title: Cold denaturation of the hammerhead ribozyme.
abstract: Cold denaturation is a thermodynamic phenomenon resulting from a difference in the heat capacities, DeltaCp, of the folded and unfolded states of a macromolecule. Whereas this phenomenon has been extensively studied in proteins, it has been thought not to occur in nucleic acids due to a negligible DeltaCp of folding. Questioning the validity of this assumption, the low-temperature structure of the hammerhead ribozyme, a small catalytic RNA, was investigated by circular dichroism spectroscopy. In the presence of 10 mM Mg2+ at pH 5.0 and 40% methanol, a cold unfolding event likely corresponding to tertiary structure loss was observed with a Tm of -20 degrees C. In 500 mM NaCl at pH 6.6, and 40% methanol, large-scale unfolding of the ribozyme at both hot (Tm = 53 degrees C) and cold (Tm = -1 degrees C) temperatures occurred. Fitting of these data to a two-state model allowed determination of DeltaCp = 3.4 kJ mol-1 K-1, corresponding to >/=0.18 kJ K-1 (mol base pair)-1, in good agreement with recently published calorimetric values for DNA duplexes. These results constitute the first direct observation of cold denaturation of a nucleic acid, and point to the importance of DeltaCp terms in the thermodynamics of nucleic acid folding.
authors.affiliation: Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, USA.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Mikulecky
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Feig
mesh_terms: D001483:Base Sequence; D002942:Circular Dichroism; D003080:Cold Temperature; D009690:Nucleic Acid Conformation; D016337:RNA, Catalytic / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D016337:RNA, Catalytic; C427204:hammerhead ribozyme