  We use molecular dynamics simulations to study the thermodynamics and
kinetics of alanine dipeptide isomerization at the air-water interface.
Thermodynamically, we find an affinity of the dipeptide to the interface. This
affinity arises from stablizing intramolecular interactions that become
unshielded as the dipeptide is desolvated. Kinetically, we consider the rate of
transitions between the alpha-L and beta conformations of alanine dipeptide and
evaluate it as a continuous function of the distance from the interface using a
recent extension of transition path sampling, TPS+U. The rate of isomerization
at the Gibbs dividing surface is suppressed relative to the bulk by a factor of
3. Examination of the ensemble of transition states elucidates the role of
solvent degrees of freedom in mediating favorable intramolecular interactions
along the reaction pathway of isomerization. Near the air-water interface,
water is less effective at mediating these intramolecular interactions.
