Does DNA acid fixation produce left‐handed Z structure?

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Abstract
The effects of acetic acid (HCOOCH3) on the solution structure of calf‐thymus DNA are studied at pH 7.3−2.5 with acid/DNA(P) (phosphate) molar ratios (r) of , 1, 2, 10, 20 and 40. Fourier Transform infrared (FTIR) difference spectroscopy is used to establish correlations between spectral changes and base protonation, DNA conformational transition and structural variations of the acid‐DNA complexes in aqueous solution. The FTIR difference spectroscopic results showed that protonation of cytosine and subsequent unpairing of the G‐C base pairs begins at pH 4‐3 and continues up to pH 2.5, where a complete base separation and base unstacking occur. Similarly, protonation of A‐T base pairs starts at pH 4‐3 and is completed at pH 2.5, where base separation and base unstacking are observed. The protonation of the G‐C base pair leads to the formation of Hoogsteen‐type H‐bonding, before a complete G‐C disruption. The biopolymer protonation leads to the formation of several non‐B‐DNA structures, including left‐handed Z conformation.