Low frequency (collective) modes in proteins (~10-600 cm-1) have received considerable attention because it is thought that domains or sub-domains in proteins are involved in correlated motions on picosecond timescales. It is clear that motions at these frequencies make an important contribution to the mean-square displacement of atoms from their equilibrium (vibrational) positions. Thus, these motions are the first steps in functionally important motions leading to large-scale conformational changes. Such changes are critical to structural and functional processes like protein folding, cooperativity, and protein-protein or protein-ligand interactions involved in electron transfer, enzyme catalysis, and signaling. Poly-amino acids provide a model system for understanding collective modes in proteins. By polymerizing different amino acid monomers, varying secondary and tertiary structures can be created and studied with far infrared absorption spectroscopy. They provide a controlled method for identifying and characterizing any ?breathing modes? attributable to various ?-helical or ?-sheet type structures in proteins. We have also obtained the far-infrared spectra of poly-L-phenylalanine, poly-L-alanine, poly-L-tryptophan, and poly-L-leucine from 10 to 295 K in 20 K increments. The results demonstrate (1) there are several (ranging from 4-10, depending on the poly-amino acid) low frequency modes in the far-infrared region, (2) the bands below ~200 cm-1 all increase in frequency with decreasing temperature, and (3) this temperature-dependence increases with decreasing band frequency. These results suggest that these modes are Hhighly anharmonic, consistent with various computational studies. Currently, curve-fitting analysis is being used to aid in the identification of various modes, assignment of their origins, and their sensitivity to temperature.
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