The heme 'cooling' following CO photolysis was investigated with picosecond time-resolved anti-Stokes Raman spectroscopy on modified myoglobins (Mb), in which the 6- or 7-propionate is selectively replaced by a methyl group. The time constants of population decay of vibrationally excited states for two modified Mbs became significantly larger compared with those of native Mb. This is the first experimental evidence for appreciable contribution of each heme-propionate side chain to the vibrational energy transfer from the heme to the surroundings. (c) 2006 Elsevier B.V. All rights reserved.