Conformational study of tyramine and its water clusters by laser spectroscopy.

Tyramine and its monohydrated clusters have been investigated by several laser spectroscopic methods in a pulsed molecular beam. The conformational structures and their effects on hydration have been revealed by resonant two-photon ionization (R2PI), UV-UV ion-dip, and ab initio calculations. UV rotational band contour spectra of the S1 <-- S0 origin bands enabled determination of ethylamine side chain conformations for all seven stable conformers of tyramine. When coexpanding tyramine with a mixture of Ar and water vapor, we have found two kinds of conformational effects on hydration. One is sensitive to conformation of the ethylamine chain and the other to the orientation of the OH group, particularly in the most stable pair of conformers. UV-UV ion-dip spectra detected seven stable conformers of the monohydrated clusters, of which hydrogen-bonding structures, spectral shifts, and origin band intensity distributions are well explained by considering tyramine as a hybrid of phenylethylamine (PEA) and phenol. Monohydration of the most stable gauche conformer pair (cis and trans) of tyramine leads to more detailed conformational assignments regarding the orientation of the phenolic OH group. Cyclic hydrogen-bonding linkage formed in the monohydrated cluster pair is found to be sensitive to the orientation of the phenolic OH group. One of the cluster pair, in which tyramine has the gauche-cis conformation, is more stabilized by the cyclic hydrogen bonding and its origin band intensity becomes stronger than that of the other.