When exposed t.o a low pH, in various ionic strength conditions and in sufficiently dilute solutions, DNA undergoes a transition, revealed by an increase in opt,ical density. A careful analysis shows that., associated with this transition, there is an effective decrease in absorbance, overcompensated by an increase in scattering. The conditions for the new transition can be summarized conveniently by a graph in a pH-Na + molarit,y diagram. If the pH of a DNA solution is progressively lowered at constant N a f concentration, one finds first the melt.ing transition (I), and, at lower pH values, the new transit.ion (11). If the same experiment is performed on pre-denatured DNA, only transition I1 will be found. If native DNA is bronght, directly to the low pH conditions, wit,hont allowing it t,o denatnre irreversibly at intermediate pH values, transition I1 is reversible (wit,h a small hyst,eresis effect). DNA, initially native, neut,ralized after prolonged exposures to the low pH, recovers t.he buoyant density value of native DNA, along wit.h t,he absorption and scattering properties of the native state. The experiments are consistent, with the interpretation t.hat a new st,ate exists in yhich DNA, still double stranded, assnmes a very compact shape (of the order of 1500 A in diameter for T L DNA), with a hyperchromicity value of 10-147, above t,he nat.ive value. Nearly monodisperse suspensions of I)NA molecule in this apparent state may be obtained only at very low concentrations (-0.23 pg/ml). At 1 pg I)NA/ml aggregation is noticbeable. The possible ronnection with t,he condition of intraphage DNA is disciissed.
* For a disclissiorl 011 the h y p o d ~~~~i i i i ~: ellec( prc?c:etliiig t r:uisili(m, I, see Kefr. I , 2, 16.