ABSTRACT:Poly(L-valine) (PLV) belongs to the class of polyamino acids having a bulky side chain. It preferably adopts fl-sheet conformation, although high molecular weight PL V may form an IX helix as well. In the present communication we report a detailed study of the phonons and their dispersion for f/-PLV and the spectra of oligovalines in relation to its polymeric form. The N-deuterated analog of f/-PLV has also been studied to confirm the correctness of assignments and force-field. Density-of-states and heat capacity have been obtained. The heat capacity is in very good agreement with the experimental measurements of Roles et al.

No abstract

ABSTRACT:In continuation to our work on the oligomapping of the dispersion curves of poly(L-valine) in P form, the present work is a study of normal modes and their dispersion in the rx form. The Urey Bradley force field used in Wilson's GF matrix method has been transferred from the p form and refitted to explain the FTIR spectra. The characteristic features of the dispersion curves involve repulsion with character exchange and tendency of certain pair of modes involving the motion of c. atom to come closer near the helix angle. Predictive value of heat capacity is obtained from the density-of-states. It is observed that the mixed modes make the maximum contribution.KEY WORDS Conformation I rx Helix I Crossing Over I Density-of-States IIn an earlier publication, 1 the authors have reported a study of the normal vibrations, their dispersion and oligomapping of the dispersion curves of poly (L-valine) in the f3 form. In continuation of this work, we report in the present communication a study of phonon dispersion in the a form of this biopolymer. Predictive values of heat capacities calculated from the density of states are also presented. To the best of our knowledge no such studies have been reported so far for IX poly(L-valine).Contrary to the earlier conclusions from calculations and experiments that poly(L-valine) (PL V) did not exist in IX-helical form, 2 -5 Scheraga and others 6 on the basis of conformational mapping predicted the existence of Poly(L-valine) in right handed a-helical conformation. Experimental studies on block copolymers of L-valine and D,L-lysine in solution by Epand et al. 7 confirmed this theoretical prediction. High molecular weight PL V can be prepared by polymerizing the N-carboxyanhydride of L-valine in anisole. 8 Raman spectroscopic studies 9 on polymers of L-valine for the degree of polymerization (DP) greater than 500, also indicated the existence of PL V in IX helical form, although a helical content is not directly proportional to DP. The existence of right handed IX PL V (Figure 1) could be attributed to the rotation about the Ca-CfJ bond (x 1 ). The set of x's is an important variable in deciding the minimum energy configuration that the polymer adopts. The rotation around the Ca-CfJ bond moves the bulky side groups (methyl groups) away from the amide groups and thus it relieves the steric hinderance being caused by the proximity of the side-chain and main-chain groups. Earlier non-bonded interactions are now gradually replaced by new ones. This facilitates the formation of IX helix. THEORYNormal mode analysis of a PLV has been performed according to Wilson's GF matrix method 10 as modified by Higgs 11 for an infinite chain. Details about his calculation have been discussed in the paper on phonon dispersion in f3 poly(L-valine). 1 The spectra of high molecular weight poly(L-valine) in the 4000-200 em -1 fre-1 To whom all correspondence should be addressed. 474quency range was obtained from Prof. Takuya Yamashita (Faculty of Pharmaceutical Sciences, Tokushima University, Ja...

No abstract

No abstract