Polarization transfer solid-state NMR for studying soft matter: From surfactants to the stratum corneum

Sammanfattning: Popular Abstract in English The word “motion” usually brings to mind movement from point A to point B. In the world of chemistry, this kind of motion is typically referred to as translational motion. Its consequences are experienced by us every time we smell a scent in the air or watch a dye spread in water. The fragrance is nothing else than molecules that fly in the air. Colour is molecules swimming in the water. In solid materials, translational motion of molecules is hindered or impossible, but it is wrong to think that the molecules do not move at all. In fact, in many materials that appear solid to the eye, molecules fidget quite a lot. Such kind of motion affects the properties of those materials, for example: if we try to bend a plastic spoon, it will break. However, if we put a plastic spoon into a glass of hot water and try to bend it there, it will actually bend. What happened? The polymer molecules, from which the spoon is made, move about more if they are hot, taking away the brittleness of the spoon and making it supple. The plastic spoon is an example of what we call “soft matter”. Another example of soft matter is the outer layer of human skin, stratum corneum. In normal conditions, the molecules making up stratum corneum are quite rigid, acting as a barrier to both, water trying to get out of the body, as well as something from the outside getting in. However, in very humid conditions the molecules in the skin will wriggle and more water, or things from the outside, will be able to go through. When medication has to be applied through skin, increasing humidity is often used to enhance the efficacy of the therapy. In the case of the spoon it was temperature that changed the properties of the material, while in the case of skin – the amount of water. When designing new soft matter materials, such as the spoon, or understanding their properties, water content and temperature are the most important external conditions. It is so, because those two change on daily basis and a successful material usually has ideal properties: we want a rigid plastic spoon, but rather flexible car tire. Those properties have to stay the same on a rainy night when the temperature is lower and the humidity of the air considerably high, as well as during a sunny afternoon, or in an air-conditioned room, where the humidity of the air can be very low. When talking about biological material, also largely encompassed within the soft matter, it is not so much the question of “designing”, but more of “understanding”. Knowing how much the molecules fidget or what makes them stop is very helpful when trying to understand what makes the organisms tick. In the long run, this knowledge can lead to designing therapies for diseases, such as Alzheimer’s or Parkinson’s. Nuclear magnetic resonance (NMR) is an experimental method, that can be used to measure molecular mobility and predict or explain material properties at different temperatures and water contents. As the name suggests, we work with nuclei, which are the cores of atoms, that are put into a magnetic field and we can measure their resonance frequencies at that field. Not all nuclei react to magnetic fields, but those that do can be manipulated to provide information about their state. This information, depending on the exact NMR experiment, can include not only the description of the molecular fidgeting, but also the translational motion and molecular structure, making NMR a versatile and powerful tool. The advantage of NMR is that no modification of the investigated material is required, because the molecules that make up the material we want to measure are composed of atoms that we can use as our informants. Also, NMR is non-invasive, which means that the magnetic field does not change the investigated substance and no destruction of the material occurs during the experiment. In this thesis, molecular wriggling is described in the terms of rate and directional preference by using two experiments that act as mobility filters. Conducting one experiment leads to response only from the nuclei in mobile parts of molecules, while the other provides information on which nuclei reside in rigid parts of molecules. Obtaining the spectrum from only one of them, or comparing the intensities when both experiments produce a spectrum, presents insight into the degree of fidgeting.