The individual H-bond energies have been calculated at the B3LYP/D95** level for linear chains of H-bonding formamides containing from 2 to 15 monomeric units. The cooperative effect upon the strongest H-bonds (those nearest the center of the 15-formamide chain) approaches 200% that of the dimer. The cooperative interaction far exceeds that expected for electrostatic interactions. The large variation in the calculated H-bonding enthalpies cannot readily be modeled using pairwise nearest-neighbor potentials. The energetic data obtained from the DFT calculations have been empirically fit using parameters based upon the chain length (n) and the H-bond type (k) which corresponds to the position of the H-bond in the chain (k ) 1 for terminal, 2 for penultimate H-bonds, etc.). Each type (k) of H-bond asymptotically approaches a limiting interaction energy specific for that type. As k becomes larger, the initial H-bond for that type becomes more stable, but the cooperative contribution to that type becomes less. The results are discussed with respect to their utility for improving the modeling of peptide structure and protein folding.

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The cooperative contributions to the H-bonding interaction energies of the adenine-thymine and guaninecytosine base pairs have been evaluated using molecular orbital theory. The energies of the individual bonds in each base pair were ascertained by using models structures that keep one H-bond at a time intact by rotating one base with respect to the other about the axis of each H-bond to form structures with the bases perpendicular to each other. The energies of the individual H-bonds calculated in this way are compared with those of the planar base pairs. Optimized geometries were obtained using ab initio molecular orbital theory with electron correlation (MP2/D95**) and density functional theory (B3LYP/D95**). The cooperative contributions are of similar magnitude for each base pair. However, since the A‚T overall interaction is weaker, the cooperative interaction provides 31% of its stability versus only 12-16% for G‚C. The relatively smaller cooperative contribution to G‚C is due to the difficulty of forming three optimal H-bonds between two rigid molecules. Structural modifications that might strengthen one H-bond tend to weaken another. As a result the central H-bond of G‚C appears to be compressed by the attractive interaction in the two outer H-bonds. To the extent that these observations can be generalized, they should be important to the design of materials that utilize H-bonding motifs for self-assembly.