Some Synthetic DNA Pairs Show pH-Dependent "Memory" Effect

Thiele and colleagues, writing in Molecular Biology Reports in 1973, examined how certain lab-made double-stranded DNA-like molecules respond to acid and base conditions. Using a technique called circular dichroism, which reads structural changes in molecules through how they interact with polarized light, the team tracked three synthetic double helices at 25°C across a range of pH levels.

Two of the pairings, built from alternating purine and pyrimidine building blocks, switched shape smoothly and reversibly as acidity changed. The third pairing, made of one strand with alternating adenine and guanine bases matched to a strand of alternating thymine and cytosine, behaved differently: it got locked into an unusual, unstable structural state once acidified and then returned to neutral pH. This "trapped" form only relaxed back to its normal structure when heated to roughly 60°C, and researchers could only recreate the trapped state by cycling the whole sample down to a strongly acidic pH of 2.5 and back.

The findings come from pure physical-chemistry experiments on synthetic polymers in solution, not from cells, tissue or any living organism. The abstract gives no information on how widely this structural quirk applies beyond the specific sequences tested.