HYDROGEN

JUDITH KLINMAN, UNIVERSITY OF CALIFORNIA, BERKELEY

To be the number one--not to mention the most abundant and lightest--element on the periodic table is a weighty responsibility. But hydrogen does not disappoint. Its role in the universe is indisputable, but for me its attraction lies in its two long-lived isotopes: one stable, deuterium (D), and one radioactive, tritium (T). In fact, I can't imagine the emergence of modern chemistry and biochemistry without the benefit of these isotopes.

My first exploration of deuterium was as a graduate student with Edward Thornton, when we used solvent D₂O to examine mechanisms of carbonyl reactivity in the condensed phase. But biology beckoned, and the application of hydrogen isotopes to biochemical processes proved irresistible. Working subsequently with Irwin Rose as a postdoctoral researcher, I used enzymes to prepare methyl groups that were chiral by virtue of the presence of H, D, and T. The key to unlocking the cryptic stereochemistry for the production and utilization of chiral methyl groups by metabolic enzymes lay with the difference in reaction rate between H and D.

FUEL FOR THOUGHT The vast majority of matter in the universe--and the primary fuel for stars--is hydrogen. NASA PHOTO

Theories of kinetic isotope effects had been advanced in the preceding years. These attributed isotopic differences in rate to differences in the energy of the ground-state stretching modes of the labeled bonds, which were recognized as arising solely from the altered masses of the isotopes. Eureka--with the expectation that the potential energy surface of the labeled bond would be unaffected--biochemists had found the long-awaited nonperturbing probe with which to analyze the nature of catalysis in enzymatic reactions.

A decade and a half of exciting activity followed. Suddenly, it was possible to dissect enzyme reactions into their individual steps, learning in the process how enzymes alter the reaction-barrier height together with the internal thermodynamics for the interconversion of bound reactants and products.

Around the mid-1980s, scientists began to realize that all was not as it had seemed--experimental anomalies indicated breakdowns in classical theories of kinetic hydrogen isotope effects. This was met with a healthy blend of curiosity and resistance. Both experimentalists and theorists then opened up a new vista in the chemistry and biochemistry of hydrogen.

Returning to our roots, my laboratory called upon the three isotopes of hydrogen--H, D, and T--to examine the origin of the kinetic anomalies in the hydride-transfer reaction catalyzed by alcohol dehydrogenase. From a comparison of H/T and D/T kinetic isotope effects, it was possible to demonstrate that the entire methylene carbon at the reactive position of the alcohol substrate was showing a pattern that had been predicted for hydrogen tunneling. Clearly, the wave property of the hydrogen atom was dominating its reactivity at room temperature.

Soon, an arsenal of experimental tools was developed for investigating tunneling in enzyme-catalyzed reactions. In every system studied, the quantum property of the reacting hydrogen was apparent under physiological temperatures. This extended to the functionally high temperatures of a thermophilic enzyme, where tunneling could be demonstrated above 50 °C.

Suddenly, the dominant theories in physical organic chemistry for the interpretation of the reactivity of hydrogen required reexamination. Initial ideas focused on the addition of a tunneling correction to classical theory, while an alternative view took its lead from the decades of work on electron tunneling. In the latter, the barrier to reaction is dominated by the heavy-atom reorganization energy, a prerequisite for effective wave-function overlap of the tunneling particle. While these divergent views were initially difficult to differentiate, data began to emerge that were incompatible with a tunnel correction, necessitating new theoretical frameworks for the interpretation of hydrogen reactivity in condensed phase.

One important distinction that enters into the quantum behavior of the electron and hydrogen is the huge mass differential of these particles. By virtue of its much smaller wavelength, movement of hydrogen between reacting centers is expected to be exquisitely sensitive to small changes in donor/acceptor distance. In the context of enzyme-catalyzed reactions, this has led to the realization that motions within a protein backbone must be invoked in the context of the hydrogen-transfer process and, furthermore, that optimal catalysis may require coupling of protein motions that are both proximal and remote from the enzyme active site. In the past several years, investigators have turned toward identifying the structural context for these modes, together with their amplitudes and time constants.

This recent history of hydrogen illustrates beautifully the interplay between advances in enzyme catalysis and solution chemistry. To a researcher at the interface of chemistry and biology, the power of interdisciplinary research to advance our knowledge is extremely gratifying.

Judith Klinman is a professor and former chair of chemistry and professor of molecular and cell biology at the University of California, Berkeley. She has received the ACS Repligen Award for her work on the chemistry of biological systems.

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