Hydrogen bond flexibility

Of course, it is somewhat artificial to simply impose a single fixed H-bond length on a system such as (H20-H+-0H2). In the first place, this particular system has an equilibrium interoxygen distance of about 2,4 A, short enough that the transfer potential has a single central minimum. We are more 

The force constant or stiffness of the spring is represented by k. r refers to the distance between the two starred hydrogens which serve as anchors, and Vq is their separation in the absence of any forces other than the spring itself. By choosing different values of k and r, it is possible to extract the sensitivity of the proton transfer barrier to both the stiffness of the forces holding the two O atoms apart and their equilibrium separation in the protein. 

The explicit dependence of the transfer properties upon the distance separating the two anchor atoms is reported in Table 2. All proton transfers took place within the context of a spring of uniform stiffness k=8 mdyn/A. As listed in the first row, when the spring is 4.0 A long, i.e. when the two anchor atoms would be 4.0 A apart in the absence of other forces, the transfer potential has no effective barrier, and the two O atoms can approach to within about 2,45 A of one another. When the spring is stretched to 4.2 A, a barrier appears, albeit only 1.2 kcal/mol high. The equilibrium separation of the two O atoms is 2.59 A, which contracts to 2.48 A in the transition state where the proton is equidistant between them. A further stretch of 0.3 A in 

Dependence of parameters of proton transfer potentials (A and kcal/mol) upon the length of spring. Force constant k is equal to 8 mdyn/A. 

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Neutron Diffraction Studies on Proteins Give Insight into Local Hydrogen-Bonding Flexibility... 

The reaction of di- and polyisocyanates with hydroxyl-bearing polyesters and polyethers, co-reactants in many isocyanate adhesive applications, produces the strong, polar, hydrogen-bonded, flexible polyurethanes which, when wet, intimately contact and show strong attraction for a variety of surfaces. 

Simplified models for proteins are being used to predict their stmcture and the folding process. One is the lattice model where proteins are represented as self-avoiding flexible chains on lattices, and the lattice sites are occupied by the different residues (29). When only hydrophobic interactions are considered and the residues are either hydrophobic or hydrophilic, simulations have shown that, as in proteins, the stmctures with optimum energy are compact and few in number. An additional component, hydrogen bonding, has to be invoked to obtain stmctures similar to the secondary stmctures observed in nature (30). 

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