Complexes of (dG-dC)(20) with Mn2+ ions: A study by vibrational circular dichroism and infrared absorption spectroscopy

The B-Z transition of the synthetic oligonucleotide, (dG-dC)(20), induced b y Mn2+ ions at room temperature, was investigated by absorption and Vibrati onal Circular Dichroism (VCD) spectroscopy in the range of 1800-800 cm(-1). Metal ion concentration was varied from 0 to 0.73 M Mn2+ (0 to 8.5 moles o f Mn2+ per mole of oligonucleotide phosphate, [Mn]/[P]). While both types o f spectra showed considerable changes as the Mn2+ concentrations were raise d, differences between the two were often complementary in their expression and extent, those displayed by VCD being more clearly evident due to the i nversion of the opposite helical sense from the right-handed to the left-ha nded conformation. The main phase of the transition occurred in the metal i on concentration between 0.8-1.1 [Mn]/[P]. Gradual changes that took place in the spectra were interpreted in terms of simultaneous processes that dep ended on metal ion concentration, namely B-Z transformation, binding of Mn2 + to phosphates and to nitrogen bases, and partial denaturation. Below simi lar to 0.6 [Mn]/[P], only a small portion of the oligonucleotide adopted th e Z conformation within a 3 hour period, whereas conversion was completed i n the same time interval for concentrations between 0.9-1.2 [Mn]/[P]. At [M n]/[P] > 1.7, complete transition to the Z-form took place immediately on a dding Mn2+. Applying VCD spectroscopy in combination with conventional infr ared absorption proved most useful for corroborating changes in the absorpt ion spectra, and for detecting in an unique manner, not attainable by absor ption methods, conformational changes that lead to the inversion of the hel ical sense of the oligonucleotide.