Under certain conditions, DNA condenses into a cholesteric liquid crystalline phase, yielding solutions that give CD signals that are orders of magnitude larger than usually observed for DNA.' DNA in this state-called + DNA-may display either positive or negative CD signals. +-DNA displays a large negative CD spectrum, and forms in the presence of salt and neutral polymers such as poly(ethyleneglyco1) (PEG)?,3 ++ DNA shows a large positive CD spectrum, and forms in the presence of multivalent cations or high concentrations of ethanol?*4 The sign of the CD signal is thought to reflect the packing arrangement of DNA helices within the cholesteric phase, with +-CD spectra arising from a left-handed twist between DNA molecules and ++ CD spectra arising from a right-handed twist?t5 The preferred packing arrangement appears to be stringently controlled by the solution conditions. Heretofore there has been no instance in which uncomplexed DNA has adopted the ++ form in NaCl/PEG solutions-such solutions have always favored the formation of +-Reported here is the novel observation that poly(ciA)-poly(dT) forms the ++ condensate in the presence of NaCl/PEG. This is the first DNA to do so under such solution conditions, and most probably reflects an unusual packing arrangement of poly(dA)-poly(dT) molecules within the cholesteric phase in comparison with standard B DNA. Poly(dA)-poly(dT) has been previously shown to be unusual in many aspects of its solution b e h a v i ~r , ~-' ~ perhaps a reflection of its nonstandard right-handed ~t r u c t u r e . ' ~-' ~
The results presented here show that poly(dA)-poly(dT) is unusual in its condensation behavior as well, and demonstrate that subtleties of the secondary structure of DNA must dictate the manner in which it packs into higher ordered structures.
DNA?
MATERIALS AND METHODS
Polynucleotides
Poly(dA)-poly(dT) (lot no. OH617860) and poly(dAdT)-poly(dAdT) (lot no. PM717870) were purchased from Pharmacia (Piscataway, NJ) and used without further purification. Samples were dissolved in 6 m M Na,HP04, 2 m M NaH,PO,, 1 m M Na, EDTA, 0.185M NaC1, pH 7.0 (BPES buffer), and dialyzed exhaustively against the same buffer before use. The quality of the polynucleotides was checked by thermal denaturation with uv absorbance detection and by differential scanning calorimetry. T, values, enthalpy values, and hypochromism values determined by these methods were in excellent agreement with literature values. Polynucleotide concentrations (in terms of base pairs) were determined by uv absorbance, using extinction coefficients of czW = 12,OOOM-' cm-' for poly(dA)-poly(dT) and cZ6, = 13,200M-' cm-' for poly(dAdT)-poly(dAdT).