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ABSTRACT:Poly(L-lysine) is a polypeptide having~ bulky hydrophobic side chain. A systematic study of the full vibrational dynamics, including phonon dispersion, for the a-helical form of this biopolymer has been reported using Wilson's GF matrix method as modified by Higgs for an infinite polymeric chain. The calculated frequencies explain well the IR and Raman spectra. A comparison of the amide modes with other a-helical polypeptides is reported here. Spectral frequencies have been obtained for the N-deuterated species to check the validity of force field and correctness of assignments. Heat capacity has also been calculated from dispersion curves via density-of-states using Debye relation in the temperature range 50-500 K.The calculated values are found to be in close agreement with the recent experimental data of Roles et al. [Biopo/ymers, 33, 753 (1993)).KEY WORDS Vibrational Dynamics / Phonon Dispersion / Density-of-States / Heat Capacity / Proteins are considered linear polymers of amino acids with a polypeptide backbone and occasional disulfide cross-linking. Synthetic linear polyamino acids are therefore used as model compounds for the study of more complex proteins. Poly(L-lysine) (Plys) is a very attractive model for studying the protein conformations because it can form extensively any of the three most common secondary structures, namely ()(-helix, antiparallel /J-sheet and the so called random coil, depending on pH and temperature of the medium and the relative humidity of the atmosphere. In the solid state, it is mainly in ()(-helical form. 1 Plys in aqueous solution, below pH 10.5 in which NH 2 group is in a protonated state, exists in random coil conformation. Above this pH the NH 2 group gets deprotonated and acquires ()(-helical conformation below 30°C whereas at higher temperature /J-sheet conformation is preferred over ()(-helix. 2 • 3 The ()(-helix conformation is present at relative humidity of 76% and up while the /J-sheet conformation is observed between 33% and 76% relative humidity. Crystals in ()(-helical conformation are assumed to be hexagonally packed while in /J-sheet they form an orthorhombic unit cell. 4 The free energy difference between the ()(-helix and /J-sheet conformations is very small. 5 The addition of solvents like methanol to neutral solutions of Plys favors helix formation. 6 In the presence of specific ions like Cl0 4 , SCN-etc., Plys with protonated side chains acquires the ()(-helical conformation. 7 -9 Carrier et al. 10 have studied pressure induced changes in the secondary structure of Plys and concluded that in solution both the /J-sheet and unordered polypeptide undergo a reversible, pressure induced conformational change to ()(-helix. The conversion occurs at a much higher pressure from the unordered conformer (9 kbar) than from the /J-sheets (2 kbar). An increase of poly lysine concentration at high pH slows down these conformational transformations.Various techniques such as X-ray diffraction, Raman and infra red (IR) spectroscopy are used to determine the ...

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