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Chemistry and toxicology of cigarette smoke and biomarkers of exposure and harm – how tobacco smoke causes disease: the biology and behavioral basis for smoking-attributable disease – ncbi bookshelf

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In water or in the droplets of particulate matter in tobacco smoke, the distribution of nicotine among its three forms depends on the pH of the solution. Increasing acidity of the solution increases the fraction of protonated molecules conversely, increasing basicity increases the fraction in the unprotonated (free base) form (Figure 3.3). Because all forms of nicotine are highly soluble in water, all of the nicotine entering the respiratory tract from one puff of tobacco smoke easily dissolves in lung fluids and blood. However, because unprotonated nicotine from tobacco smoke particles is volatile, whereas protonated nicotine is not, a higher percentage of unprotonated nicotine in a puff results in a higher rate of nicotine deposition in the respiratory tract (Pankow 2001 Henningfield et al. 2004). The exact nature and effects of the increased rate of deposition depends on the chemical composition and the size of particles in the tobacco smoke, as well as topographic characteristics of smoking, such as puff size and duration and depth of inhalation. Increased rates of deposition in the respiratory tract lead to increased rates of nicotine delivery to the brain, which intensify the addictive properties of a drug (Henningfield et al. 2004). The conventional view has been that a sample of particulate matter from tobacco smoke is not usually so acidic that the diprotonated form becomes important. In water at room temperature, the approximate dividing line between dominance by protonated forms or by the unprotonated form is a pH of 8 (Gonz&#x000e1 lez et al. 1980). At higher pH, the fraction of unprotonated nicotine (&#x003b1 fb) is greater than the fraction of protonated nicotine (Pankow 2001). At pH 8, the two fractions are present in equal percentages. At any lower pH, the fraction of protonated nicotine is greater.

Because a typical sample of particulate matter from tobacco smoke collected from a cigarette or cigar is mostly nonaqueous liquid, it is not possible to take conventional pH measurements to determine nicotine distribution between the monoprotonated and unprotonated forms (Pankow 2001). However, it is possible to measure the concentration of unprotonated nicotine in a sample of tobacco smoke particulate (cp,u), because that level produces a directly proportional concentration of unprotonated nicotine in the gas phase, which is measurable (Pankow et al. 1997, 2003 Watson et al. 2004). Measuring the concentration of nicotine in a sample of tobacco smoke in the particulate phase (cp,t) allows calculation of the fraction of unprotonated nicotine &#x003b1 fb cp,u/cp,t (Pankow et al. 2003). To simplify the discussion of &#x003b1 fb values in tobacco smoke, Pankow (2001) introduced the term &#x0201c effective pH&#x0201d (pHeff), which refers to the pH needed in water to obtain the &#x003b1 fb value in a sample of particulate matter from smoke. Reported values of &#x003b1 fb for smoke from commercial cigarettes at 20oC were 0.006 to 0.36 (Pankow et al. 2003 Watson et al. 2004), which corresponds to pHeff values at 20oC in the range of 5.8 to 7.8.

The fraction &#x003b1 fb for particulate matter in tobacco smoke is important because the rapidity with which inhaled nicotine from tobacco smoke evaporates from the particulate phase and deposits on the linings of the respiratory tract is directly proportional to the &#x003b1 fb value for the smoke (Pankow et al. 2003). According to numerous tobacco industry documents, increasing levels of unprotonated nicotine in tobacco smoke was known to increase smoke &#x0201c strength,&#x0201d &#x0201c impact,&#x0201d &#x0201c kick,&#x0201d and/or &#x0201c harshness&#x0201d (Backhurst 1965 Dunn 1973 Teague 1974 Ingebrethsen and Lyman 1991). Because of similar mechanisms, nicotine replacement therapy delivering gaseous nicotine caused throat irritation at delivery levels per puff that were similar to those reached by smoking a cigarette rated by using the FTC regimen at approximately 1 mg of total nicotine delivery thus, cigarette design is focused on a balance between smoke &#x0201c impact&#x0201d and irritation. Some researchers have suggested that the irritation and harshness of smoke at higher pH makes it harder for smokers to inhale this smoke into the lungs (Brunnemann and Hoffmann 1974).

The value of &#x003b1 fb for particulate matter in each puff of smoke from one brand of cigarette or cigar strongly depends on the overall proportion of acids to bases in the puff (Pankow et al. 1997). As already noted, nicotine itself is a base. The natural acids in tobacco smoke (e.g., formic acid, acetic acid, and propionic acid) can protonate nicotine and tend to reduce &#x003b1 fb from its maximum of 1.0. The natural bases (e.g., ammonia) tend to neutralize the acids and keep more nicotine in the unprotonated form.

Variability in the acid base nature of commercially available tobacco leaf is considerable. Flue cured (&#x0201c bright&#x0201d ) tobacco is typically viewed as producing acidic smoke. Air cured (&#x0201c burley&#x0201d ) tobacco is typically viewed as producing basic smoke. Simple adjustment of the tobacco blend can therefore produce a considerable range of acid or base content in tobacco smoke. In acidic smoke, &#x003b1 fb can be 0.01 or lower (e.g., 1 percent unprotonated nicotine), and in basic smoke, the &#x003b1 fb can be relatively high (e.g., 0.36 36 percent unprotonated nicotine ) (Pankow et al. 2003 Watson et al. 2004).

Tobacco additives that are bases increase &#x003b1 fb values in mainstream smoke, and these additives are discussed extensively in tobacco industry documents (Henningfield et al. 2004). The documents reveal that a variety of basic additives have been considered, including ammonia and ammonia precursors. Conversely, some manufacturers also were interested in reducing harshness to a minimum and investigated acidic additives such as levulinic acid as &#x0201c smoothing&#x0201d agents. In that context, the natural basicity of a specific blend and the harshness of the smoke can be reduced by acidic additives such as levulinic acid, which tend to reduce &#x003b1 fb (Guess 1980 Stewart and Lawrence 1988).

In summary, nicotine in cigarette smoke exists in either a protonated or unprotonated form, depending on a number of factors, including the presence of natural acids and bases, the tobacco blend, tip ventilation, and the use of additives. Cigarette design ensures that the smoke has enough unprotonated nicotine to rapidly transfer nicotine into the body but not so much of it as to be too harsh for the smoker to continue to smoke.