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Counts400003000020000Unlabeledi, i+1i, i+2i, i+3εUnlabeledi, i+1i, i+2i, i+3100000175017001650160015501750170016501600Raman Shift (cm -1 )Raman Shift (cm -1 )Fig. 2. Experimental (left) and simulated (right) Raman spectra of the hexapeptide series(i,i+1: A4A5; i,i+2: A2A4; i,i+3: A2A5) in TFE solution (peptide concentration: ca. 100mg/ml).Results and DiscussionTheoretical IR absorption and VCD (Figure 1), and Raman (Figure 2) simulations wereperformed on sequences identical to the synthesized ones with φ, ψ torsion anglesconstrained to an ideal 3 10 -helical geometry (-60°, -30°), but with all the other coordinatesfully optimized.All calculations were carried out for peptides in vacuo using the DTF BPW91/6-31G*levels of theory. The simulations predicted the conformational dependence of the relativeseparations of 13 C=O and 12 C=O features, and the exciton splitting of the 13 C=O band in thedoubly labeled species to be in agreement with those seen experimentally (Figures 1 and 2),with the exception that fraying at the N- and C-termini causing a change in diagonal forcefield are not well represented in the theoretical modeling.Experimental spectra for the octapeptide (A4A5) confirmed the source of deviation forthe related hexapeptides. Comparison of IR absorption and VCD intensity patterns helpedsort out the vibrational coupling constants sensed in the 13 C=O modes. The isotopic labeledgroup vibrations are coupled to each other most strongly when they are degenerate and areeffectively uncoupled from those of the unlabeled groups.References1. Toniolo, C., Benedetti, E. Trends Biochem. Sci. 16, 350-353 (1991).2. Toniolo, C., Crisma, M., Formaggio, F., Peggion, C. Biopolymers (Pept. Sci.) 60, 396-419 (2001).589