Future Possibilities for IRRAS

Currently, IRRAS spectra may be acquired with good signal-to-noise ratios (up to about 1,000:1) from stable Langmuir films in a reasonable data collection time (arbitrarily chosen to be less than 10 min). Bands with RAs of 0.0001 may be readily detected and frequency shifts of 0.2 cm-1 may be monitored, ^is sensitivity may be utilized in several ways. One application at the limit of current feasibility is isotope labeling of peptides. Substitution of 13C in the peptide bond C=0 group is a useful technique to locate particular secondary structure elements, ^e approach is illustrated in Fig. 3.13. ^e peptide (similar to residues 9-36 in SP-B) WLARALIK RIQAMIPKGA*LA*VA*VA*QVCR, was synthesized in two variants, one with normal 12C=0 bonds, the second with 13C=0 bonds at the indicated alanine residues (Flach et al. 2003). ^e IRRAS spectra of both peptides are shown in the figure, ^e spectrum of the unlabeled peptide shows two major amide I components at around 1,626 and 1,645 cm"1, due to antiparallel P-sheet structure and a mixture of a-helical and random coil, respectively. Isotope labeling enables us to locate the ^-structure within the sequence, ^us, in the spectrum of the labeled peptide, the presence of an amide I component at 1,597 cm-1, coupled with the disappearance of the 1,626 cm-1 band, directly identifies the location of the P-sheet (i.e., at the indicated sites of the

Fig. 3.12. Experimental dichroic ratios from IRRAS spectra of KL4 (P-sheet) in DPPC monolayers (circles) are compared with calculated values for^ = 90, 70, and 60°. The experimental data were best fit with % = 90° and ^=35°

Fig. 3.12. Experimental dichroic ratios from IRRAS spectra of KL4 (P-sheet) in DPPC monolayers (circles) are compared with calculated values for^ = 90, 70, and 60°. The experimental data were best fit with % = 90° and ^=35°

Isotope labeling in the SP-B fragment

WLARALIKRIQAMI PKGA*LA*VA*VA*QVCR

Isotope labeling in the SP-B fragment

WLARALIKRIQAMI PKGA*LA*VA*VA*QVCR

1700 1650 1600 1550

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