ABSTRACT:Poly(L-glutamic acid) is one of the polyamino acids containing a bulky side chain. At the room temperature and at pH < 5 it exists in ex helical conformation but at pH > 7 the ex helix is reduced to the random coil state. Normal mode analysis of poly(L-glutamic acid) has been reported by Sengupta and Krimm but no phonon dispersions have been reported so far. As a consequence both the thermodynamical studies and fuller interpretation of spectra remain incomplete. In the present communication we report complete normal mode analysis and full dispersion curves for poly(L-glutamic acid). A comparative study of ex and /32 forms of poly(L-glutamic acid) has been carried out in order to identify and to understand the spectral features due to two different conformations. Heat capacity has also been calculated by using density-of-states (g(v)). For the calculation of normal modes the Urey Bradley force field has been used. The normal modes for N-deuterated systems have also been studied to check the validity of the assignments and force field. Our results are in good agreement with the observed experimental data.
Normal modes and their full dispersions are obtained for the right-handed a-helical poly(L-tyrosine). Optically active modes corresponding to the zone center and at the helical angle are assigned. Both the assignments and the force field are supported by the spectra of N-D species. In addition, modes corresponding to the regions of high density of states and other characteristic features of dispersion curves such as repulsion and exchange of character are discussed. Conformation sensitive modes of other a-helices are compared. Heat capacity is obtained from the dispersion curves via density of states and it is found in agreement with the measured data.
ABSTRACT:The dispersion curves for /J2 sheet form of poly(L-glutamic acid) (PLG) are reported alongwith full normal coordinate analysis. Several earlier assignments have been revised. Characteristic features of the dispersion curves, such as repulsion, exchange of character etc., are discussed. Separate contributions to the heat capacity due to purely skeletal, pure side chain, and mix-modes have also been calculated via density-of-states. It is found that the maximum contribution comes from the mixed modes and minimum from the pure backbone modes.
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