Clusters acidic

Figure 1.10 Comparison of the activation energy of protonation of toluene by (a) an acidic cluster versus that of activation of the same molecule by (b) Mordenite (MOR)...

Controllable Assembly, Structures, and Properties of Lanthanide-Transition Metal-Amino Acid Clusters... 

Abstract Amino acids are the basic building blocks in the chemistry of life. This chapter describes the controllable assembly, structures and properties of lathanide(III)-transition metal-amino acid clusters developed recently by our group. The effects on the assembly of several factors of influence, such as presence of a secondary ligand, lanthanides, crystallization conditions, the ratio of metal ions to amino acids, and transition metal ions have been expounded. The dynamic balance of metalloligands and the substitution of weak coordination bonds account for the occurrence of diverse structures in this series of compounds. 

In the following sections, our work on the amino acid clusters will be reviewed according to the effects of the various reaction conditions on self-assembly. 

Except for the factors mentioned above, such as the reactant ratio employed, variation of lanthanide and transition metal, crystallization conditions, and the presence of a secondary ligand, there are several other factors that can affect the controllable assembly of the lanthanide-transition metal-amino acid cluster compounds. 

Aleshin and coworkers (49) have reported the X-ray crystal structure at 2.2-A resolution of a G2-type variant produced by Aspergillus awamori. Meanwhile, an attempt was made to determine the amino acid residues that participate in the substrate binding and catalysis provided by G2 of A. niger (52). The results of the chemical approach indicated that the Asp-176, Glu-179, and Glu-180 form an acidic cluster crucial to the functioning of the enzyme. This conclusion was then tested by site-specific mutagenesis of these amino acid residues, which were replaced, one at a time, with Asn, Gin, and Gin, respectively (53). The substitution at Glu-179 provided an inactive protein. The other two substitutions affected the kinetic parameters but were not of crucial importance to the maintenance of activity. The crystal structure (49) supports the conclusion that Glu-179 functions as the catalytic acid but Asp-17 6 does not appear to be a good candidate for provision of catalytic base. Thus, there still exists considerable uncertainty as to how the disaccharide is accepted into the combining site for hydrolysis. Nevertheless, the kind of scheme presented by Svensson and coworkers (52) almost surely prevails. 

Takats, Z. Nanita, S. C. Cooks, R. G. Schlosser, G. Vekey, K. Amino Acid Clusters Formed by Sonic Spray Ionization. Anal Chem. 2003a, 75, 1514-1523. 

Takahasi, O., Yamanouchi, S., Yamamoto, K., and Tabayashi, K. (2006). Theoretical study of the x-ray absorption spectra of small formic acid clusters. Chem. Phys. Letters 419, 501-505. 

The shift of the sp states towards the interface with the acidic cluster can also explains the remarkable difference between the H in the atop position adsorbed at the PtyNa20 and at the Pt4/F20 cluster. H is tilted much more towards the interface region in the case of the acidic F20 support (Figure 2). Since the Pt 6s,p states are very important in the Pt-H bonding, the hydrogen atom tries to follow the metal sp states. Therefore, the Pt 6s,p states do not only determine the Pt-H bond energy, but also influence the geometry. 

Gas phase molecular aggregates that contain acid molecules have been produced with free jet expansion techniques and detected by using electron impact ionization mass spectrometry. The clusters of aqueous nitric acid paralleled many properties of the condensed phase. Multiple nitric acid molecules were found in the clusters that were sufficiently dilute. The acid molecule was absent in the ionized clusters involving HC1 and only water was evident. Experiments also demonstrated the reactivity of ammonia with aqueous nitric acid and sulfur dioxide clusters and of sulfur trioxide with water clusters. The natural occurrence of acid cluster negative ions offers a means to probe the gas phase acid loading of the atmosphere through laboratory and field studies of the ion chemistry. 

Electrostatic deflection experiments have shown the acid-water adducts to be polar. However, larger clusters exhibit defocusing behavior indicative of a polarizable, but essentially nonpolar, species. Only for the pure acetic acid clusters, specifically the... 

---