Comparison of poly(dG-dC).poly(dG-dC) conformations in oriented films and in solution.

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Abstract
The structures of B and Z forms of poly(dG-dC).cntdot.poly(dG-dC) in oriented hydrated films were investigated by IR spectroscopy. The IR linear dichroism spectrum of poly(dG-dC).cntdot.poly(dG-dC) in B form (high relative humidity and 2 M Na+) was recorded. The spectrum of the Z form was recorded at a lower relative humidity and higher Na content (4 M). The experimental conditions were similar to those used in X-ray diffraction studies of left-handed poly(dG-dC).cntdot.poly(dG-dC) fibers. A qualitative agreement existed between the calculated values of the angles characteristic of the orientation of PO2- groups with respect to the helical axis in the Z1 form and the experimental values. The IR spectra of B and Z forms show some significant differences in H2O and 2H2O. The deuteration rates of exchangeable protons involved in H bonds between guanine and cytosine residues were deduced from the changes in absorbance near 1700 cm-1. In the B form, the exchange half-time is of the order of 20 min. In the Z form, the exchange half-time of some protons is very slow, of the order of 24 h. Because of the similarity between these values and those previously reported for the high-salt and low-salt form of poly(dG-dC).cntdot.poly(dG-dC) in solution, it was concluded that the high-salt form of poly(dG-dC).cntdot.poly(dG-dC) in solution belongs to the Z family.