Drinks daydrinks day

Figure 7.

1169.

1 6 The effects of different alcoholic beverages on mortality. From data reported by Gr0bbxk M et al. (1995) British Medical Journal 310: 1165-

Table 7.2 Prudent upper limits of alcohol consumption

Royal College of Physicians

Women

21 units (— 168 g alcohol)/week 14 units (— 112 g alcohol)/week

Department of Health (1995)

Women

Table 7.3 Amounts of beverages providing 1 unit of alcohol

8 g absolute alcohol '/2 pint of beer (300 mL) 1 glass of wine (100 mL) Single measure of spirits (25 mL)

its outermost shell, is extremely unstable and very highly reactive. Such compounds are known as free radicals.

Free radicals usually exist for only extremely short periods of time, of the order of nanoseconds (10-9 seconds) or less, before they react with another molecule, either gaining or losing a single electron in order to achieve a stable configuration. However, this reaction, in turn, generates another molecule with an unpaired electron. Each time a radical reacts with a molecule to lose its unpaired electron and achieves stability, it in turn generates another radical that is again short-lived and highly reactive. This is a chain reaction.

To show that a compound is a free radical, its chemical formula is shown with a dot

(*) to represent the unpaired electron — for example, the hydroxyl radical is *OH.

If two radicals react together, each contributes its unpaired electron to the formation of a new, stable bond. This means that the chain reaction, in which reaction of radicals with other molecules generates new radicals, is stopped. This is quenching of the chain reaction, or quenching of the radicals. As radicals are generally so short-lived, it is rare for two radicals to come together to quench each other in this way.

Some radicals are relatively stable. This applies especially to those which are formed from molecules with aromatic rings or conjugated double-bond systems. A single unpaired electron can be distributed or delocalized through such a system of double bonds, and the resultant radical is less reactive and longer-lived than most radicals. Compounds that are capable of forming relatively stable radicals are important in quenching radical chain reactions. Their radicals frequently have a long enough life to permit two such stable radicals to come together, react with each other and so terminate the chain. Vitamin E (sections 7.4.3.3 and 11.4)) and carotene (section 7.4.3.4) are especially important in quenching radical reactions in biological systems.

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