Complexation free energy

Variational transition-state theory (VTST), as its name implies, variationally moves the reference position along the MEP that is employed for the computation of the activated complex free energy, either backwards or forwards from the TS sttuctme, until the rate constant is minimized. Notationally... 

Katritzky, A.R. Fara, D.C. Yang, H. Karelson, M. Suzuki, T. Solov ev, V.P. Varnek, A. Quantitative structure-property relationship modeling of beta-cyclodextrin complexation free energies. J. Chem. Inf. Comp. Sci. 2004, 44 (2), 529-541. 

Schneider, H.J. Rudiger, V. Raevsky, O.A. The incremental description of host-guest complexes Free energy increments derived from hydrogen bonds applied to crown ethers and cryptands. Org. Chem. 1993, 58 (14), 3648-3653. 

Table 3.17 shows the complexation free energies for various ammonium ions with three corand receptors. It is immediately apparent that NH4+ is bound more strongly than either of its alkylated analogues, and methylammonium is bound more strongly than f-butylammonium, with a particularly large difference between NH4+ and CH3NH3+ complexes of 3.95 and 3.96 (10 kj mol-1). 

Table 3.17 Complexation free energies for corand receptors with ammonium ions.

Table 3.18 Complexation free energies for spherand-type receptors with ammonium ions.12...

The design of preorganised, complementary anion binding hosts exhibiting maximum complexation free energy and selectivity must follow from an appreciation of some of the fundamental characteristics of anions. Listed below are some of the issues that are important to anion binding in particular. 

Figure 6.42 Complexation free energy of cryptophane-C and -E for various guests at 300 K (reproduced with kind permission from Springer Science + Business Media from Section Key Reference 1993).

Figure 6. Evidence for the cation-7t. effect in cyclophane complexes (all complexation free energies AG [kJ/mol], in water).

Figure 7. Complexation free energies AG of the porphyrin macrocycle with benzoic acid derivatives, showing the absence of hydrophobic effects with R = CH3, and the large effect of polarizable groups such as R = N02.

Scheme 6-1. Thermodynamic cycle used to calculate the complexation free energy...

For transition metal coordination compounds AG° involves terms related to the ionization potential / of the free metal ion, the difference in complexation free energy A(AGc), involving the oxidized and the reduced free metal ion, and the difference in solvation free energy A(AGaq), involving the complexes in the two oxidation states ... 

Schneider, H.-J., RUdiger, V. and Raevsky, O.A. (1993). The Incremental Description of Host-Guest Complexes Free Energy Increments Derived from Hydrogen Bonds Applied to Crown Ethers and Cryptands. J.Org.Chem., 58,3648-3653. 

P. Raiteri, A. Laio, F. L. Gervasio, C. Micheletti, and M. Parrinello (2006) Efficient reconstruction of complex free energy landscapes by multiple walkers metadynamics. J. Phys. Chem. B 110, pp. 3533-3539... 

A remarkably simple one-step synthesis of a peptide receptor in 13% yield has been reported by Yoon and Still (Figure 69) [94], This molecule was shown to bind y-acylated amino acids and dipeptide amides with high enantioselectivity. For example, L- and D-enantiomers of N-Boc-Gly-Val-NHMe (Boc=t-butoxycarbonyl, Val = valine) showed complexation free energies of... 

For the seemingly simplest case of spherical metal ion complexation, the hole-size fit often, but not necessarily, holds. Fig. 2.5 illustrates the classical case where the cavity diameter of an ionophore determines the selectivity of cation complexation according to its radius [31]. As long as sufficient contact between the metal ion and the donor atom of the hgand is possible, the complexation free energy will be just a linear function of the number of such interactions and their donor quality... 

A fundamental limitation for fhe apphcation of geometric fitting procedures is that fhe complexation free energies are fhe sum of enthalpic and entropic contributions, with the consequence that selectivity can be inversed at different temperatures. Positive cooperativity between different interactions in a complex will usually lead to tighter association at fhe expense of motional freedom and fhus of entropy [51]. The interplay and often observed compensation of enthalpic and entropic contributions have been discussed in several reviews [10, 52, 53], particularly with emphasis on biological systems, and cannot be dealt with in detail here. Unfortunately, many published enthalpy-entropy compensations are blurred by possible artifacts, as the two underlying parameters do not represent independent variables [54]. 

Fig. 2.13 Decrease of complexation free energy AC with increasing number of single bonds in complexes like those illustrated...

Fig. 2.14 Complexation free energies with ligands of variable flexibility and transition metal ions (see [53]).

Tab. 2.1 Complexation free energies (—AC, kJ/mol) . (a) For anions with model amides 1—3 (b) Of these anions with carbohydrate models C and C...

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