Ionic H-Bonds

Problem Use the data in Tables 5.7 and 5.8 and Figs. 5.5 and 5.7 to estimate the relative importance of donor-acceptor interactions in neutral versus ionic H-bond complexes. 

Fig. 12.6 A transfer-NOE derived space-fill model of the polymyxin B (PMB) -lipid A complex. Bidendate ionic H-bonds between pairs of the y-amino groups of Dab residues of PMB and the phosphates of lipid A, as well as hydrophobic interactions between the methyloctanoate group of PMB and the polyacyl domain of lipid A stabilize the interactions...

Fig. 12.15 Model of the interaction of DS-96 (in green) with LPS. Note bidentate ionic H-bonds (salt-bridges, interrupted lines) between the amines and the phosphate groups on lipid A (sticks), and additional salt-bridges between the internal secondary amines and the carboxylates of the KDO residues of LPS...

A more subtle feature described by Pauling later became known as the cooperativity effect a chain of n consecutive H-bonds is generally stronger than n individual H-bonds in isolation from one another. He delved into the unusual characteristics of very short, ionic H-bonds such as (FHF)", questioning whether the equilibrium position of the proton is precisely midway between the two F nuclei. Pauling also indicated that H-bonds did not have to involve two different molecules it was deemed possible for two groups within the same molecule to form a H-bond with one another. 

The triply bonded nitrile and aUcyne groups also engage in ionic H-bonds. Unlike the case when the atoms involved are F or O, the H-bonds in which the C or N atoms of these species participate tend to be noncentrosymmetric, that is, the proton transfer potential contains a pair of minima. The exception is (C=NH-N=C) which is strongly bound by some 27 kcal/mol and which appears to contain a centrosymmetric H-bond. The H-bonds are all weaker when the triply bonded species are replaced by their double-bonded analogs. For example, whereas R(C--C) is equal to 3.35 A for (HC CH"C=CH) , with a dissociation energy of 11 kcal/mol, the bond stretches to 3.7 A for (H2C=CH2 CH=CH2) and the H-bond weakens to 5 kcal/mol. 

Regardless of whether the ionic H-bond is of the cationic or anionic type, the H-bond elongates as the electronegativity of the atoms involved diminishes. The shortest is the interfluorine distance of 2.3 A in (HFH-- FH)+ and (FH "F) the binding energy in the latter is in excess of 40 kcal/mol. Ionic H-bonds involving F, Cl, and O are centrosymmetric ... 

S is on the borderline, while N and P bonds are characterized by double-well potentials. Methyl substitution has little effect upon the nature of these ionic H-bonds. 

One can rationalize a simple relationship between the strength of an ionic H-bond (A—H B) on one hand and the difference in proton affinity between the two partners A and E on the other. The Marcus formulation provides a convenient framework for predicting the El-bond energy of an arbitrary system based on knowledge of the interaction energy in a symmetric system (A=B), and the difference in proton affinity between the two partners. This model has proven successful in a series of interoxygen and internitrogen H-bonds. 

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