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Additions and Corrections
The Journal of Physical Chemistry, Vol. 85, No. 5, 1981
the intercalation geometries of the two dyes differ. Specifically, the suggestion that the two amino groups of proflavin are hydrogen bonded to phosphate groups on 0.25’ opposite chains6J0has received support from X-ray diffraction data.39 This model could also explain the sen0.20sitivity of the CD of the intercalated PF species to salt 0.15concentration either in terms of the effect of changes in the counterion binding to phosphate groups on the hy0.10drogen bonding or in terms of conformational changes in DNA that take placez2as a result of the changing electrical environment. Thirdly, as may be seen in Figure 10 where K2CFis plotted against r, the ratio of the bound dimer to bound Figure 10. The ratio of the concentrations of the monomeric to dimeric bound dye specks plotted agalnst r: (A) acrMlne wange, (B) proflavln. monomer concentrations is essentially independent of the Circles, triangles, and squares denote solutions that are, respectively, salt concentration. This implies that, in contrast to in0.001, 0.1, and 1.0 mol d r r 3 with respect to NaCI; abcissa (both A tercalative binding, the dimerization process is little afand B), r ; ordinate, K P C F . fected by the electrical environment in solution; a conclusion that needs to be borne in mind in proposing models plots. We note that more extensive binding data would for that process. Thus,the reason why less dimer is formed be required if one wished to test, and differentiatebetween, with increasing salt concentration is primarily that, as more complex models of the intercalation process.16J’ already discussed above, there are less intercalated moNevertheless, the results are consistent with a model which nomers under these circumstances. This effect is also reduces maximum intercalation with increasing concenapparent in Figure 10 which shows that the DNA becomes tration of salt in solution. Taken together with the ob“saturated” with bound dyes at decreasing values of r with served dependence of Kl on salt concentration the results increasing salt concentration; measurements on the sysimply the existence of competitive binding between intems studied here, for example, could not be extended tercalated dyes and cations such as Indeed, about r = 0.08 in 1.0 mol dm-3 salt solution. We X-ray structural studies of dinucleotide-A0 c o m p l e x e ~ ~ ~beyond ~~ believe that the insensitivity of the dimerization to external have already shown the presence of sodium ions salt concentration is consistent with our assumption re“intercalated” between base pairs. ferred to before, that the optical activity of the noninSecondly, we wish to consider the significance of the tercalated species and that of the dimer is essentially inmonomeric CD spectra obtained at r = 0 for the systems dependent of salt. studied here. In particular, the CD of the intercalated A 0 Finally, we wish to point out that the apparent specifspecies is negative and independent of the salt concenicity of nonintercalated dye binding to DNA40implies, in tration, while that of PF is positive with a magnitude that terms of the model used here, that the dimerization process increases with salt (Table I). Since the electronic transition is base specific. transition involved is the same long-axis-polarized(?T*,?T) for both dyes,23p24 these differences provide evidence that Acknowledgment. This work was supported by the Australian Research Grants Committee. 0.30.
(36) Pauluhn, J.; Zimmermann, H. W. Ber. Bunsenges. Phys. Chem. 1978,82,1265. (37) Seeman, N. C.; Rosenberg, J. M.; Suddath, F. L.; Kim, J. J. P.; Rich, A. J. Mol. Biol. 1976,104, 145. (38) Wang, A. H-J.; Quigley, G. J.; Rich, A. Nucleic Acids Res. 1979, 6, 3879.
(39) Neidle, S.; Achari, A.; Taylor, G. L.; Berman, H. M.; Carrell, H. L.; Glusker, J. P.; Stallings, W. C. Nature (London) 1977, 269, 304. (40) Ramstein, J.; Dourlent, M.; Leng, M. Biochem. Biophys. Res. Commun. 1972,47, 874.
ADDITIONS AND CORRECTIONS 1981, Volume 85 Chr. Brauchle, D. M. Burland,* and G. C. Bjorklund: Hydrogen Abstraction by Benzophenone Studied by Holographic Photochemistry. Since we did not receive galley proofs for this Letter, several small errors went uncorrected in-the published manuscript. The following corrections should be noted: Page 124. Footnotes 6 and 7 should read: (6)D. M. Burland, G. C. Bjorklund, and D. C. Alvarez, J. Am. Chem. SOC.,102,7117 (1980). (7) G.C. Bjorklund, D. M. Burland, and D. C. Alvarez, J. Chem. Phys., 73,4321 (1980). Page 127. In column 1, line 17, the quantity 102 W/cm2 should be 1.02 W/cm2.
