SynopsisThe hydration of DNA is an important factor in the stability of it.s secondary structure. Methods for measuring the hydration of DNA in solution and the results of various techniques are compared and discussed critically. The buoyant density of native and denatured T-7 bacteriophage DNA in potassium trifluoroacehte (KTFA) solution has been memured as a function of temperature between 5 and 50°C. The buoyant density of native DNA increased linearly with temperature, with a dependence of (2.3 f 0.5) X 10-4 g/cc-"C. DNA which has been heat denatured and quenched at 0°C in the salt solutiori shows a similar dependence of buoyant density on temperature a t temperatures far below the T,= and above the T,. However, there is an inflection region in the buoyant density versus 1' curve over a wide range of temperatures below the T,. Optical density versus temperature studies showed that this is due to the inhibition by KTFA of rccovcry of secondary structure on quenching. If the partial specific volume is assumed to be the same for native and denatured DNA, the loss of water of hydration on denaturation is calculated to be about 20% in KTFA at a water activity of 0.7 at 25°C. By treating the denaturation of DNA as a phase transition, an equation has been derived relating the destabilizing effect of trifluoroacetate to the loss of hydration on denaturation. The hydration of native DNA is abnormally high in the presence of this anion, and the loss of hydration on denaturation is greater than in CsCl. In addition, trifluoroacetate appears to decrease the A H of denaturation. 132sReceived February 1, 196s

The dependence of the net hydration of DNA on it,s base composition has been measured by density gradient ultrncentrifugation of three 1)l"u'A's in a series of cesium and lithium salt solutions of different water aativiticxs. 1t;strapolation to zero water activity showed the tkyeridcucc of thc partial specific volume 0 1 1 base composition to be very small for CsDX.4 and zero for 1,iI)XA. At least 907& of the dependence of buoynut density on base composition ran be nccountrd for on the basis of a diffcrcntial hydration, with a mole of adcnine-thymine pairs binding about 2 moles more water than a molc of gunnine-cytosine pairs in CsCI.*Ahhrrviations: G = guauine, C = cytosine, . I = atknosiirc., T = thyrni(1iiw. 134.5

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