This unique volume presents a comprehensive but accessible introduction to the field of ultrafast two-dimension infrared (2D IR) vibrational echo spectroscopy based on the pioneering work of Professor Michael D Fayer, Department of Chemistry, Stanford University, USA. It contains in one place a qualitative introduction to the field of 2D IR spectroscopy and a comprehensive set of scientific papers that underlie the qualitative discussion. The introductory material contains several detailed illustrations, and is based on the Centenary Lecture at the Indian Institute of Science given by Professor Fayer July 16, 2008 as part of the celebration of the 100th anniversary of the founding of IIS in Bangalore, India. The second part of the volume contains reprints of Fayer's relevant papers. The compilation will be very useful because it presents the historical background, motivation, methodology, and experimental results at a level that is accessible to the non-expert. The reprints of the scientific papers, from review articles to detailed theoretical papers, provide rigorous supporting material so that the reader can delve as deeply as desired into the subject.

Temperature-jump (T-jump) two-dimensional infrared spectroscopy (2D IR) is developed, characterized, and applied to the study of protein folding and association. In solution, protein conformational changes span a wide range of timescale from nanoseconds to minutes. Ultrafast 2D IR spectroscopy measures time-dependent structural changes within the protein ensemble by probing the frequency changes associated with amide I backbone vibrations. Combining 2D IR with a perturbing laser-induced T-jump enables the study of conformational dynamics from 5 ns to 50 ms. To access a finer time-sampling of the conformational evolution, a one-dimensional variant of 2D IR, heterodyne-detected dispersed vibrational echo spectroscopy (HDVE), is implemented. The framework for interpreting transient HDVE and 2D IR spectra is developed, and we propose a method to remove the linear absorption distortions along both frequency axes. We first present the T-jump 2D IR spectra of a dipeptide to reveal the general amide I baseline response expected in the absence of conformational change. To facilitate the analysis of T-jump data, singular value decomposition (SVD) is employed for reducing noise, identifying the number of distinguishable states, and separating spectral changes based on shared timescales. Finally, T-jump 2D IR spectroscopy is applied to study the unfolding of ubiquitin, disordering of the 12-residue p-hairpin peptide trpzip2 (TZ2), and the dissociation of insulin dimers to monomers. Experimental results for ubiquitin highlight the importance of linear absorption corrections for interpretation of the data. In response to the T-jump, 2D IR results indicate p-sheet structure melts in ubiquitin with a small amplitude (~10 gs) and large amplitude (17 ms) response. Isotope-labeling T-jump experiments on TZ2 allow for the proposal of a free energy surface in which transitions from a native and misfolded state proceed through a disordered hub-like state with a 1-2 gs timescale. Multiple timescales are observed in the T-jump induced dissociation of insulin. Based on their spectral features and concentration dependence, the insulin timescales can be assigned to dissociation, disordering, and oligomerization processes. With these applications, we demonstrate the capability of T-jump 2D IR spectroscopy to reveal detailed molecular dynamics.

Proteins are complex molecular machines that facilitate the chemical reactions fundamental to life. Their functions are encoded in a linear sequence of amino acids, of which only 20 species are found in nature. Yet the functional and structural diversity accessible through these building blocks is vast. Molecular and atomic-level protein studies have been crucial to our understanding of health and treatment of disease, with increasingly sophisticated experimental and computational methods continuing to provide new information with which to advance medicine. However, the requirement for more detailed understanding of proteins has risen through the emergence of multi-antibiotic-resistant bacteria and also through the potential to design synthetic proteins of novel function. Paradigms of protein function have evolved significantly since early studies, though few all-encompassing descriptions have been proposed, owing to the complex, dynamic structures of these large biomolecules. Presently, the relationship between protein structural motions at different timescales appears to hold vital significance to the elusive aspects of biological mechanisms. No single measurement technique is capable of accessing the multitude of timescales over which protein motions occur, and thus concerted investigation is necessary. Observation of dynamics at the femtosecond-picosecond timescale has only recently become possible through the development of new experimental techniques, allowing a new class of protein motions to be investigated. In this thesis, the advanced technique of two-dimensional infrared spectroscopy (2DIR) is employed to study three biomolecular systems with implications to ubiquitous protein interactions. The aims of these investigations are, firstly, to demonstrate the suitability of 2DIR spectroscopy in gathering novel dynamic information from biological systems that is not accessible via other methods, and secondly, to derive the potential physical significance of these dynamics as they relate to biological function. A description of the underlying theory of 2DIR is presented in this Chapter, along with the considerations that must be made in the application of such a technique to complex biological case-studies. In Chapter (2), descriptions are given for the experimental setups used to acquire infrared spectra, specifically, Fourier transform infrared (FTIR), pump-probe and 2DIR spectroscopies. In Chapter (3) the catalytic-site dynamics of two closely-related haem proteins are each studied by monitoring the vibrational evolution of a nitric oxide (NO) probe molecule bound to the haem centre. A comparison of the active site dynamics is performed in order to correlate the observed differences with discrepancies between the protein reaction mechanisms. Chapter (4) explores the potential of a coenzyme with high protein-binding promiscuity to serve as an intrinsic reporter of the dynamics that occur at substrate binding sites. Infrared analysis and categorisation of the free coenzyme molecule is performed in order to establish its effectiveness as a probe. In Chapter (5), method-development strategies are proposed for the extraction of 2DIR data from large, complex protein-protein systems, with the objective of expanding the range of interactions on which 2DIR can effectively report. Both well-established and novel strategies are employed, and the potential and limitations of the technique are discussed in the context of these demanding case-studies. Chapter (6) draws together conclusions and an overview of progress made and discusses future directions.

Understanding the structure and dynamics of proteins is essential to understanding their roles and functions in these physiological processes. In this thesis, I describe the implementation of an ultrafast nonlinear spectroscopic technique, two-dimensional infrared (2D IR) spectroscopy to probe the structure and dynamics of ion channels and amyloid fibers. Regarding ion channels, I describe the combination of semisynthesis, 2D IR spectroscopy and molecular dynamic (MD) simulations in addressing the longstanding question of ion permeation through the selectivity filter of a potassium ion channel. I show that ions and water alternate through the filter and that these ions cannot occupy adjacent binding sites. Furthermore, 2D IR experiments revealed a flipped state that is predicted by MD simulations but not observed in x-ray crystallography. In another aspect of this work, we show that the collapsed state of the filter is structurally different in low K+ and low pH. Moreover, our work also reveals how the large conformational motions of the protein are coupled to structural changes in the selectivity filter, as evidenced by a change in the ion occupancy. In a second research direction, I developed an optical technique to quantify photoactivatable fluorophores with fluorescence microscopy. This technique allows for the quantification of a limitless number of fluorophores, and corrects for stochastic events such as fluorescence intermittency. This work can be extended to the study of amyloids, where determining the number of proteins in a prefibrillar aggregates is necessary for understanding their roles in amyloid related diseases. Finally, using 2D IR spectroscopy we describe the effect of common solvents on the anharmonicity of small molecule chromophores. The data indicates that the carbonyl anharmonicity, and, subsequently, the Stark tuning rate, is an intrinsic property of the carbonyl vibrational probes, which have important implications on the interpretation of carbonyl vibrational frequency shifts in the condensed phase.