Structures and Interaction Energies of Nucleic Acid Base−Amino Acid Complexes. Methylcytosines−Acrylamide Model

The interaction energies between the cytosine methylated derivatives (mCyt, m2 1, N Cyt, m3 1, N , N Cyt) and acrylamide, which models the side chains of natural amino acids asparagine and glutamine, were measured using the temperature-dependent field ionization mass spectrometry method. The experimental enthalpies of the dimer formation are −57.0 kJ mol-1 for the acrylamide−m1Cyt dimer, −58.7 kJ mol-1 for the acrylamide−m2 1, N Cyt dimer and −45.7 kJ mol-1 for the acrylamide−m3 1, N , N Cyt dimer. The dimer structures were determined using quantum-chemical calculations. The calculated structures are almost planar with antiparallel orientations of the monomer dipoles and they are stabilized by intermolecular N−H···O and N−H···N H-bonds. The calculated interaction energies are in good agreement with the experimental data. The results indicate that the interaction energy between acrylamide and m1Cyt is significantly higher than that between acrylamide and m1Uracil. This result is relevant to the mechanism of the protein−nucleic acid recognition.