Fig. 3.13. IRRAS spectra oftwo forms of the peptide, WLARALIKRIQAMIPKGA*L A*VA*VA*QVCR, unlabeled with normal 12C=0 bonds, and labeled with 13C=0 bonds at the indicated alanine residues labeling). To extend this approach, increases in sensitivity will enable the detection of spectral changes in larger proteins labeled at a single site.

^e prospects for enhanced sensitivity center around improved hardware. IR sources are currently standard broadband IR sources, e.g., globars of sintered silicon carbide. On the horizon, the routine availability of tunable IR diode lasers is expected in the next few years, ^ese will provide a source with several orders of magnitude more photons at each frequency, ^ere are several experiments which will benefit from the anticipated increased spectral quality from the improved sources, ^e availability of spectra at better signal-to-noise ratios will permit detection of smaller spectral changes than is currently feasible. However, this in itself is a limited gain, since the interpretation of such spectral changes is sparse. Improved theoretical understanding of spectra-structure correlations will also be required.

A second appealing possibility is the incorporation of array detectors into the IRRAS instrumentation to more rapidly acquire IRRAS spectra of high quality. Rabolt, Chase, and their coworkers (Pellerin et al. 2004; D.B. Chase, personal communication) have demonstrated the feasibility of this powerful technique by using a conventional source and spreading a grating-dispersed IRRAS spectrum onto an IR array detector. Usable IRRAS data have been acquired in as little as 8.7 ms. ^e prospect of combining the ability to rapidly acquire IRRAS spectra with the spatial resolution that planar array detectors are known for will provide opportunities for new investigations.

Acknowledgment, ^e work from Rutgers University described in this chapter was carried out with the generous support of a grant to R.M. from the National Institutes of Health (GM-29864).

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