Low vibrational modes in a range of 80-400 cm(-1) for bacteriochlorophyll a are excited and observed as beating dynamics in two-dimensional electronic spectra. A coupled multi-vibrational mode displaced oscillator model is proposed to account for the vibronic coherence. We found that these low frequency vibrational modes are coupled. By comparing the fitted lifetime of the vibrational modes appearing in the beating dynamics for bacteriochlorophyll a and a protein-bound bacteriochlorophyll a dimer B820 probed by transient grating method, it is suggested that the protein scaffold provides a protection effect on the vibronic coherence where no excitonic coherence has be excited. (C) 2017 Elsevier B.V. All rights reserved.