Resonance Raman Examination of the Electronic Excited States of Glycylglycine and Other Dipeptides: Observation of a Carboxylate→Amide Charge Transfer Transition

Abstract

We have examined the UV resonance Raman and the VUV absorption spectra of aqueous glycylglycine and other dipeptides. We observe strong resonance Raman enhancement of the amide I, II, and III bands and the amide CαH bending mode in a manner similar to that we observed previously with excitation within the π→π* transition of N-methylacetamide (Chen, X. G.; Asher, S. A.; Schweitzer-Stenner, R.; Mirkin, N. G.; Krimm, S. J. Am. Chem. Soc. 1995, 117, 2884). However, in addition, we observe strong resonance Raman enhancement of the ca. 1400 cm-1 symmetric COO- stretching vibration, whose 206.5 nm Raman cross section is increased 20-fold compared to that of the carboxylate in sodium acetate, for example. Addition of a methylene spacer between the amide and carboxylate groups causes the resonance Raman enhancement of this symmetric COO- stretch to disappear. The UV resonance Raman excitation profiles, the Raman depolarization ratio dispersion, and the VUV absorption spectra of glycylglycine and other dipeptides demonstrate the existence of a new 197 nm charge transfer band which involves electron transfer from a nonbonding carboxylate orbital to the amide-like π* orbital. This transition occurs at the penultimate carboxylate end of all peptides and proteins.