Spectral echo formation

In equation (2) f 1 corresponds to the period of echo formation while t2 corresponds to the time during which the echo is sampled coi and a>2 correspond to the dispersion within individual J-spectra and to the normal spectral spread respectively. The resulting two-dimensional spectrum S((Ui,a)2) is of course made up of real and imaginary parts according (26) to equation (3) where terms such as 5 (co 1,0)2) are defined by equation (4), the superscripts s and c referring to sine and cosine respectively. 

We have presented two types of nonlinear IR spectroscopic techniques sensitive to the structure and dynamics of peptides and proteins. While the 2D-IR spectra described in this section have been interpreted in terms of the static structure of the peptide, the first approach (i.e., the stimulated photon echo experiments of test molecules bound to enzymes) is less direct in that it measures the influence of the fluctuating surroundings (i.e., the peptide) on the vibrational frequency of a test molecule, rather than the fluctuations of the peptide backbone itself. Ultimately, one would like to combine both concepts and measure spectral diffusion processes of the amide I band directly. Since it is the geometry of the peptide groups with respect to each other that is responsible for the formation of the amide I excitation band, its spectral diffusion is directly related to structural fluctuations of the peptide backbone itself. A first step to measuring the structural dynamics of the peptide backbone is to measure stimulated photon echoes experiments on the amide I band (51). 

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