communication, DeVoe refers to the well-known classical equation for a pair of coupled oscillators with regard to a given direction of the electric field. However, in our treatment of the dimer, it is explicitly stated that our equation refers to isotropic oscillators and averaged over all directions of the electric field vector. We have, in fact, calculated the absorption coefficient for the frequency corresponding to the maximum absorption in the monomer. However, in the case under consideration, this also corresponds to the frequency of maximum absorption in the dimer. In fact, the absorption due to the longitudinal dipole oscillations in the dimer is shifted in frequency (without change of intensity or width) and that due to the transverse oscillations is shifted in the opposite sense, but the mean frequency for the isotropic case (one longitudinal plus two transverse polarizations) is not changed. In the hypothetical case of a very narrow band, this would appear as a splitting, or if the monomer band is already visibly split this would result in opposite displacements of the components. In the cases under consideration we were concerned with a situation corresponding to nucleic acid, where the absorption bands would be too broad (half-width ∼ 300 A) for this splitting to be observed. Thus the net effect here is a reduction of the absorption at the peak, which, however, should be compensated by some additional broadening elsewhere so as to preserve the total oscillator strength; there is therefore no violation of the sum rule.