294 Raymond Latarjet 



any one of its units. Although the concept of energy transfer 

 within a single fibre is familiar, that of transfer between 

 fibres, the links of which are still unknown, carries more 

 subtle implications. It recalls the transfer between enzyme 

 and substrate postulated by Setlow (1955) in order to explain 

 the increased radiosensitivity of hyaluronidase when combined 

 with hyaluronic acid. 



However, it remains possible that, in such a cluster, a hit 

 results in the formation of a very stable cross-link between the 

 aggregated fibres, and that this cross-link prevents successful 

 incorporation by the asssay bacterium. 



(3) Protection against direct and indirect effects. It has been 

 suggested recently (Alexander and Charlesby, 1954) that 

 radiant energy could be transferred from an absorbing macro- 

 molecule to surrounding solutes. This process would bring 

 some protection against the direct effect of radiation upon the 

 macromolecule. We have observed that: 



(a) In the liquid state, TP is much more resistant in 10 per 

 cent than in • 1 per cent yeast extract. 



(b) It is still more resistant in frozen 10 per cent and 0-1 

 per cent yeast extract as well. 



We are inclined to interpret these results as follows: 



(a) In the liquid state, increasing the concentration of 

 yeast extract gives additional protection of TP against 

 the indirect effects of the radicals produced in water. 

 The form of the DNA fibre, with a great surface : volume 

 ratio, explains the high sensitivity to the indirect effect. 



(b) In the solid state, there is no noticeable energy transfer 

 from DNA to the components of the extract. In the 

 present case, there is no protection against the direct 

 effect, as far as physical processes which take place in 

 the solid state are concerned. 



(c) Freezing provides additional protection by preventing 

 migration of active distant radicals from water to DNA, 

 without preventing them from recombining before the 

 system is melted. 



