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  • Title: Circular dichroism and thermal melting of two small DNA restriction fragments of the same molecular weight.
    Author: Stellwagen NC.
    Journal: Biochemistry; 1984 Dec 18; 23(26):6311-9. PubMed ID: 6085005.
    Abstract:
    The thermal melting and circular dichroism of two 147 base pair restriction fragments of pBR322 have been studied. The fragment with the higher GC content, 12B, melts at a higher temperature than the other fragment, 12A, as expected. The melting temperatures are proportional to the logarithm of the concentration of NaCl or tris(hydroxymethyl)aminomethane (Tris) buffer, between 1 mM and 0.2 M added salt. In 1 mM Tris buffer, the melting temperatures of the two fragments are nearly equal. The circular dichroism spectra of fragments 12A and 12B in 0.2-10 mM Tris buffer are characteristic of B-form DNA. In 81% ethanol, the circular dichroism spectra of the two fragments are characteristic of A-form DNA. With 1 mM Tris buffer as the supporting electrolyte, fragment 12A exhibits a very sharp B----A transition, with a midpoint at 79% ethanol. However, a biphasic transition is observed for fragment 12B, with midpoints at 73% and 80% ethanol. This biphasic transition may represent the conversion of separate domains of fragment 12B from the B conformation to the A conformation; half of this fragment is much more GC rich than the other half. Methods are also described for preparing polymers of the 12A and 12B fragments.
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