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• W98531902 abstract "Gramicidin is a naturally occurring pentadecapeptide that forms aggregates capable of transporting monovalent cations through membranes and lipid bilayers. It is synthesized in Bacillus brevis by a complex polymerase system. Its alternating L,D peptide composition is a unique feature of gramicidin. No other naturally occurring peptide has an alternating L and D sequence of more than three or four residues. Crystals of gramicidin grown from methanol, ethanol, and propanol were found to contain left handed Although they are commonly referred to as helical structures, it is important to note that they are actually the worlds smallest – a two stranded antiparallel wrapped into a tight barrel with a left handed twist . These crystal structures agree fully with NMR analysis of gramicidin in many organic solvents, including methanol [1,2]. The crystal structures of and complexes of gramicidin grown in methanol contain a completely different from the left handed one seen in the uncomplexed crystals [3]. In the monovalent cation complexes the antiparallel is wrapped around the ions in the opposite sense forming a double stranded right handed An identical conformation is also found in an entirely new crystal form of gramicidin, grown from glacial acetic acid solution [3]. A similar structure of the complex was reported based on the NMR spectra of gramicidin in methanol/chloroform [4]. The crystal structures of the ion complexes not only confirm and agree with all of reported NMR assignments but provide information on stable positions of the cations in the channel. These structures also provide unexpected evidence of an unusual mode of ion coordination consistent with facile transport of ions across lipid bilayers and membranes. The complexed structures reveal separate partially solvated ions distributed over three binding sites. The channel is electrostatically negative on the interior as required for cation passage, while the exterior is neutral, a requirement for membrane insertion. The “coordination” of the ion is achieved by interaction with the orbitals of the carbonyls which do not point toward the ions. It is encouraging to find left-handed conformers of gramicidin in a solution of methanol and in crystals grown from methanol and right-handed conformers in a salt solution of methanol and in crystals grown from that solution (Fig. 1). Clearly, the addition of salt to a methanolic solution of gramicidin induces a remarkable transformation from a left handed coiled ribbon to a right handed coiled ribbon. This change requires the disruption of 30 hydrogen bonds in the uncomplexed structure and formation of an entirely different set of 26 hydrogen bonds in the monovalent cation complexed form. An examination of the hydrogen bonding in the two structures reveals that in the uncomplexed structures, 16 hydrogen bonds form between the L residues creating a right handed This two-stranded sheet then wraps up to form a left handed barrel stabilized by additional hydrogen bonds between seven pairs of D-residues. In the complexed structures 14 hydrogen bonds between the D-residues of the antiparallel create a left handed that wraps up into a right-handed barrel stabilized by ion coordination and 10 hydrogen bonds between five pairs of L-residues." @default.
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• W98531902 date "2006-05-16" @default.
• W98531902 modified "2023-09-27" @default.
• W98531902 title "Gramicidin A: Structure and dynamic properties" @default.
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• W98531902 doi "https://doi.org/10.1007/0-306-46881-6_156" @default.
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