Total protonic potential difference

The maximal value that the proton-motive force can reach is limited by the redox potential difference ( A q) between the electron donor and acceptor and by the total number of protons (n) translocated per two electrons transferred ... 

The proposed mechanism involves two main aspects. First, the trans-membrane concentration difference of H20-H+ determines the direction and rate of the ballistic proton flux and the corresponding amount of ATP synthesis or hydrolysis. Second, in the direction of ATP synthesis, a threshold electric potential difference is obligatory for compensation of a certain dissipation of the proton s kinetic energy, (e.g. with a total energy of 0.5 proton volt, 0.1 volt compensates for 20% loss). Thus, the ballistic proton mechanism elucidates the thermodynamic concept of a proton-motiveforce in the chemiosmotic hypothesis [42, 43]. It accounts quantitatively for the elementary energetic event it bypasses the problem of trans-membrane proton transportation and it differentiates between independent roles of chemical potential and electrical potential of H20-Fi+ across the membrane [44]. 

The acid-base chemistry of nicotine is now well known and investigations have shown that nicotine in tobacco smoke or in smokeless tobacco prodncts can exist in pH-dependent protonated or nnprotonated free-base forms. In tobacco smoke, only the free-base form can volatilize readily from the smoke particnlate matter to the gas phase, with rapid deposition in the respiratory tract. Using volatility-based analytical measurements, the fraction of nicotine present as the free-base form can be quantitatively determined. For smokeless tobacco products, the situation differs because the tobacco is placed directly in the oral cavity. Hence, the pH of smokeless tobacco prodncts can be measured directly to yield information on the fraction of nicotine available in the nnprotonated free-base form. It is important to characterize the fraction of total nicotine in its conjugate acid-base states as this dramatically affects nicotine bioavailability, because the protonated form is hydrophilic while the nnprotonated free-base form is lipophilic and thus readily diffuses across membranes (Armitage and Turner 1970 Schievelbein et al. 1973). As drug delivery rate and addiction potential are linked (Henningfield and Keenan 1993), increases in delivery rate due to increased free-base levels affect the addiction potential. 

No wonder the electrochemical behavior of O2 is totally different in aprotic media as proton, driving ORR with electron, is lacking in nonprotic solvents when potential sources, like water, are carefully eliminated. 

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