As shown in Figure 6B , the far-UV CD spectrum of N TAILW518 is almost perfectly superimposable on that of N TAILHN , thus indicating that the introduction of the tryptophan residue does not affect the overall secondary structure content of the protein. In order to investigate whether the variant form retained the ability to undergo induced folding in the presence of XD, we have added various molar excesses (ranging from 1 to 4) of XD to N TAILW518 , and recorded the corresponding far-UV CD spectra (data not shown). These latter studies indicate that the tyrosine to tryptophan substitution does not affect the ability of the protein to undergo induced folding in the presence of the partner, thus supporting the biochemical relevance of this variant form.