Figure 4.

The end-to-end collision rates of all constructs save our two longest poly-adenine constructs exhibit Arrhenius temperature dependencies (i.e., ln k is linear in 1/T at R² > 0.97 for all but our two longest poly-adenine constructs). The steeper curves generally observed for poly-adenine suggest that the energy barrier for end-to-end collisions in this polymer is much greater than that of poly-thymine, presumably due to base-stacking effects absent in the latter (58). The two longest poly-adenine constructs, in contrast, exhibit biphasic (bilinear) behavior; whereas they behave like shorter adenine chains at lower temperatures (to the right) their barrier becomes effectively indistinguishable from that of the equivalent length of poly-thymine above some crossover temperature that is, itself, chain-length dependent. This nonlinear behavior and the chain-length dependence of the crossover temperature are consistent with the barrier friction mechanism (see discussion in text).