Repulsive/attractive interaction among compact DNA molecules as judged through laser trapping: difference between linear- and branched-chain polyamines

Yusuke Kashiwagi, Takashi Nishio, Masatoshi Ichikawa, Chwen-Yang Shew, Naoki Umezawa, Tsunehiko Higuchi, Koichiro Sadakane, Yuko Yoshikawa, Kenichi Yoshikawa

It is well known that polyamines induce a folding transition from an elongated coil to a compact globule state for giant DNA larger than several tens of kbp (kilo base pairs). Here, we studied the interaction between compact DNA molecules in the presence of linear and branched-chain isomers of polyamines. We compared the stability of the assembly among plural number of compact DNA molecules generated by laser trapping. As a result, the assembly of compact DNAs with a linear-chain polyamine is stable even after the laser is switched off. On the other hand, the assembly of DNAs with a branched-chain polyamine disperses into individual compact DNAs when the laser is switched off. Thus, compact DNAs with linear- and branched-chain polyamines attract and repel each other, respectively. This difference in the effects of linear and branched polyamines is discussed in terms of the steric interaction between negatively charged double-strand DNA and cationic polyamines.

DOI

Temperature-driven volume phase transition of a single stimuli-responsive microgel particle using optical tweezers

Deepak K. Gupta, D. Karthickeyan, B. V. R. Tata, T. R. Ravindran

Poly(N-isopropylacrylamide) (PNIPAM)-based microgels respond to temperature and exhibits a transition from swollen to deswollen state upon variation of temperature, which is known as volume phase transition (VPT). Dynamic light scattering (DLS) is a popular technique to identify the volume phase transition temperature (VPTT) of microgel particles, which measures variation of particle size with temperature in a suspension having microgel particle concentration of 107–108 particles/cm3. Here, we employ optical tweezers to trap a single microgel particle and identify its VPTT by measuring the lateral trap stiffness, κ as a function temperature. It is shown that near the VPTT, κ increases gradually upon increasing temperature, which is due to a gradual decrease in particle size with simultaneous increase in its refractive index.

DOI