Effects of Na2SO4 on hydrophobic and electrostatic interactions between amphipathic alpha-helices.
The effects of 2 molal Na2SO4 at neutral pH on hydrophobic and electrostatic interactions between amphipathic alpha-helices were investigated by circular dichroism spectroscopy. The amphipathic peptides that were studied included LEK (acetyl-LEELKKKLEELKKKLEEL-NH2) and LEE (acetyl-LEELEEELEELEEELEEL-NH2). In phosphate buffer at neutral pH, only LEK adopted a predominantly alpha-helical conformation, attributable to glu-lys+ interactions where a major contribution is evidently a hydrogen bond (Biochemistry 32: 9668-9676). Despite the presence of lys+ in the e and g' positions of the abcdefg heptad repeat, LEK exhibited mean-residue ellipticities at 222 nm ([theta]222) which were dependent on peptide concentration, indicating the presence of a coiled coil. In the presence of 2 molal Na2SO4 at 25-75 degrees C, the helical content of LEK increased, with the greatest increase observed at 75 degrees C. The value of the ellipticity ratio R ([theta]222/[theta]208) of LEK in 2 molal Na2SO4 also increased, indicating a stronger interhelical association. At 50 degrees C and 75 degrees C, LEK remained predominantly alpha-helical. In phosphate buffer at neutral pH, LEE was mainly random coil. In the presence of 2 molal Na2SO4, however, the peptide formed alpha-helices that associated to form a coiled coil. At 50 degrees C and 75 degrees C, LEE became predominantly random coil but the remaining alpha-helices were still associating. These results are consistent with the strengthening of interhelical hydrophobic interactions and the absence of screening of helix-stabilizing and helix-destabilizing electrostatic interactions in amphipathic alpha-helices by Na2SO4.
Lazo
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Department of Dermatology, University of Iowa, Iowa City 52242, USA. noel-lazo@uiowa.edu
Downing
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DT
D000595:Amino Acid Sequence; D002868:Chromosomal Proteins, Non-Histone; D002942:Circular Dichroism; D008840:Microfilament Proteins; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D055672:Static Electricity; D013431:Sulfates / Q000737:chemistry*
D002868:Chromosomal Proteins, Non-Histone; D008840:Microfilament Proteins; D010455:Peptides; D013431:Sulfates; C094772:centromere protein F; C012036:sodium sulfate