To verify whether the maximum or the minimum Fukui function site is better for protonation reactions or an altogether different local reactivity descriptor, viz., the charge is necessary, we calculate the Fukui functions (using a finite-difference approximation as well as a frozen-core approximation) and charges (Mulliken, Hirshfeld, and natural population analysis schemes) of several hydroxylamine derivatives, their sulfur-containing variants, and amino acids using B3LYP/6-311G(d,p) technique. While the Fukui functions provide the wrong selectivity criterion for hard−hard interactions, the charges are found to be more reliable, vindicating Klopman's idea. It is transparent from the present results that the hard−hard interactions are better explained in terms of charges, whereas the Fukui functions can properly account for soft−soft interactions known to be frontier-controlled.

Spin-polarized density functional theory is used to analyze chemical reactivity from a more general point of view, which distinguishes between the changes produced by charge transfer between the interacting species (changes in the total number of electrons, N = + TVj where f refers to spin-up or a and , to spin-down or ß) and the changes produced by the redistribution of the electronic density of each of the interacting species (changes in the spin number, Ns = Af -7V|). It is found that the response of the system to changes in N and the external potential is given in terms of the chemical potential, the hardness, the electronic density, and the Fukui function, while the response of the system to changes in 7VS and an external magnetic field is given in terms of a new set of parameters which we have named the spin potential, the spin hardness, the spin density, and the spin Fukui function. Making use of the Kohn-Sham approach to density functional theory, it is shown that the generalized Fukui functions can be reduced to a set of spin-polarized classical frontier orbitals by imposing frozen core approximations.

We studied the energetics of finite and infinite polyalanine chains in the α-helical and extended structure by employing density-functional theory. On the basis of these results we extracted the energy of hydrogen bonds (hb's) and their interactions by taking the full peptide−peptide connectivity (backbone) of proteins into account. We focus on two limiting cases:  an isolated hb and one within an infinite α-helical chain. In the infinite chain the cooperativity within an infinite network of hb's strengthens each individual bond by more than a factor of 2. This effect has important consequences for the stability of α-helices.

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