Glycine-water complexes

Balabin, R. M. The first step in glycine solvation The glycine—water complex, J. Phys. Chem. B 2010,114,15075-15078. 

Chaban GM, Gerber RB. Anharmonic vibrational spectroscopy of the glycine-water complex calculations for ab initio, empirical and hybrid quantum mechanics/molecular mechanics potentials. J Chem Phys 2001 115 1340-1348. 

Balabin RM (2010) The first step m glycine solvation the glycine-water complex. J Phys ChemB 114 15075... 

Alonso JL, Cocinero EJ, Lesarri A, Sanz ME, Lopez JC (2006) The glycine-water complex. Angew Chem Int Ed 45 3471... 

Su J, Zhang K, Schwarz WHE, Li J. Uranyl-Glycine-Water Complexes in Solution Comprehensive Computational Modeling of Coordination Geometries, Stabilization Energies, and Luminescence Properties. Inorg Chem. 2011 50 2082-2093. 

Basch, H., and W. J. Stevens. 1990. The Structure of Glycine-Water H-Bonded Complexes. Chem. Phys. Letters 169, 275-280. 

Alonso et al. examined the glycine-one water complex using laser ablation molecular beam Fourier-transform microwave spectroscopy. By comparison of the observed rotational parameters and those predicted from an MP2/6-311+G(d,p) optimization, they concluded that the only observed isomer is that formed of the neutral isomer and water, 44a. Balabin examined the glycine-one water complex using IR spectroscopy. While he too found 44a, he was able to detect a small amount of 44b and 44c. An interesting side note is that anharmonic corrections were necessary in order to match up the computed (MP2) frequencies with the experimental values. These experiments indicate that more than one water molecule is needed to stabilize the zwitterion tautomer. 

The table shows that the neutral glycine - molecule of water complex is the most stable. Moreover, the energy barrier which drives the state of transition is much greater than that which is obtained when Quantum Mechanics is used to reproduce the autoionization of the glycine with the presence of the solvent by means of an intramolecular process. This data suggests that even in the case where a larger number of water molecules are included in... 

Figure 3.4. TJV-vis absorption spectra of 3.10c in water and in water containing 3.0 mM of Cu (glycine) complex, 3.0 mM of Cu(N-methyl-Ftyrosine) and 3.0 mM of Cu(L-abrine).

The concentration of chlorine dioxide, chlorite and total oxidants was determined on site using a portable colorimeter (Palintest Photometer 5000) and a modification of the DPD test in which any chlorine species are complexed with glycine to ensure only chlorine dioxide reacts with DPD. The chlorite and total oxidants are then determined on a fresh sample by acidification and neutralisation in the presence of potassium iodide. The initial dose level was set at 0.3ppm chlorine dioxide injected in the water feed to the cold... 

It has been tacitally assumed in this discussion that the second-order formation rate constants measure the simple water substitution process. Although this must apply when unidentate ligands replace coordinated water, a composite process could describe the replacement by multidentate ligands. However, consideration of rate constants for successive formation and dissociation processes suggests that the overall rate of complex formation with flexible bidentate (and probably multidentate) ligands such as diamines, dipyridyl, glycine is probably determined by the rate of expulsion of the first water molecule from the metal aqua ion (56, 80, cf. 3 and 84). 

The result for glycine is comparable to that of Hammes and Steinfeld and the water dissociation rate constant is probably about 10 times 12. As the charge decreases from +1 to —2 there is a decrease in the kn value rather than the very large increase which would be expected if a charge decrease made the coordinated water much more labile. In fact, the rate drops more or less in order of the statistical number of coordinated water available for substitution. On the other hand, with the +2 polyamines the dien complex shows a substantial increase in rate of water substitution. The individual coordinated water appears to be about 100 times more labile than in Niaq+2. 

A comparison with the glycine work suggests that it is the nitrogen coordination rather than the carboxylate coordination which accelerates the water substitution of the metal complex. It appears that three nitrogens coordinated to Ni(II) gives a labile system which is diminished by having four or five nitrogens coordinated. 

The enhanced reactivity in the cupric ion-catalyzed hydrolysis cannot be due solely to the electrostatic effect of an attack of hydroxyl ion on a positively charged a -amino ester, since the introduction of a positive charge, two atoms from the carbonyl group of an ester, increases the rate constant of alkaline hydrolysis by a factor of 103 (10), whereas there is a difference of approximately 106 between the cupric ion-catalyzed and the alkaline hydrolyses of DL-phenylalanine ethyl ester. The effective charge on the cupric ion-glycine (buffer)-ester complex is +1, so that the factor of 106 cannot be explained by an increase in charge over that present in the case of betaine. Furthermore, the reaction cannot be due to attack by a water molecule on a positively charged a-amino acid ester, since the rate constant of the acidic hydrolysis of phenylalanine ethyl ester is very small. It thus seems... 

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