荧光标记的聚（N-异丙基丙烯酰胺）链在水溶液中的折叠动力学

Kinetics of Coil-to-Globule Transition of DansyI-Labeled Poly（N-isopropyl- acrylamide） Chains in Aqueous Solution

使用1．54μm的激光脉冲（脉宽约10ns）诱导丹磺酰基标记的热敏性线性聚N-异丙基丙烯酰胺（PNIPAM）发生线团到小球的转变．当聚合物链中NIPAM单体与丹磺酰基基团的摩尔比由110增至300时，共价键合到聚合物主链上的丹磺酰基发色团对PNIPAM相转变行为的影响会随之减小．PNIPAM链塌缩经历成核过程（伴随着初始珍珠的形成，快弛豫时间Tfast=0．1ms）和珍珠的增长粗化阶段（慢弛豫时间Tslow=0．5ms），这与之前使用水溶性的-苯胺-8-萘磺酸铵盐作为荧光探针研究PNIPAM在水溶液中的折叠动力学得到的结果类似．Tfast在很宽的分子量范围内都与分子量无关，而Tslow则随链长增加而略有增大．

The coil-to-globule transition of thermally sensitive linear poly（N-isopropylacrylamide） （PNIPAM） labeled with dansyl group is induced by 1.54 μm laser pulses （widths10 ns）. The dansyl group is used to follow the transition kinetics because its fluorescence intensity is very sensitive to its micro-environment. As the molar ratio of NIPAM monomer to dansyl group increases from 110 to 300, the effect of covalently attached dansyl fluorophores on the transition decreases. In agreement with our previous study in which we used 8-anilino- l-naphthalensulfonic acid ammonium salt free in water as a fluorescent probe, the current study reveals that the transition has two distinct stages with two characteristic times, namely, Tfast≈0.1 ms, which can be attributed to the nucleation and formation of some ＂pearls＂ （locally contracting segments） on the chain, and tslow≈0.5 ms, which is related to the merging and coarsening of the ＂pearls＂.Tfast is independent of the PNIPAM chain length over a wide range （Mw=2.8× 10^6-4.2 × 10^7 g/mol）. On the other hand, Tslow only slightly increases with the chain length.