The receptor binding affinity of monocyclic [Ala3,Xaa11]endothelin-1 analogs correlates with inducible helix length

Characterization of a partially folded intermediate of papain induced by fluorinated alcohols at low pH

A systematic investigation of the effects of aqueous 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and 2,2,2-trifluoroethanol (TFE) on the structure of acid-unfolded papain (EC. 3.4.22.2) was made using circular dichroism (CD), intrinsic tryptophan fluorescence, and 1-anilino 8-sulfonic acid (ANS) binding. At pH 2, papain exhibits substantial secondary structure as beta-sheet and is relatively less denatured as compared to 6 M guanidine hydrochloride (GdnHCl) but loses the persistent tertiary structure of the native state. Addition of HFIP and TFE caused an induction of alpha-helical structure as evident from the increase in the mean residue ellipticity value at 208 and 222 nm. Induction was 20% more in HFIP than TFE. Interestingly, at 13% (v/v) HFIP and 30% (v/v) TFE a near-UV CD spectrum approaches the native-like spectral features. Tryptophan fluorescence studies indicate the change in the environment of the tryptophan residues on the addition of HFIP and TFE to acid-unfolded papain. Maximum ANS binding occurs at 13% (v/v) HFIP and 30% (v/v) TFE, suggesting a compact "molten globule"-like conformation with enhanced exposure of hydrophobic surface area. Acid-unfolded papain in presence of 13% (v/v) HFIP and 30% (v/v) TFE showed the recovery of enzymatic activity by 54 and 61%, respectively. Thermal stability of these states was assessed by changes in fluorescence emission maximum and absorbance at 292 nm. Temperature-induced unfolding of papain at pH 2 was non-cooperative and the transition curves were biphasic in nature. Temperature-induced unfolding of HFIP and TFE-induced state was weakly cooperative in comparison to cooperative transition of native.

Formation of a molten-globule-like state of myoglobin in aqueous hexafluoroisopropanol

The effects of aqueous hexafluoroisopropanol (HFIP) media on the structure of myoglobin are reported. Circular dichroism (CD) spectra of this alpha-helical protein in as little as 4% (v/v) HFIP indicate that native-like amounts of secondary structure remain while rigid tertiary structure is lost. However, thermal studies suggest some residual cooperativity of unfolding in this state. At much higher HFIP concentrations, the helicity exceeds the native value and the protein behaves as a series of independent helices which do not interact with each other. We did not observe cold denaturation of myoglobin, even though this phenomenon has been observed for molten globule states of myoglobin, as well as for monomeric amphipathic alpha-helices when moderate quantities of HFIP are present. The pH dependence of trifluoroethanol-induced disruption of tertiary structure revealed that the degree of disruption increases as the enthalpic advantage of the folded state is diminished at low pH.

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 be distinguished from alpha helices by circular dichroism. The differentiating feature is stated to be a [theta]222:[theta]208 ratio (R2) distinctly smaller than unity. This has been reported for a C(alpha)alpha'-disubstituted homooctamer [Toniolo et al. (1996), J. Am. Chem. Soc. 118, 2744-2745] and for alanine-rich systems of 16-21 residue length with modest fractional helicity [Millhauser (1995) Biochemistry 34, 3873-3877]. We report here the changes in the CD spectrum produced by inserting aminoisobutyric acid (Aib) residues into the helical domain of human pancreatic amylin. In order to examine this effect at comparable net fractional helicities, CD spectra were measured for each species during the course of a helicity titration by trifluoroethanol addition. The addition of five Aib residues gave results of particular interest. At low net fractional helicity, this Aib-rich system displays a diminished pi-->pi* (circa 208 nm) rotational strength 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 the diminished intensity of the minimum at 208 nm at low net helicity when 3(10) conformations should contribute, we urge extreme caution in using a [theta]222:[theta]208 ratio smaller than unity as a diagnostic for 3(10) helices.