Surface enhanced Raman spectra of Phenylalanine (Phe) in Ag colloidal solution have been recorded for Phe solutions at different pH. Spectral line-shape analyses of three enhanced modes have been carried out between pH 4.5 and 10.5. The variation of line-width with pH reveals two mechanisms in solution: (i) the fluctuation of pH in microscopic volume and (ii) the motional narrowing caused by the intermolecular ionic interaction. The different charge states of the reference molecule are responsible for the observed bond softening with decrease in pH. The 'Raman shift' and the 'Raman activity' of the vibrational modes with maximum enhancement have been explained by carrying out DFT calculations. (c) 2006 Elsevier B.V. All rights reserved.