Carboxyphosphate,
a suspected intermediate in ATP-dependent carboxylases,
has not been isolated nor observed directly by experiment. Consequently,
little is known concerning its structure, stability, and ionization
state. Recently, carboxyphosphate as either a monoanion or dianion
has been shown computationally to adopt a novel pseudochair conformation
featuring an intramolecular charge-assisted hydrogen bond (CAHB).
In this work, additive and subtractive correction schemes to the commonly
employed open–closed method are used to estimate the strength
of the CAHB. Truhlar’s Minnesota M06-2X functional with Dunning’s
aug-cc-pVTZ basis set has been used for geometry optimization, energy
evaluation, and frequency analysis. The CHARMM force field has been
used to approximate the Pauli repulsive terms in the closed and open
forms of carboxyphosphate. From our additive correction scheme, differential
Pauli repulsion contributions between the pseudochair (closed) and
open conformations of carboxyphosphate are found to be significant
in determining the CAHB strength. The additive correction modifies
the CAHB prediction (Δ<i>E</i><sub>closed–open</sub>) of −14 kcal/mol for the monoanion and −12 kcal/mol
for the dianion to −22.9 and −18.4 kcal/mol, respectively.
Results from the subtractive technique reinforce those from our additive
procedure, where the predicted CAHB strength ranges from −17.8
to −25.4 kcal/mol for the monoanion and from −15.7 to
−20.9 kcal/mol for the dianion. Ultimately, we find that the
CAHB in carboxyphosphate meets the criteria for short-strong hydrogen
bonds. However, carboxyphosphate has a unique energy profile that
does not result in the symmetric double-well behavior of low-barrier
hydrogen bonds. These findings provide deeper insight into the pseudochair
conformation of carboxyphosphate, and lead to an improved mechanistic
understanding of this intermediate in ATP-dependent carboxylases
