Changes in the temperature, pH, ionic strength, or denaturant concentration of aqueous solutions of the monomeric non-a-helical peptide acetylYEAAAKEAPAKEAAAKAamide generate changes in its dichroic spectrum characteristic for a conformational transition. This transition has the characteristic features of a residue PII/unstructured conformational equilibrium in which PII denotes an extended left-handed helical conformation and unstructured denotes all the remaining conformations in a random coil ensemble. Replacement of the proline residue facilitates population of residues in an a-helical conformation. However, the elliptcity values for these non-proline peptides merge with the ellipticity of the proline peptide as the population of residues in the a-helix conformation is diminished. This convergence suggests that all residues in a host/guest peptide series of the same length share a common PII/unstructured conformational equilibrium in a given solvent. We propose that the fractional helix content of peptides within such a series may be estimated by using a two-state calculation in which the ellipticity for the non-a-helix conformations is provided by a peptide having a central proline guest residue.Keywords: ellipticity; helix content; peptide; PI1 conformation; random coil A variety of studies has examined the effect of a guest amino acid residue on the helical content of a host peptide of defined sequence (e.g., O'Neil & DeGrado, 1990;Lyu et al., 1990Lyu et al., , 1993Forood et al., 1993;Park et al., 1993; Chakrabarrty et al., 1994;Doig & Baldwin, 1995;Munoz et al., 1995). Such studies have been used to rank order the apparent helix propensity of the 20 common amino acid residues and to generate residue statistical parameters for prediction of helical content from sequence.We have utilized the sequence acetylYEAAAKEAXAKEAA AKAamide to synthesize a series of host/guest peptides, each containing a different guest residue X at sequence position 9 (Merutka et al., 1990;Park et al., 1993). These host/guest peptides generate a nested set of dichroic spectra having an isodichroic point at 203 nm and -16,500 deg cm2 dmol" at pH 7 and 0". The alanine 9 guest peptide has the most negative ellipticity at 222 nm, -24,300 deg cm2 d m o l~ I , and a dichroic spectrum characteristic for peptide solutions having a high fractional population of residues in the helical conformation. The proline 9 guest peptide has the least negative ellipticity at 222 nm, +I20 deg cmz dmol", and a dichroic spectrum characteristic for a random coil conformation.The proline 9 guest peptide was used subsequently to measure the effects of a variety of denaturing solvent conditions on the dichroic spectrum of a random coil. We were surprised to observe Reprint requests to: Earle Stellwagen, Department of Biochemistry, University of Iowa, Iowa City, Iowa 52242; e-mail: cmdste@blue.weeg. uiowa.edu. that these solvent conditions generated a nested set of dichroic spectra characteristic for a conformational transition. We suggest that these dic...

Comparative CD measurements were made on the model helical peptides acetylYEAAAKEAXAKEAAAKAamide and acetylYEAAAEKAXAKEAAAKAamide in which X represents a nonaromatic nonionic residue. The former peptide contains three potential i, i + 4 complementary ion pairs at neutral pH, while the latter peptide contains one potential complementary and two potential antagonistic i, i + 4 ion pairs. The effect of pH and ionic strength on the mean residue ellipticity of these peptides was measured at 222 nm and 0 degrees C. These measurements were analyzed assuming a common two-state helix/coil transition and only i, i, + 4 ion-pair interactions. The analyses suggest that the central ion pairs do modulate helical content while the peripheral ion pairs do not, presumably due to the location of the peripheral ion pairs in the frayed ends of the helix. The complementary central ion pair stabilizes the helix by about 0.4 kcal/mole and the antagonistic central ion pair destabilizes the helix by about 0.2 kcal/mole.