Stabilization of RNA oligomers through reverse micelle encapsulation.

The cellular milieu is rich in diversity of both simple and complex molecules and is also quite crowded. By contrast, typical sample concentrations employed for in vitro investigation of biophysics and structural biology make use of purified macromolecules in simple buffer systems at concentrations that range from micromolar to millimolar. Although this formulation has proven to be compatible with a wide range of biological and structural studies, it is quite different from the relatively crowded conditions typically found within cells. The importance of these crowding effects for proteins has been recognized for some time, but the equivalent analysis is underexplored in nucleic acids. Encapsulation with surfactant-based reverse micelles has emerged as an effective biophysical tool, allowing study of the influence of ionic strength, pH, hydration, and crowding on biologically active macromolecules over a wide range of conditions. We have encapsulated an oligonucleotide model of TAR RNA from HIV and the 5' stem loop oligonucleotide of the U4 snRNA. Observation of imino (1)H resonances is an established method for evaluating the stability of nucleic acid oligonucleotides, implying the presence of stacked, hydrogen bonded base pairs. Inspection of (1)H NMR spectra of the RNA molecules reveals that the intensity of several of the imino resonances increases upon encapsulation. Additional resonances not observed in spectra of the oligonucleotides free in solution support the suggestion that the molecules have gained stability. These results indicate that RNA oligonucleotides may acquire significant stability in the presence of cellular levels of crowding.