title: DNA cholesteric pitch as a function of density and ionic strength.
abstract: The nature of chiral interactions among chiral biopolymers, such as DNA, protein alpha-helices, and rodlike virus particles, remains elusive. In particular, a satisfactory model connecting molecular chiral interactions and the pitch of the resulting chiral mesophases is lacking. We report the measurement of short-fragment (146-bp) DNA cholesteric spherulite pitch as a function of osmotic pressure, average DNA interaxial spacing, and salt concentration. We determined cholesteric pitch and interaxial spacing by polarizing optical microscopy and x-ray scattering, respectively, from which the twist-angle between DNA molecules can be calculated. Surprisingly, we found that decreasing ionic strength resulted in weaker chiral interactions between DNA chains, as evidenced by the decrease in the twist-angle, and consequent increase in the cholesteric pitch, for a fixed interaxial spacing. We propose that this behavior can be explained by increased smearing-out of the helical charge pattern along DNA as the Debye screening length is increased.
authors.affiliation: Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA, USA.
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Stanley
authors.forename: Helen
authors.initials: H
authors.lastname: Hong
authors.forename: Helmut H
authors.initials: HH
authors.lastname: Strey
mesh_terms: D004247:DNA / Q000032:analysis / Q000737:chemistry* / Q000648:ultrastructure*; D004548:Elasticity; D007477:Ions; D008859:Microscopy, Polarization / Q000379:methods*; D009038:Motion; D009690:Nucleic Acid Conformation; D009997:Osmotic Pressure; D012492:Salts / Q000737:chemistry*; D013237:Stereoisomerism; D013314:Stress, Mechanical; D013329:Structure-Activity Relationship; D014961:X-Ray Diffraction / Q000379:methods*
chemical_list: D007477:Ions; D012492:Salts; D004247:DNA