A student is trying to determine the mechanism for a reaction that uses ATP to activate a carboxylate ion, which then reacts with a thiol. If the carboxylate ion attacks the g -phosphorus of ATP, the reaction products are the thioester, ADP, and phosphate. However, whether it attacks the a -phosphorus or the b -phosphorus of ATP cannot be determined from the reaction products because the thioester, AMP, and pyrophosphate would be the products in both reactions. The mechanisms can be distinguished by a labeling experiment in which the enzyme, the carboxylate ion, ATP, and radioactively labeled pyrophosphate are incubated, and ATP is isolated. If the isolated ATP is radioactive, attack occurred on the a -phosphorus. If it is not radioactive, then attack occurred on the b -phosphorus. Explain these conclusions.
VSEPR Valence Shell Electron Pair Repulsion Theory (VSEPR) is a qualitative theory that allows us to predict the geometry of molecules based upon their Lewis structures. This model looks at the repulsion of different electron regions (either bonding or non-bonding) and chooses a geometry that minimizes their repulsions. Valence Bond Theory The valence bond model is a quantum mechanical model for understanding bonding in molecules. However, now it is mostly used by chemists as a means to describe the bonding with organic systems qualitatively. The idea is that bonds form between atoms as a result of the overlap of atomic orbitals. This leads to what we would call a localized bonding picture. You can point to a bond in a molecule and say what types of atomic orbitals lead to that parti