Generalized Anomeric Interpretation of the “High-Energy” N−P Bond inN-Methyl-N‘-phosphorylguanidine:  Importance of Reinforcing Stereoelectronic Effects in “High-Energy” Phosphoester Bonds

Abstract

Electronic structure calculations have been performed on a model N-phosphorylguanidine, or phosphagen, to understand the stereoelectronic factors contributing to the lability of the "high-energy" N-P bond. The lability of the N-P bond is central to the physiological role of phosphagens involving phosphoryl transfer reactions important in cellular energy buffering and metabolism. Eight protonated forms of N-methyl-N'-phosphorylguanidine have been energy minimized at levels of theory ranging up to B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) to investigate the correlation between protonation state and N-P bond length. Selected forms have also been minimized using the CCSD/6-311++G(d,p) and QCISD/6-311++G(d,p) levels of theory. Bulk solvation energies using the polarized continuum model (PCM) with B3LYP/6-311++G(d,p) test the influence of the surroundings on computed structures and energies. The N-P bond length depends on the overall protonation state where increased protonation at the phosphoryl group or deprotonation at the unsubstituted N'' nitrogen results in shorter, stronger N-P bonds. Natural bond orbital analysis shows that the protonation state affects the N-P bond length by altering the magnitude of stabilizing n(O) --> sigma*(N-P) stereoelectronic interactions and to a lesser extent the sigma(N-P) --> sigma*(C-N'') and sigma(N-P) --> sigma*(C-N) interactions. The computations do not provide evidence of a competition between the phosphoryl and guanidinium groups for the same lone pair on the bridging nitrogen, as previously suggested by opposing resonance theory. The computed n(O) --> sigma*(N-P) anomeric effect provides a novel explanation of "high-energy" N-P bond lability. This offers new mechanistic insight into phosphoryl transfer reactions involving both phosphagens and other biochemically important "high-energy" phosphoester bonds.

Focus on Phosphoarginine and Phospholysine

“…The G°'s for hydrolysis of ATP and phosphocreatine are ~ -7.3 kcal/mol and ~ -10.3 kcal/mol, respectively, but under physiological conditions the differences are probably less significant [19]. It has been argued previously that the large negative free energy of hydrolysis of the phosphagens is due to difference in stabilities of + P-O versus + P-N bonds [20], which has led to the use of the term "high energy bonds".…”

“…Recent high level calculations have shown that in phosphoguanidines the guanidinium group dominates in the competition for the central nitrogen's lone pair of electrons and delocalisation onto the phosphoryl group is negligible (Fig. 14), and therefore opposing resonance cannot explain the large negative free energy of hydrolysis of the phosphagens [19]. Fig.…”

“…The value for phospholysine is probably fairly similar. Thus, phospholysine is thermodynamically much less stable than MgATP, which has a G°' hydrolysis of -(6-7) 3 kcal/mol [18] (~ -7.3 kcal/mol for ATP, [19]). Whether nature takes advantage of the high energy of phospholysine in phosphoryl transfer reactions remains to be seen.…”

“…The two most common phosphagens are phosphoarginine (19), used by most invertebrates, and the vertebrate equivalent, phosphocreatine (21) (Fig. 12).…”

“…It has been argued previously that the large negative free energy of hydrolysis of the phosphagens is due to difference in stabilities of + P-O versus + P-N bonds [20], which has led to the use of the term "high energy bonds". This terminology is somewhat misleading as free energy of hydrolysis does not always correlate with bond energy (the energy required to break a bond, not hydrolyse it [19]). Furthermore, as described earlier for phospholysine, there is little overall difference in the sum of energies of all bonds broken versus those formed, so this explanation is not valid [21].…”