We report the results of direct measurements by atomic force microscopy of solvent-driven structural transitions within polyadenylic acid (poly(A)). Both atomic force microscopy imaging and pulling measurements reveal complex strand arrangements within poly(A) induced by acidic pH conditions, with a clear fraction of double-stranded molecules that increases as pH decreases. Among these complex structures, force spectroscopy identified molecules that, upon stretching, displayed two distinct plateau features in the force-extension curves. These plateaus exhibit transition forces similar to those previously observed in native double-stranded DNA (dsDNA). However, the width of the first plateau in the force-extension curves of poly(A) varies significantly, and on average is shorter than the canonical 70% of initial length corresponding to the B-S transition of dsDNA. Also, similar to findings in dsDNA, stretching and relaxing elasticity profiles of dspoly(A) at forces below the mechanical melting transition overlap but reveal hysteresis when the molecules are stretched above the mechanical melting transition. These results strongly suggest that under acidic pH conditions, poly(A) can form duplexes that are mechanically stable. We hypothesize that under acidic conditions, similar structures may be formed by the cellular poly(A) tails on mRNA.
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