Oxygen sensitivity tolerance

Because nitrogenase proteins are oxygen-sensitive (Section I,A,2), mechanisms must exist to either exclude O2 from the vicinity of the enzyme or, alternatively, make the susceptible sites 02-tolerant (Dalton and Postgate, 1969a). Both these mechanisms do exist although the latter (conformational protection) has only been shown in Azotobacter. These mechanisms have been discussed exhaustively in earlier reviews (Postgate, 1971, 1974 Yates... 

Fig. 4 Structures of the (a) [4Fe3S] proximal cluster found in oxygen-tolerant [NiFe]-hydrogenases and (b) [4Fe4S] proximal cluster found in oxygen-sensitive [NiFe]-hydrogenases. Reprinted with permission from Macmillan Publishers Ltd Nature 479(7372) 249-252, copyright 2011...

Nitrogen flushing is still the major application of MAP but the level of about 0.5% oxygen is already below the economic minimum achievable and, depending on the physical form of the food, it is often desirable to achieve lower levels, especially if the aim is to avoid antioxidant or preservative use (Abe and Kondoh, 1989). Estimates of the maximum tolerable oxygen uptake by some foods for a shelf-life of 1 year at 25°C have been tabulated by Koros (1990). This varies from 1-5 ppm for instant coffee, 5-15 ppm for dried foods and snacks and 50-200 ppm for oils that normally contain antioxidant. Aseptically packaged juices and dairy products are also very oxygen-sensitive. 

The main starting materials used in this area of chemistry are Cp2TiCl2 and Cp2ZrCl2, both of which are commercially available and may be handled in air. All the other metallocene complexes described are moisture and, in most cases, oxygen sensitive, and therefore all reactions must be carried out under anhydrous and oxygen-free conditions. Hydroxyl groups are not tolerated in... 

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