120 MACROMOLECULAR COMPLEXES 



water at — 130°C, which had been set aside bv the biologist as a 

 "sanctuary" protecting all life processes from further change, corre- 

 sponds in fact to the upper limit of the vigorous new field of low- 

 temperature chemistry. This discipline concerns itself mainly with 

 the wide variety of low-temperature reactions which occur below 

 150° K and are characterized by low activation energies of 5 kcal/ 

 mol or less. The studies of Klein and Scheer (1958) have demon- 

 strated that numerous chemical reactions involving the addition of 

 hydrogen atoms to solid olefins readily occur at —195° C with 

 measurable activation energies, and also evidence of diffusion proc- 

 esses which are of considerable importance at low temperatures. 

 Recently, these authors have shown that even at 20° K h\'drogen 

 atoms will react with solid oxvgen. Free radicals and many other 

 unstable chemical species will exhibit considerable reactivity at 

 70° K (Broida, 1957), and the trapping of these transient interme- 

 diates by freezing into an inert solid at liquid helium temperatures 

 (Minkoff, 1959) has been the subject of considerable investigation. 

 Free radicals and other intermediates with unpaired electron-spin 

 play an important role in many enzymatic reactions and in photo- 

 synthetic processes (Calvin, 1959a). The possibility of stabilizing 

 these highly reactive intermediates in intact biological systems, 

 which can be adequately achieved only at liquid helium tempera- 

 tures, would therefore amplv justify its application. 



Normal liquid helium I shows poor heat conductivity and is 

 therefore less suitable than its low-temperature phase, known as 

 liquid helium II, for direct, rapid cooling of biological specimens. 

 Helium II, which is obtained by evaporating ordinary liquid helium 

 under reduced pressure until the temperature falls below the critical 

 X-point (2.19° K), exhibits the unique properties of heat supercon- 

 ductivity and superfluidity (Allen, 1952; Mendelssohn, 1956). Un- 

 der certain conditions, the bulk liquid helium II will conduct heat 

 10,000 times better than will copper, and it can flow rapidlx- through 

 the finest capillaries without any viscous drag. These phenomena 

 are restricted to a narrow temperature range, and transfer of heat 

 from a specimen into the bulk liquid is actually impeded by a 

 poorly conducting boundary layer (Allen, 1952; Mendelssohn, 

 1956). However, despite the existence of this thermal boundary re- 

 sistance, helium II appears to be the most effective refrigerant for 

 direct cooling of thin biological specimens to temperatures which 

 are 100° C lower than those obtained with the standard isopentane- 



