Thermodynamics of 27 oligonucleotides with internal A.C mismatches at two different pHs were determined from UV absorbance versus temperature melting profiles. The data were combined with four literature values and used to derive nearest-neighbor parameters for all 16 trimer sequences with internal A.C mismatches at pH 7.0 and 5. 0. The results indicate that the contribution of single A.C mismatches to duplex stability is strongly dependent on the solution pH and the nearest-neighbor context. On average, the protonation of an internal A.C mismatch stabilizes the duplex by 1.39 kcal/mol for DeltaG degrees37 and 7.0 degreesC for the TM. The nearest-neighbor parameters predict DeltaG degrees37, DeltaH degrees, DeltaS degrees, and TM of oligonucleotides presented in this study with average deviations of 6.3%, 11.0%, 12.2%, and 1.8 degreesC, respectively, at pH 7.0 and 4.7%, 5.9%, 6.1%, and 1.3 degreesC, respectively, at pH 5. 0. At pH 7.0, the contribution of single A.C mismatches to helix stability ranges from 2.25 kcal/mol for TCA/AAT to 1.22 kcal/mol for GCG/CAC. At pH 5.0, however, the contribution of A+.C mismatches ranges from 1.09 kcal/mol for TCT/AAA to -0.43 kcal/mol for GCC/CAG. Implications of the results for replication fidelity and mismatch repair are discussed.