Conformational characteristics of deoxyribonucleic acid-butylamine complexes with C-type circular dichroism spectra....

The derivatives of calf thymus DNA in which n-butylamine is covalently attached as described in the preceding paper in this series [Chen, C. Y., Pheiffer, B. H., Zimmerman, S. B., & Hanlon, S. (1983) Biochemistry (preceding paper in this issue)] were examined by Raman spectroscopy. As previously mentioned, these complexes exhibit profoundly decreased rotational strengths of the positive band of the circular dichroism (CD) spectrum above 260 nm, with the most heavily substituted (ca. 0.12 mol of amine/mol of nucleotide) resembling that of DNA in 11 m LiCl. Raman spectra of all complexes and their controls in the form of either fibers at 98% relative humidity or gels at 40 mg/mL in 20 mM NaCl, pH 7, show typical B-type spectra with no evidence of significant amounts of C, A, Z, or disordered forms. We have thus concluded that the assignment of the nonconservative CD spectrum of DNA typically observed in concentrated electrolyte solutions to a C form is in error. Both these Raman data and the X-ray results reported in the previous paper indicate that the structure giving rise to the C CD spectrum has B-form backbone geometry.