Polyelectrolyte properties of biopolymers: conductivity and secondary structure of polyriboadenylic acid and its salts in solutions

Polyriboadenylates of alkali metals were obtained from (1) K(+)-poly(A) (salts 1) and (2) H(+)-poly(A) (salts II) by the ion-exchange method. The conductivity of these salts as well as of H(+)-poly(A) were studied. Salts I and II of the same counterion were shown to have significantly different conductivity coefficients (f) and polyion conductances (lambda 0p). the charge density parameter (xi) was 1.3 and 2.5, respectively, with lambda 0p equal to 44 and 83 ohm-1 cm2 mole-1 for poly(A)-I and poly(A)-II salts, respectively. This is credited to the difference in the conformations of corresponding polyions. The linear dependence of equivalent conductivity on the square root of polymer concentration (Kohlrausch coordinates), earlier obtained for DNA, is also satisfied for the studied polynucleotides. A comparison of the slopes of straight lines in Kohlrausch coordinates for poly(A), simple electrolytes, and for earlier studied polyribouridylic acid salts lends credence to the concepts, developed by a number of authors, that DNA can act as a "buffer" against the ion-ion interaction in concentrated electrolyte solutions. Using the approximation that the polyion conductance is independent of the counterion nature, parameter f (agreeing in this case with Eisenberg parameter phi) has been shown to decrease as the polynucleotide concentration is increased; the decrease is caused by the relaxation effect. The transference numbers of counterions, which have negative values in poly (A)-II solutions, grow with the increase in polymer concentration; the higher the xi, the more apparent is this increase. This is explained by the increase in the fraction of conductivity along the polyion chains ("surface" conductivity) with the growth of polyelectrolyte concentration.

DNA probes: applications of the principles of nucleic acid hybridization

Nucleic acid hybridization with a labeled probe is the only practical way to detect a complementary target sequence in a complex nucleic acid mixture. The first section of this article covers quantitative aspects of nucleic acid hybridization thermodynamics and kinetics. The probes considered are oligonucleotides or polynucleotides, DNA or RNA, single- or double-stranded, and natural or modified, either in the nucleotide bases or in the backbone. The hybridization products are duplexes or triplexes formed with targets in solution or on solid supports. Additional topics include hybridization acceleration and reactions involving branch migration. The second section deals with synthesis or biosynthesis and detection of labeled probes, with a discussion of their sensitivity and specificity limits. Direct labeling is illustrated with radioactive probes. The discussion of indirect labels begins with biotinylated probes as prototypes. Reporter groups considered include radioactive, fluorescent, and chemiluminescent nucleotides, as well as enzymes with colorimetric, fluorescent, and luminescent substrates.

Membrane bilayer assembly in neural tissue of rat and squid as a critical phenomenon: influence of temperature and membrane proteins

Cell membrane bilayers have been reconstructed in vitro utilizing total lipid extracts from rat neural tissue (forebrain, cerebellum, brainstem and spinal cord) and from the optic lobe and fin nerve of the squid Loligo pealei. In agreement with the critical state theory of bilayer assembly (Gershfeld, N.L. 1986. Biophys. J. 50:457-461; Gershfeld, NL.L. 1989. J. Phys. Chem. 93:5256-5261), these lipid extracts spontaneously formed purely unilamellar structures in aqueous dispersion, but only at a critical temperature, T*, which was species dependent. For all the rat tissues T* = 37 +/- 1 degrees C; for squid neural extracts T* = 15.5 +/- 1.4 degrees C. These values correspond to 'physiological' temperatures for both organisms, implying that their lipid metabolism is geared to permit spontaneous assembly of unilamellar membranes at the ambient temperature in the tissues. Membrane protein composition had little or no effect on critical bilayer formation.