O=C1CSC(c2ccccc2O)N1c1nc2cc3sc(N4C(=O)CSC4c4ccccc4O)nc3cc2s1
An acid's negative base-10 logarithmic dissociation constant (pka) follows directly from its underlying thermodynamics; pKa is directly proportional to its standard Gibbs free energy change. What is the pka value of this molecule?