title: Generalized anomeric interpretation of the "high-energy" N-P bond in N-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.
authors.affiliation: Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306, USA.
authors.forename: Eliza A
authors.initials: EA
authors.lastname: Ruben
authors.forename: Michael S
authors.initials: MS
authors.lastname: Chapman
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Evanseck
mesh_terms: D006146:Guanidines / Q000737:chemistry*; D008958:Models, Molecular; D010755:Organophosphates / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D006146:Guanidines; D010755:Organophosphates