Spectroscopy of Neurotransmitters and Their Clusters:  Phenethylamine and Amphetamine Solvation by Nonpolar, Polar, and Hydrogen-Bonding Solvents

Abstract

Clusters containing a phenylethylamine (PEA) or amphetamine (AMP) molecule and a solvent species such as Ar, CH 4 , CF 3 H, CO 2 , H 2 O, and other small molecules are formed in a supersonic jet expansion. Spectral studies of the solvation and related chemistry of PEA and AMP are pursued by using both fluorescence and mass spectroscopy techniques. To help analyze the experimental results, ab initio and atom-atom LennardJones-Coulomb (LJC) potential calculations are employed to calculate cluster geometries and binding energies. The LJC potential parameters for the 10-12 hydrogen-bonding potentials have been reevaluated on the basis of new ab initio partial atomic charge values and new experimental binding energies and geometries. The observed dependence of the relative spectral intensities of PEA and AMP conformers and their clusters on the cooling conditions (backing pressure and coolants employed) suggests that these species undergo population redistribution in the cooling and clustering process. The amount of excess energy (binding energy) available to the forming cluster plays a major role in the conformational conversion of PEA and AMP during cluster formation. If strong interactions (hydrogen bonding) exist between the solute and the solvent, such conversion/ redistribution processes occur among all conformers and their clusters. The conversion/redistribution process is restricted within the anti or gauche conformer sets and their clusters for weakly interacting solute/solvent pairs. All PEA and AMP clusters studied experience complete fragmentation upon ionization. The observed gradual dependence of photo ion intensity on the ionization laser energy suggests a significant change in geometry for both PEA and AMP, as well as their clusters, upon ionization. Consequently the high vertical ionization energy leads to an excess energy in the vibrational modes of the ions, causing fragmentation of the clusters. The clusters can fragment along two different general paths: (1) simple loss of the solvent molecules and (2) breaking the R-carbon bond of PEA or AMP, with additional loss of solvent molecules in some cases. Those clusters with weaker solute/solvent binding tend to fragment through solvent loss, while those forming hydrogen bonds tend to favor the R-carbon bond cleavage. Reactions are observed for PEA and AMP with NO. NO can completely quench PEA and AMP monomer spectra.

Molecular and supramolecular chirality: R2PI spectroscopy as a tool for the gas‐phase recognition of chiral systems of biological interest

“…Precious structural information has been gathered on biologically relevant chiral molecules, such as neurotransmitters, 35,[77][78][79][80] amino acids, 81,82 small peptides, [83][84][85][86][87][88] and sugars, [89][90][91][92] by the use of the R2PI, IR-R2PI, and UV-R2PI spectroscopies. The same techniques have been employed for investigating the structure and the conformational behavior of covalently bound diastereoisomers.…”

“…Detailed conformational studies on the simplest members of the class of neurotransmitters, as amphetamine, dopamine, noradrenaline, ephedrine, pseudoephedrine and others, has been accomplished 35,[77][78][79] , 97 by the analysis of the of the S 1 /S 0 transition origins of their various conformers and by IR-R2PI determination of ground state frequencies. An important aspect of these studies concerns the conformational changes of the neurotransmitter molecule caused by monosolvation.…”

“…Sometimes, the vertical ionization energy is even higher than the fragmentation thresholds and this causes extensive fragmentation of the system even at threshold ionization. 35 In addition to ionization or dissociation, the R2PI ionized molecule or cluster can undergo more or less extensive fragmentations. The fragmentation thresholds can be measured by scanning the energy of the ionizing/fragmenting (hm 2 ) photon and looking at the appearance thresholds of the produced ionic fragment (Figs.…”

The Gas-Phase Dipeptide Analogue Acetyl-phenylalanyl-amide: A Model for the Study of Side Chain/Backbone Interactions in Proteins

“…Because of possible pollution of the NAPA mass channel by fragmented peptide hydrates or peptide dimers, the composition of the species responsible for the weak signals was carefully checked by monitoring the arrival times after the opening of the pulsed valve. 42, 43 The UV spectrum of NAPA ( Figure 2) is rich and, with the help of the IR/UV measurements, three spectral systems labeled A, B, and C can be distinguished. The most intense system A is composed of a low-frequency (18 cm -1 ) vibrational progression accompanied by much weaker bands.…”

Homolytic CαCβ Bond Cleavage in a Chiral Alkylarene Radical Cation: Effects of Asymmetric Microsolvation

“…[5,6] Mass-resolved resonant twophoton ionization spectroscopy (R2PI-TOF) on a supersonically expanded molecular beam represents a highly accurate tool for determining the energetics of radical ions and their microsolvated derivatives. The energetics of solvation at the microscopic level has been obtained by measuring vibronic spectra, bond-dissociation energies, and IPs (ionization potentials) of various molecular systems.…”