| Comments |
P.998 2nd paragraph: "DNA Conformation: The double helical nature of DNA has now been convincingly established (ref 9), and except for minor details the experimental evidence confirms the
canonical B-DNA structure: an antiparallel, right-handed double stranded helix with internal Watson-Crick base pairing. The refined structure was shown by model building to fit the X-ray diffraction pattern of DNA fibers at high humidity. This approach and the results have been discussed extensively elsewhere (refs 1-3, primary source 10, ref 11) [investigators] only list here, in Table 1, some useful molecular parameters." P.1000 2nd paragraph: "Environmental effects: As shown in Table 1, DNA takes on different conformations in different environments in the fiber. For example, depending on the type and amount of salt or the amount of hydration, a B, C, or A structure is observed. This polymorphism has been recently
discussed (ref 32). Circular dichroism (CD) spectra of isotropic DNA films prepared under conditions similar to those of the fibers have been measured (ref 33). Correlation between optical properties and geometrical structure suggests that transitions of one form into another can be observed directly. Changes in the near UV CD spectrum of DNA induced by variations in salt concentration, type of salt, solvent composition, temperature, or in the DNA itself (superhelicity, collapse) have been reported and interpreted by many authors (refs 34, 35). Although useful as a characterization of the different transitions, the approach is limited by the fact that interpretation in terms of defined conformational changes is rather difficult and controversial (refs 34, 36, 36a, 37). [An example is the decrease in the 275 nm CD band observed at high salt, and attributed to a B-form to C-form transition. This interpretation would require a change in the winding angle of up to 2.6 deg (see Table 1), whereas a direct determination of that change by ethidium bromide sedimentation velocity titration gives a maximum value of 0.8 deg (ref 36).]" |