Hydration profiles of aromatic amino acids: conformations and vibrations ofl-phenylalanine–(H₂O)_nclusters

Abstract

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine-(H(2)O)(n) clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe-(H(2)O)(1), and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe-(H(2)O)(2), only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe-(H(2)O)(3) the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the pi-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation.

Cold collisions catalyse conformational conversion

“…Recently, it has been experimentally shown that the abundance of a particular conformer of the amino-acid phenylalanine in a supersonic expansion critically depends on the carrier gas that is being used. 33 It seems likely that the above described model applies here as well. Also, it is interesting to note that there presently is a large activity in the field of cold collisions.…”

The x-ray absorption spectroscopy model of solvation about sulfur in aqueous L-cysteine

“…Similar water molecule networks have been reported for other dissolved aqueous amino acids. [73][74][75][76][77] Such a network would constitute an extended stable shell of water molecules about cysteine. Any durably located bridging network of hydrogen-bonded water molecules with a S Cys -O w1 -O w2 angle of 150…”

Mid-IRspectra of different conformers of phenylalanine in the gas phase

“…Second, and more interesting, is the observation of variations in the relative intensities of conformers A and E. When using argon as a carrier gas, conformer E is much depleted and A increased, resulting in an intensity pattern that is more similar to the previous R2PI spectrum of Simons et al 4 A similar observation for phenylalanine has recently been made by others. 33 It is known that the nature of the carrier gas can affect conformer distributions, and a general explanation has been given recently and illustrated with the conversion between two conformers of the weakly bound benzene dimer catalyzed by the interaction with rare gas atoms. 34 In this model, the binding energy of the rare gas atom to the molecule in a short lived collision complex is temporarily used to overcome barriers.…”