Picosecond coherent Raman investigation of the relaxation of low frequency vibrational modes in amino acids and peptides

Picosecond Coherent Raman Scattering (ps CARS) is used to study vibrational dynamics in low temperature hydrogen bonded crystals including various amino acids and peptides. At low temperature this technique can be used to determine the vibrational lifetime T₁ when the ps CARS decay is exponential, and a lower limit to T₁ when it is not. Vibron (Ω = 150-1600 cm^-1) lifetimes are less than 10 ps, whereas librons (Ω = 30-120 cm^-1), which are torsional oscillations of the amino acid or peptide chains, have long lifetimes (10 ps < T₁ < 5 ns). The theory of anharmonic processes is used to calculate the frequency dependence of T₁ for low temperature librons. Most crystals show T₁ ∝ Ω^-4, which is characteristic of spontaneous decay to two counterpropagating acoustic phonons at Ω/2. The crystal l-alanyl-l-tyrosine · 3H₂O shows T₁ ∝ Ω^-2, which is characteristic of decay to one libron and one acoustic phonon. This mechanism dominates when the unit cell is large and complex. The temperature dependent ps CARS data is discussed and is consistent with a mechanism involving absorption of acoustic phonons. Finally the relationship between the crystal dynamics and protein dynamics, and preliminary results on crystalline lysozyme are discussed.