PROTONATED POLYNUCLEOTIDES .7. THERMAL TRANSITIONS BETWEEN DIFFERENT COMPLEXES OF POLYINOSINIC ACID AND POLYCYTIDYLIC ACID IN ACID MEDIUM

The interaction between poly (I) and poly (C) in acid medium has been studied by potentiometric titration, mixing curves and thermal denaturation. Phase diagramms as a function of ionic strength, pH, and temperature have been established. From these data it is shown that the acid titration of the complex poly (I) · poly (C) passes through a triple‐stranded intermediate poly (I) · poly (C) · poly (C+) to yield finally the protonated double‐helical complex poly (I) · poly (C+). The mixing curves indicate the sole presence of the three‐stranded complex in the intermediate zone. On the basis of the pK's the coexistence between the three‐stranded complex with the neighboring double‐stranded structure is demonstrated in a narrow rang of pH and ionic strength. The geometry of the base arrangements, their conformation and the sense of the strands are discussed in the light of the data presented. A Hoogsteen‐type pairing between the bases for poly (I) · poly (C+) is favored, although the reverse Hoogsteen pair cannot be excluded. Copyright © 1969 John Wiley & Sons, Inc.