Vibrational Energy Dynamics of Glycine,N-Methylacetamide, and Benzoate Anion in Aqueous (D2O) Solution

Abstract

Ultrafast infrared-Raman spectroscopy is used to study vibrational energy dynamics of three molecules in aqueous solution (D(2)O) that serve as models for the building blocks of peptides. These are glycine-d(3) zwitterion (GLY), N-methylacetamide-d (NMA), and benzoate anion (BZ). GLY is the simplest amino acid, NMA a model compound with a peptide bond, and BZ a model for aromatic side chains. An ultrashort IR pulse pumps a parent CH-stretch on each solute. Anti-Stokes Raman monitors energy flow through the solutes' strongly Raman-active transitions. Stokes Raman of D(2)O stretching functions as a molecular thermometer to monitor energy dissipation from solute to solvent. A three-stage model is used to summarize the vibrational energy redistribution process and to provide a framework for discussing energy dynamics of different molecules. The initial CH-stretch excitation is found to be delocalized over some or all of the solute molecule in NMA and BZ but not in GLY. The overall time constants for energy dissipation are 7.2 ps for GLY, 4.9 ps for NMA, and 8.0 ps for BZ. CH-stretch energy in GLY is redistributed in a nearly statistical manner among observed GLY vibrations. In NMA the energy is distributed among about one-half of the observed vibrations, and in BZ much of the observed energy is channeled along a CH-stretch to the ring stretch pathway. The strongly Raman-active vibrations accurately represent the flow of vibrational energy through NMA but not through GLY or BZ.