Adventitious chemistry at reduced water activities: free radicals and polyhydroxy agents

Free radicals have been associated with loss of viability of lyophilized bacteria exposed to oxygen. Free radical concentration was proportional to the log of the oxygen pressure in the sample. Sugars, such as lactose or sucrose, preserved viability and inhibited free radical production. Lyophilized tissue, particularly liver and spleen, also reacted with oxygen to produce free radicals, which appear to be associated with ascorbic acid in the tissues. Pure ascorbic acid in air does not produce free radicals, but when mixed with protein before lyophilization it reacts with oxygen in air. When a mixture of sodium ascorbate and phenylalanine or tryptophan is lyophilized, free radicals identical to those observed in tissue are obtained. Propyl gallate and di- or trihydroxybenzoates also react with oxygen when lyophilized with phenylalanine, but the g value of the free radical is significantly less than that obtained with ascorbate. A number of amino acids and similar nitrogenous compounds act as catalysts to form propyl gallate free radicals. As with the bacterial or tissue preparations, various sugars or similar carbohydrates inhibited free radical production by either ascorbate or gallate. In the absence of water the free radicals produced by the action of oxygen on lyophilized samples are stable for years. The rate of free radical production is increased by small amounts of moisture (exposure to moist air), but at humidities over 30% rh the radicals are unstable.