Polymer and Protein Physics : Simulations of Interactions and Dynamics

Sammanfattning: Proteins can, without any exaggeration, be called the "building blocks of life". Their physical properties depend not only on the chemical structure but also on their geometric shape. In this thesis, I investigate protein geometry using several different methods.We start with a coarse-graining model to study the general behavior of polymers. For this reason, we utilize an effective Hamiltonian that can describe the thermodynamic properties of polymer chains and reproduce secondary and tertiary structures. To investigate this model, I perform classical Monte Carlo simulations using my software package.Another problem we address in this thesis is how to distinguish thermodynamic phases of proteins. The conventional definition of phases of polymer systems uses scaling laws. However, this method needs the chain's length to be varied, which is impossible to do with heteropolymers where the number of sites is one of the system's characteristics. We will apply renormalization group (RG) theory ideas to overcome this difficulty. We present a scaling procedure and an observable through which RG flow can define a certain polymer chain's phase.Another part of the thesis is dedicated to the method of molecular dynamics. Our focus is on a novel experimental technique called Single Particle Imaging (SPI). The spatial orientation of the sample in this method is arbitrary. Scientists proposed to use a strong electric field to fix the orientation since most biological molecules have a non-zero dipole moment. Motivated by this, we investigate the influence of a strong electric field's ramping on the orientation of protein ubiquitin. For the same question of SPI and using the same protein, we study the reproducibility of unfolding it in a strong electric field. With the help of a new graph representation, I show different unfolding pathways as a function of the electric field's value and compare them with thermal and mechanical unfolding. I show that the RG flow observable can also detect the different ubiquitin unfolding pathways more simply.The study described in this thesis has two types of results. One is a very concrete type, which can be utilized right away in the SPI experiments, like MS SPIDOC on the European XFEL. The other type of results are more theoretical and opens up a new field for further research. However, all of them contribute to protein science, an area vital for humanity's ability to protect us from threats such as the current COVID-19 pandemic.