Anomeric Effect in “High Energy” Phosphate Bonds. Selective Destabilization of the Scissile Bond and Modulation of the Exothermicity of Hydrolysis

Abstract

A natural bonding orbital (NBO) analysis of phosphate bonding and connection to experimental phosphotransfer potential is presented. Density functional calculations with the 6-311++G(d,p) basis set carried out on 10 model phosphoryl compounds verify that the wide variability of experimental standard free energies of hydrolysis (a phosphotransfer potential benchmark) is correlated with the instability of the scissile O-P bond through computed bond lengths. NBO analysis is used to analyze all delocalization interactions contributing to O-P bond weakening. Phosphoryl bond lengths are found to correlate strongest (R = 0.90) with the magnitude of the ground-state n(O) --> sigma*(O-P) anomeric effect. Electron-withdrawing interactions of the substituent upon the sigma(O-P) bonding orbital also correlate strongly with O-P bond lengths (R = 0.88). However, an analysis of sigma*(O-P) and sigma(O-P) populations show that the increase in sigma*(O-P) density is up to 6.5 times greater than the decrease in sigma(O-P) density. Consequently, the anomeric effect is more important than other delocalization interactions in impacting O-P bond lengths. Factors reducing anomeric power by diminishing either lone pair donor ability (solvent) or antibonding acceptor ability (substituent) are shown to result in shorter O-P bond lengths. The trends shown in this work suggest that the generalized anomeric effect provides a simple explanation for relating the sensitivity of the O-P bond to diverse environmental and substituent factors. The anomeric n(O) --> sigma*(O-P) interaction is also shown to correlate strongly with experimentally determined standard free energies of hydrolysis (R = -0.93). A causal mechanism cannot be inferred from correlation. Equally, a P-value of 1.2 x 10(-4) from an F-test indicates that it is unlikely that the ground-state anomeric effect and standard free energies of hydrolysis are coincidentally related. It is found that as the exothermicity of hydrolysis increases, the energy stabilization of the ground-state anomeric effect increases with selective destabilization of the high-energy O-P bond to be broken in hydrolysis. The anomeric effect therefore partially counteracts a larger resonance stabilization of products that makes hydrolysis exothermic and needs to be considered in achieving improved agreement between calculated and empirical energies of hydrolysis. The avenues relating the thermodynamic behavior of phosphates to underlying structural factors via the anomeric effect are discussed.

Phosphate monoester hydrolysis at tricopper site: The advantage and disadvantage of closely assembled trimetallic active sites

“…S10). The value of K a = 2.16×10 5 M −1 for the 1:1 complexation indicated a strong binding affinity of HPO 4 2− at the tricopper site of 1 (Fig. S11) [53].…”

“…Therefore, such inhibition effect of phosphate was not critical in the catalysis with 2. In addition, the binding constant of HPO 4 2− with 1 in 1:1 (v/v) acetonitrile-water solution was measured by titration experiments (Fig. S10).…”

“…It is necessary to point out that the binding constant in buffer solution is difficult to be determined (buffer solution has a strong absorbance in the same region). However, it is expected that the binding constant of HPO 4 2− with 1 should become smaller due to the competition of buffer anions at the Cu centers [76]. The pH dependence of NPP hydrolysis provides additional evidence for phosphate-induced inhibition of tricopper complex 1.…”

“…The diverse biological roles of phosphates are a result of the contrast between the extreme stability of P O C bonds in phosphate mono-and diesters and the lability of P O P bonds in phosphate anhydrides [2][3][4]. Therefore, the cleavage of P O bonds is critical for phosphate metabolism, and the hydrolysis is the key to control many biological processes involved with phosphate and its derivatives.…”

“…Considerable efforts have been made to synthetically mimic metalloenzymes for phosphate hydrolysis [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38], and trimetallic complexes were paid special attention and were found to function as artificial phosphatases/nucleases with unique activity [39][40][41][42][43][44][45]. Humphreys et al reported that a trinuclear copper complex based on the triethylamine-linked tris-dpa ligand L3 (Scheme 2) was effective in DNA hydrolysis [46,47]; Komiyama et al showed that tricopper complex of N,N,N ,N ,N ,N -hexa[(2-pyridyl)methyl]-1,3,5-tris(aminomethyl)benzene (L4) had notable hydrolytic activity for ribonucleotides [48]; Ungaro and co-workers have synthesized a series of di-and trinuclear copper complexes of tacn ligating units attached to the 1,2-, 1,3-, and 1,2,3-positions of the upper rim of a calix [4]arene platform (L5), and they demonstrated that cooperation between metal ions was significant for the cleavage of P O C bonds in phosphate esters [49,50]. Despite such achievements, however, studies on substrate recognition and binding and on hydrolysis product inhibition are rare from a structural point of view.…”