EFFORTS TOWARD DERIVING THE CD SPECTRUM OF A 3(10) HELIX IN AQUEOUS-MEDIUM

There have been two recent reports suggesting that 3(10) helices can b e distinguished from alpha helices by circular dichroism. The differen tiating feature is stated to be a [theta](222):[theta](208) ratio (R2) distinctly smaller than unity. This has been reported for a C-alpha,C -alpha'-disubstituted homooctamer [Toniolo et al. (1996), J. Am. Chem. Sec. 118, 2744-2745] and for alanine-rich systems of 16-21 residue le ngth with modest fractional helicity [Millhauser (1995) Biochemistry 3 4, 3873-3877]. We report here the changes in the CD spectrum produced by inserting aminoisobutyric acid (Aib) residues into the helical doma in of human pancreatic amylin. In order to examine this effect at comp arable net fractional helicities, CD spectra were measured for each sp ecies during the course of a helicity titration by trifluoroethanol ad dition. The addition of five Aib residues gale results of particular i nterest. At low net fractional helicity, this Aib-rich system displays a diminished pi --> pi(parallel to) (circa 208 nm) rotational streng th versus the less Aib-rich species. However, NMR data and comparisons of CD difference spectra suggest that fluoroalcohol-induced extension of the short Aib-rich helix is in the form of an alpha helix. Given t he diminished intensity of the minimum at 208 nm at low net helicity w hen 3(10) conformations should contribute, we urge extreme caution in using a [theta](222):[theta](208) ratio smaller than unity as a diagno stic for 3(10) helices.