Influence of Ca super(2+) Cations on Low pH-Induced DNA Structural Transitions

A confocal Raman microspectrometer was used to investigate the influence of Ca super(2+) cations on low pH-induced DNA structural changes. The effects of Ca super(2+) cations on the protonation mechanism of opening AT and changing the protonation of GC base pairs in DNA are discussed. Based on the observation that the midpoint of the transition of Watson-Crick GC base pairs to protonated GC base pairs lies at around pH 3 (analyzing the 681 cm super(-1) line), measurements were carried out on calf thymus DNA at neutral pH and pH 3 in the presence of low and high concentrations of Ca super(2+) cations. Raman spectra show that low concentrations of Ca super(2+) cations partially protect DNA against protonation of cytosine (characteristic line at 1262 cm super(-1)) and do not protect adenine (characteristic line at 1304 cm super(-1)) and the N(7) of guanine (line at 1488 cm super(-1)) against binding of H super(+). High Ca super(2+) concentrations can prevent protonation of cytosine and protonation of adenine (little disruption of AT pairs). Analyzing the line at 1488 cm super(-1), which obtains most of its intensity from a guanine vibration, high salt was also found to protect the N(7) of guanine against protonation.