The structure of gramicidin A in dimethylsulfoxide/acetone

Abstract

It has been demonstrated by two-dimensional NMR cross-relaxation spectroscopy that gramicidin A exists in dimethylsulfoxide/acetone solution in random coil form. This contradicts earlier conclusions by Hawkes et al. Gramicidin A is a linear pentadecapeptide consisting of alternating L-and D-amino acids:It is known to affect the ionic permeability of lipid bilayers [l, 21 by forming channels that are specific for monovalent cations [3]. Because it exhibits a functional behavior similar to complex macromolecular biological structures, the gramicidin A channel has been a model of choice to study the mechanisms of ionic permeation across lipid membranes [4]. The conductance and selectivity properties of the gramicidin A channel are now well characterized [5]. However, detailed information about the transmembrane channel structure, necessary for a molecular understanding of the function of gramicidin A [6], is not yet available.Several lines of evidence indicate that the main conducting transmembrane channel structure is a head-to-head (N terminus to N terminus) dimer constructed from two helical monomers [7]. However, due to the difficulty of obtaining protein/lipid cocrystals for X-ray diffraction experiments, structural research on the gramicidin A channel has been largely restricted to indirect experimental studies. On the basis of the thallium-ion-induced carbonyl I3C chemical shifts of a series of site-specific I3C-labelled gramicidin A molecules incorporated in lysolecithin lipid vesicles, Urry et al. have argued in favor of a left-handed fi"." helix head-to-head dimer [8-111, designated by ZZ: E:; in the notation used by Bystrov and Arseniv [22]. Nevertheless, the question of the helical sense of the monomers (left-handed vs right-handed) was not solved with certainty by these investigations due to the difficulty of a quantitative interpretation of chemical shifts.Using two-dimensional proton NOE spectroscopy (NOESY), Arseniev et al. [I21 have demonstrated that the gramicidin A structure in sodium dodecyl sulfate micelle dispersion consists of a dimer of right-handed 22; i t : ; helices. Solid-state NMR experiments on gramicidin A incorporatedCorrespondence to R. R. Ernst, Laboratorium fur Physikalische Chemie, ETH-Zentrum, CH-8092 Zurich, SwitzerlandAbbreviations. 2D, two-dimensional; COSY, 2D correlation spectroscopy; Myr2GroPCho, dimyristoylglycerophosphocholine; NOESY, 2D NOE spectroscopy or 2D cross-relaxation spectroscopy; 2QF COSY, two-quantum-filtered COSY.in oriented lipid bilayers have generally confirmed the features of the head-to-head helical dimer but have been inconclusive about the handedness of the helix [13 -181. Recently, experiments with 5N-labelled gramicidin A have provided evidence on the right-handedness of the helix in lipid bilayers of dimyristoylglycerophosphocholine (Myr2GroPCho) based on an analysis of the chemical shift anisotropy tensor [19].A number of structures of gramicidin A has been reported for isotropic solutions, some of which could resemble the structure of the transme...

Vibrational circular dichroism of gramicidin D in organic solvents

“…11 The amide I absorption band maximum position is at a significantly higher frequency (1663 cm Ϫ1 ) in DMSO-d 6 , so the structure of gramicidin in DMSO-d 6 must be significantly different from that in other solvents considered above. This is consistent with the assignment of monomeric random coil structure for gramicidin in DMSO 15 and will be substantiated by VCD observations (see below). In TFE, the absorption band maximum (1646 cm…”

“…9,11,12 All four species are suggested to be present for gramicidin in ethanol. 10 For the structure of gramicidin in dimethylsulfoxide (DMSO) solutions, a monomeric structure based on a succession of ␤-turns was suggested; 13 in another study, rapid conformational order-disorder equilibrium between single strand left-handed ␤ 6.3 helix and local random coil was suggested, 14 but later this conclusion was disputed 15 and a random coil structure was suggested. The structure of gramicidin in trifluoroethanol (TFE), was suggested to be similar to that in DMSO solutions.…”

Circular Dichroism Studies of Tryptophan Residues in Gramicidin

“…In the crystal structure of this form, Trp residues are generally not in close proximity and tend to occur in orthogonal pairs [7]; this may be responsible for the rather weak ellipticity. The near-UV CD spectrum of gramicidin in TFE or in a mixture of acetone and DMSO (not shown) where the peptide has been shown to be monomeric and without periodic structure [8] is featureless with an ellipticity near zero.…”

A novel ultrashort capillary gas chromatography method using on-column injection and detection

“…Further, each of these can also contain a variant with isoleucine in place of valine at position 1. 23 Typically, gramicidin D is mainly comprised of gA (about 72%) with a balance of gC (about 19%) and minor amounts of gB (about 9%), although the actual quantities vary depending on the source. 23 Because of its very large size and inherent thermal instability, analytical separations of this polypeptide are often performed by HPLC or supercritical fluid chromatography (SFC) methods.…”

“…23 Typically, gramicidin D is mainly comprised of gA (about 72%) with a balance of gC (about 19%) and minor amounts of gB (about 9%), although the actual quantities vary depending on the source. 23 Because of its very large size and inherent thermal instability, analytical separations of this polypeptide are often performed by HPLC or supercritical fluid chromatography (SFC) methods. 24,25 Thus, it presented an interesting sample to analyze with the ultrashort GC column apparatus.…”