Bioenergetics: Experimental Demonstration of Excess Protons and Related Features

Abstract

Over the last 50 years, ever since the Nobel-prize work of Peter Mitchell’s Chemiosmotic theory, the question whether bioenergetics energy transduction occurs through localized or delocalized protons has been a controversial issue among scientists. Recently, a proton-electrostatics localization hypothesis was formulated which may provide a new and clear understanding of localized and delocalized proton-coupling energy transduction in many biological systems. The aim of this dissertation was to test this new hypothesis. To demonstrate the fundamental behavior of localized protons in a pure water-membrane-water system in relation to the newly derived pmf equation, excess protons and excess hydroxyl anions were generated and their distributions were tested using a proton-sensing aluminum membrane. The proton-sensing film placed at the membrane-water interface displayed dramatic localized proton activity while that placed into the bulk water phase showed no excess proton activity during the entire experiment. These observations clearly match with the prediction from the proton-electrostatics localization hypothesis that excess protons do not stay in water bulk phase; they localize at the water-membrane interface in a manner similar to the behavior of excess electrons in a conductor. In addition, the effect of cations (Na+ and K+) on localized excess protons at the water-membrane interface was tested by measuring the exchange equilibrium constant of Na+ and K+ in exchanging with the electrostatically localized protons at a series of cations concentrations. The equilibrium constant for sodium (Na+) cations to exchange with the electrostatically localized protons was determined to be (5.07 ± 0.46) x 10-8 while the equilibrium constant for potassium (K+) cations to exchange with localized protons was determined to be (6.93 ± 0.91) x 10-8. These results mean that the localized protons at the water-membrane interface are so stable that it requires a ten millions more sodium (or potassium) cations than protons in the bulk liquid phase to even partially delocalize them at the water-membrane interface. This provides a logical experimental support of the proton electrostatic localization hypothesis. One of the basic assumptions of proton-electrostatics localization hypothesis is that it treats liquid water as a proton conductor and that the proton conduction along the water-membrane interface might be a favored pathway for the proton energy coupling bioenergetics across biological membranes. In this study, experimental evidences discussing water acting as a proton conductor were discussed and the conductivity of water with respect to excess protons was estimated. Overall, these findings have significance not only in the science of bioenergetics but also in the fundamental understanding for the importance of water to life in serving as a proton conductor for energy transduction in living organisms.