The theory developed iri the previous paper to discuss changes in electrostatic free energies in polynucleot ide order-disorder transit ions i s extended to cases where one or more of the participatiiig species is t.itrated to some degree a. It is shown that, for any class of transition, t,he melt,ing temperature T, a t constant pH is a linear function of the logarithm of the monovalent. coiinterion concentration df, that, a t high salt t,he logarithm of the depression of bhe melt.iiig temperature by pH titration is proportional to t2he pH change, and that the stability of t,he ordered form as measured by it,s melting temperature at, neut.ral pH, is a monotonic fuiict.ion of the quantity pH, -pK, where pH,, and pK are the pH of malting and the monomer base pK, both measured under similar co~idit~ions of temperatiire and ionic strength. For the transition from double helix to roil, the dependences of T,, and dT,Jil log A1 on pII are determined experimentally and compared wit,h the qualitative predictions of the t,heory. It is found that CET,~/ d log M , a measiire of -AFel (the negative cf the electrostatic free energy change in the transition), decreases with increasing pII. I n acid solution, where the coil is more extensively proto~iat~ed than the helix, the change in electrostatic free energy in the transition is larger than a t iieiitral pH. Conversely, in alkali the electrostatic free energy change is smaller than a t iieut,ral pII.
(dT,"/d log M)neutral > (dT,,/d log lCl)a~ks~i. At sufficiently high pH, dT,/d log M is observed to become negative, indicating t,hat the electrostatic free energy change is positive in the traiisit.ion of this region. Date from the literature on t.he ionic strength dependence of the melting temperature for the acid helices of poly rA, poly rC, mid poly dC are also considered from the standpoint of the theory.
Hence (dl',Jd log