| Abstract: | Results of circular dichroism and ultraviolet absorption measurements of poly A in neutral aqueous formaldehyde solution are presented, which show that the hydroxymethylated residues retain the ability to stack, but the stacking tendency is diminished. The midpoint of the thermal denaturation profile falls from about 38°C to about 18°C on complete hydroxymethylation. Hydroxymethylation, however, appears to have only a small effect on the apparent standard enthalpy and entropy of stacking, both of which become slightly more negative. When present at concentrations greater than about 2 g/100 ml, formaldehyde has a further and somewhat more dramatic denaturing effect on poly A. This ability is probably related to the chemical similarity of aqueous formaldehyde (methylene glycol) and ethylene glycol, which is known to be an effective denaturant for nucleic acids. In the presence of 20 g/100 ml formaldehyde, a very broad thermally induced transition of poly A is observed through the loss of ultraviolet hypochromism. Only the low-temperature part of the transition is observed; it appears to be completed only at temperatures well over 100°C where the hypochromism with respect to adenosine has largely disappeared. The transition cannot be identified with the unstacking reaction usually observed through thermal denaturation experiments in the absence of formaldehyde, although the associated structure may be related in some way. The molecular nature of the structure associated with the transition is not known. |
