Novel Experiments Based on Time-Resolved Fluorescence to Probe Complex Physical Phenomena in Macromolecules and their Supramolecular Assemblies

大分子是非常大的分子，其溶液具有有趣的特性，用于各种应用，包括化妆品，药物输送，洗涤剂，油漆或提高石油采收率。反过来，大分子的理想性质是由于它们的分子结构和化学组成。新的合成进展使制备越来越复杂的大分子成为可能，这些大分子可以通过荧光在分子水平上进行探测。一种可以应用于分子水平上探测大分子的荧光技术是基于芘准分子的形成。芘是一种染料，能形成一种被称为准分子的受激复合体。准分子是两种芘染料相遇的结果。通过将芘连接到大分子上，可以用荧光探测两种芘染料之间的准分子形成过程。这些荧光实验提供了有关芘准分子形成发生的时间尺度和芘染料状态的信息。由于芘部分是共价连接到大分子上的，因此芘准分子形成的时间尺度提供了有关大分子的柔韧性和可变形性的信息。芘染料的状态表明芘是分离的，还是通过扩散形成准分子，或者是聚集的，这描述了染料如何在大分子上分布。这些影响将在本提案所提出的三个主题中加以利用。

Macromolecules are very large molecules whose solutions have interesting properties that are used in a variety of applications that include cosmetics, drug delivery, detergents, paints, or enhanced oil recovery. In turn, the desirable properties of macromolecules are due to their molecular architecture and chemical composition. New synthetic advances have allowed the preparation of increasingly complex macromolecules which can be probed at the molecular level by fluorescence. One fluorescence technique that can be applied to probe macromolecules at the molecular level is based on pyrene excimer formation. Pyrene is a dye that forms an excited complex called an excimer. The excimer is the result of an encounter between two pyrene dyes. By attaching pyrene onto a macromolecule, the process of excimer formation between two pyrene dyes can be probed by fluorescence. These fluorescence experiments provide information about the time scale over which pyrene excimer formation takes place and the state of the pyrene dyes. Since the pyrene moieties are covalently attached onto the macromolecule, the time scale of pyrene excimer formation provides information about the flexibility, and thus deformability, of the macromolecule. The state of the pyrene dyes indicates whether the pyrenes are isolated, form excimer by diffusion, or are aggregated which describes how the dyes are distributed on the macromolecule. These effects will be put to use in the three themes developed in this proposal. The first theme aims to develop a procedure to estimate the force experienced by a linear chain whose ends have been fluorescently labeled with pyrene so that they can be anchored in two surfactant micelles. Surfactants are molecules that aggregate into micelles. The micelles have an oily interior which can host pyrene while the chain is water-soluble, remains in water, and can span the two micelles if it is long enough. The solution condition can be adjusted to allow the two micelles to come closer so that they can be bridged by a shorter chain. If the two ends are located inside a same micelle, the two pyrene labels will form an excimer whereas no excimer will be formed if the two ends are in two different micelles. Fluorescence will be used to determine the fraction of chains having their two ends in a same micelle. In turn, this information will be applied to estimate the force exerted by the surfactant micelles as they pull the two chain ends apart. The second theme will probe the mobility of surfactants involved in an aggregate with DNA. DNA-surfactant aggregates are used for gene delivery where a piece of DNA is transported across the membrane of a cell. Studies have confirmed that efficient gene delivery depends critically on the arrangement on the surfactants with the DNA inside the aggregate. By using a pyrene-labeled surfactant, the mobility of the surfactant in the surfactant-DNA aggregate will be determined as a function of the surfactant arrangement in the aggregate. Finally, fluorescence measurements will be employed to probe what happens to a polymer as it collapses in solution above a critical temperature. This phenomenon is very important in Polymer Science since it is encountered during protein folding. A thermoresponsive polymer that collapses above a set temperature will be prepared and labeled with pyrene. By monitoring both the extent of pyrene aggregation and the rate of pyrene excimer formation as the polymer collapses, it should be possible to gain information about the time scale over which the thermoresponsive polymer collapses.