The present dissertation seeks to deepen our understanding of hydrogen transfers and especially C-H bond activations in enzymes. Hydrogen transfers are ubiquitous in chemistry and biology and a thorough understanding of how they occur and what factors influence them will facilitate developments in biomimetic catalysis, rational drug design, and other fields. A particular difficulty with H-transfers is the importance of nuclear quantum effects to the reaction, particularly tunneling. The overall scope of the work here aims to examine how experimental kinetic isotope effects (KIEs) can be interpreted with a particular type of tunneling model, referred to as Marcus-like models, to yield a semi-quantitative picture of the physical mechanisms of H-transfers. Previous work had used this kind of model to qualitatively interpret experimental data using a combination of intuition and generalized theories. The work here examines these theories in quantitative detail, testing and calibrating our intuition in the context of several experimental systems. The first chapter of research (ch. II) focusses on the temperature dependence of primary KIEs and how these experiments can be quantitatively interpreted as a probe for certain kinds of enzyme or solvent dynamics. The subsequent chapters (ch. III-VI) focus on the use of secondary KIEs to determine the detailed structures of tunneling ready states (TRSs) and how the dynamics of H-tunneling affect those structures. These chapters focus primarily on the TRS of the enzyme alcohol dehydrogenase, but by examining an uncatalyzed analogue to that reaction (ch. VI), the work gains some insight about similarities and differences between catalyzed and uncatalyzed reactions. In summary, the work uncovers some principles of catalysis, not just the mechanism of a catalyzed reaction. The mechanism of C-H activation presented here provides an elegant solution to problems that have been vexing to accommodate within traditional models. This work constitutes some initial steps in making Marcus-like models quantitatively useful as a supplement or even replacement for traditional models of reactivity.

The field of isotope effects has expanded exponentially in the last decade, and researchers are finding isotopes increasingly useful in their studies. Bringing literature on the subject up to date, Isotope Effects in Chemistry and Biology covers current principles, methods, and a broad range of applications of isotope effects in the physical, biolo

Isotope effects have become one of the most powerful tools available to the enzymologist for probing enzymic mechanisms. Enzyme Mechanism from Isotope Effects presents the basic theory underlying isotope effects, including the latest findings on proton tunneling and coupled atomic notions. Specific theoretical applications are emphasized in regard to the types of information that can be obtained using isotope effects. The book also examines recent theoretical treatments of the product dependence of deuterium isotope effects, multiple isotope effects and isotope effects on intermediate partitioning. Other topics include a complete discussion of methods for measuring isotope effects, including a detailed description of the use of the isotope ratio mass spectrometer to obtain isotope effects, and a review of the literature regarding mechanistic information obtained from isotope effects for individual classes of enzyme-catalyzed reactions. Enzyme Mechanism from Isotope Effects is an excellent reference source for investigators using isotope effects in their research. The book is also valuable for reference libraries and instructors teaching courses in enzyme mechanism.

Hydrogen transfer reactions are among the most fundamental organic reactions. They are involved in nearly all enzyme-catalyzed reactions necessary for the sustenance of life, and they are often involved in key steps in synthetically useful reductions. Notable among synthetically useful reactions are the class of reactions known as C-H bond functionalization. It is surprising, then, that this most fundamental chemical event is so poorly understood. To address this knowledge gap, we have developed new experimental and computational approaches that allow interrogation of the core physical nature of cyclic hydrogen transfer reactions. In general, our laboratory utilizes kinetic isotope effects (KIEs) to probe chemical reactions. This technique involves accurately determining how replacement of atoms with their heavier isotopes affects the reaction rate (or product distribution). In order to extract the most insightful information from KIE studies, suitable experiments and measurement methods need to be well-designed and carefully executed. This dissertation is composed of four chapters. The first chapter serves as a general introduction that lays out the background of the mechanistic study of cyclic hydrogen transfer, the concepts of kinetic isotope effect and a concise review of traditional and new techniques that utilize KIEs to elucidate reaction mechanisms. The remaining chapters present three unique studies that culminate in a unified view of cyclic hydrogen transfers. Each study is designed to interrogate the key physical aspects of a synthetically useful and mechanistically interesting hydrogen transfer. The first study explores the Chugaev elimination, which is used to transform alcohols into alkenes in a typically regioselective manner. In this study, we have used a combination of intermolecular and intramolecular 2H KIE measurements to demonstrate that this reaction is dominated by quantum mechanical behavior even though it occurs at elevated temperatures. Comparison between the experimental and computational results imply that the observed KIEs are influenced by the location of the bottleneck (or transition state) which is found to be dependent on the isotopic substitution. Chapter 3 explores the Cope elimination, which can be used to install alkenes adjacent to an amine, demonstrated that seemingly simple cyclic hydrogen transfer reactions can proceed simultaneously via multiple mechanisms depending upon driving force and temperatures. Our approaches in the Cope and Chugaev eliminations were then utilized to explore an exciting new C-H functionalization reaction developed by DuBois. This study demonstrates that cyclic H-transfers mediated by a Ru-nitrene species are fundamentally similar to relatively simpler syn-b eliminations in terms of the involvement of quantum mechanical phenomena. The reaction models built with prototype reactions can be generalized to more complicated metal catalyzed reactions to facilitate a better understanding of the reaction in terms of both reactivity and selectivity.

Experimental Analysis of Enzyme Mechanism Using Isotope Effects, Volume 596, the latest release in the Methods in Enzymology series, continues the legacy of this premier serial with quality chapters authored by leaders in the field. Chapters in this comprehensive update include Measurement of enzyme binding isotope effects, Chemical ligation and isotope labeling to locate dynamic effects, Measurement of heavy enzyme isotope effects, Extracting kinetic isotope effects from a global analysis of reaction progress curves, KIE of metabolic flux and enzymes, Solvent and Primary KIE on Flavin Enzymes, and The Rapid Determination of Primary Deuterium Isotope Effects on Enzyme-Catalyzed Proton Transfer at Carbon in 50/50 HOH/DOD. Readers who are interested in applying or understanding this research will find useful methods currently used for measuring isotope effects on solution and enzyme reactions. Written by pioneers of modern isotope effect research Is the only collection of modern kinetic isotope effect methods currently available