# Aqueous channels within apolar peptide aggregates: Solvated helix of the \alpha-aminoisobutyric acid (Aib)-containing peptide $Boc-{(Aib-Ala-Leu)}_3-Aib-OMe. {2H}_2O.{CH}_3OH$ in crystals

Karle, Isabella L and Flippen-Anderson, Judith and Uma, Kuchibhotla and Balaram, Padmanabhan (1988) Aqueous channels within apolar peptide aggregates: Solvated helix of the \alpha-aminoisobutyric acid (Aib)-containing peptide $Boc-{(Aib-Ala-Leu)}_3-Aib-OMe. {2H}_2O.{CH}_3OH$ in crystals. In: Proceedings of the National Academy of Sciences of the United States of America, 85 (2). pp. 299-303.

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## Abstract

Although the peptide $Boc-{Aib}^1-{Ala}^2-{Leu}^3-{Aib}^4-{Ala}^5-{Leu}^6-{Aib}^7-{Ala}^8-{Leu}^9-{Aib}^{10}-OMe$ [with a t-butoxycarbonyl (Boc) blocking group at the amino terminus, a methyl ester (OMe) at the carboxyl terminus, and four \alpha-aminoisobutyric (Aib) residues] has a 3-fold repeat of residues, the helix formed by the peptide backbone is irregular. The carboxyl-terminal half assumes an \alpha-helical form with torsion angles \phi and \psi of approximately -60 deg and -45 deg, respectively, whereas the amino-terminal half is distorted by an insertion of a water molecule between the amide nitrogen of ${Ala}^5$ [N(5)] and the carbonyl oxygen of ${Ala}^2$ [O(2)]. The water molecule W(1) acts as a bridge by forming hydrogen bonds N(5)...W(1) (2.93 A) and W(1)...O(2) (2.86 A). The distortion of the helix exposes the carbonyl oxygens of ${Aib}^1$ and ${Aib}^4$ to the outside environment, with the consequence that the helix assumes an amphiphilic character despite having all apolar residues. Neighboring helices in the crystal run in antiparallel directions. On one side of a helix there are only hydrophobic contacts with efficient interdigitation of leucine side chains with those from the neighboring helix. On the other side of the helix there are hydrogen bonds between protruding carbonyl oxygens and four water molecules that separate two neighboring helices. Along the helix axis the helices bind head-to-tail with a direct hydrogen bond N(2)...O(9) (3.00 A). Crystals grown from methanol/water solution are in space group ${P2}_1$ with a = 15.778 \pm 0.004 A, b = 11.228 \pm 0.002 A, c = 18.415 \pm 0.003 A, \beta = 102.10 \pm 0.02 deg, and two formula units per cell for $C_{49}H_{88}N_{10}O_{13}. {2H}_2O.{CH}_3OH$. The overall agreement factor R is 7.5% for 3394 reflections observed with intensities >3\sigma(F), and the resolution is 0.90 A.

Item Type: Journal Article Copyright for this article belongs to National Academy of Sciences. apolar decapeptide;hydrogen bond;helix distortion;amphiphilic helix mimic;transmembrane peptide channel Division of Biological Sciences > Molecular Biophysics Unit M.K Anitha 25 Feb 2005 19 Sep 2010 04:18 http://eprints.iisc.ac.in/id/eprint/2849