Predicting the relative stabilities of multiparticle hydrogen-bonded aggregates based on the number of hydrogen bonds and the number of particles and measuring these stabilities with titrations using dimethyl sulfoxide

This paper describes an experimental method for determining the relative stabilities of hydrogen-bonded aggregates in terms of the mole fraction of dimethyl sulfoxide (DMSO) in chloroform solution, chi(DMSO), required to cause their dissociation. It also describes three indices-I-Tm, I-G, and I-G/(N-1)--that estimate the relative stabilities of hydrogen-bonded aggregates. Each of these indices depend on two variables: HB and N. HB is the number of hydrogen bonds holding the aggregate together; N is the number of particles in the aggregate. The melting-point index, I-Tm = HB/(N-1), corresponds conceptually to a ''melting point'' for the aggregate, that is, a temperature at which it would dissociate into separate particles. This index is the most useful of the three for ''rule of thumb'' estimation of relative stability if assembly occurs cooperatively. The free energy index, I-G = 2.8HB - 16(N-1), corresponds to a free energy of assembly, Delta G, with units kcal/mol. The index I-G/(N-1) = 2.8HB/(N-1) - 16 corresponds conceptually to a free energy of association per particle, Delta G/(N-1). This third index is most useful if assembly occurs noncooperatively.