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At least 19 recordsLinked to original sources
PubMed1987

Conformational analysis of gramicidin-gramicidin interactions at the air/water interface suggests that gramicidin aggregates into tube-like structures similar as found in the gramicidin-induced hexagonal HII phase.

The energetics of interaction and the type of aggregate structure in lateral assemblies of up to five gramicidin molecules in the beta 6.3 helical conformation at the air/water interface was calculated using conformational analysis procedures. It was found that within the aggregate two types of gramicidin interaction occur. One leading to a linear organization with a mean interaction energy between monomers of -6 kcal/mol and one in a perpendicular direction leading to a circularly organization with a lower mean interaction energy of -10 kcal/mol. Extrapolation towards larger gramicidin assemblies predicts that gramicidin itself could form tubular structures similar to those found in the gramicidin-induced HII phase. The tryptophans appear to play an essential role in the tubular organization of the gramicidin aggregate, since they determine the cone shape of the monomer and contribute to the structure of the monomer and oligomer by stacking interactions. These results, which are discussed in the light of experimental observations of gramicidin self-association in model membranes and the importance of the tryptophans for HII phase formation, further support the view (Killian, J.A. and De Kruijff, B. (1986) Chem. Phys. Lipids 40, 259-284) that gramicidin is a first example of a new class of hydrophobic polypeptides which can form cylindrical structures within the hydrophobic core of the membrane.

Gramicidin↗
PubMed1997

Characterization of gramicidin S synthetase aggregation substance: control of gramicidin S synthesis by its product, gramicidin S.

An aggregation substance of gramicidin S synthetases was found and purified by DEAE-cellulose chromatography and CM-chromatography from cell debris of Bacillus brevis Nagano. It specifically aggregated and inactivated gramicidin S synthetases 1 (GS1) and 2 (GS2). On the basis of amino acid composition analysis, reversed-phase HPLC, FAB mass spectrometry, amino acid sequence analysis, and antibacterial activity, this substance (GrS-aggregation substance) was identified as gramicidin S. A gramicidin S derivative bearing a lysine residue in place of one ornithine residue was also detected as a minor component of GrS-aggregation substance. The extent of the aggregation was dependent on the concentration and relative amount of gramicidin S. The inhibition of the enzyme activities was irreversible and the inhibition was proportional to the amount of gramicidin S, like the aggregation of the enzymes. The degree of GS2 inhibition in the amino acid-dependent ATP-PPi exchange reaction varied with the amino acids of gramicidin S and increased in order of the amino acid sequence of gramicidin S. The degree of inhibition of the overall synthesis of gramicidin S was the same as that in the leucine-dependent exchange reaction.

Amino Acid Isomerases↗
PubMed1983

Transfer of D-phenylalanine from gramicidin S synthetase 1 to gramicidin S synthetase 2 in gramicidin S synthesis.

The transfer of phenylalanine from gramicidin S synthetase 1 (GS 1) to gramicidin S synthetase 2 (GS 2) was studied by the use of combinations of wild-type GS 1 with various GS 2s from a wild strain and gramicidin S non-producing mutant strains of Bacillus brevis Nagano. The combinations of mutant GS 2s lacking 4'-phosphopantetheine (from BI-4, C-3, E-1, and E-2) did not transfer D-phenylalanine from GS 1, although they could activate all the constituent amino acids. Other mutant GS 2s containing 4'-phosphopantetheine, except GS 2 from BII-3 (proline-activation lacking) accepted D-phenylalanine from intact GS 1. To ascertain more directly whether 4'-phosphopantetheine is involved in the transfer of D-phenylalanine from GS 1 to GS 2, pepsin digests of GS 2 that accepted [14C]phenylalanine were analyzed by Sephadex G-50 column chromatography and thin-layer chromatography (TLC). Radioactivity of [14C]phenylalanine was always associated with a peptide containing 4'-phosphopantetheine. Furthermore, the position of radioactivity was distinct from the position of 4'-phosphopantetheine on TLC after alkaline treatment or performic acid oxidation of the digests.

Amino Acid Isomerases↗
PubMed1987

Importance of the tryptophans of gramicidin for its lipid structure modulating activity in lysophosphatidylcholine and phosphatidylethanolamine model membranes. A comparative study employing gramicidin analogs and a synthetic alpha-helical hydrophobic polypeptide.

The importance of the tryptophan residues of gramicidin for the lipid structure modulating activity of this pentadecapeptide was investigated by studying the interaction of gramicidin analogs A, B, C (which have a tryptophan, phenylalanine and tyrosine in position 11, respectively) and tryptophan-N-formylated gramicidin (in which the four tryptophan residues have been formylated) with several phospholipid systems. In addition an alpha-helical model pentadecapeptide (P15) was studied to further test the specificity of the gramicidin-lipid interaction. DSC experiments showed that all the gramicidin analogs produced a significant decrease in the gel to liquid-crystalline transition enthalpy of dipalmitoylphosphatidylcholine. The P15 peptide was much less effective in this respect. In dielaidoylphosphatidylethanolamine the gel----liquid-crystalline transition enthalpy was much less affected by the incorporation of these molecules. In this lipid system tryptophan-N-formylated gramicidin was found to be the most ineffective. 31P-NMR and small angle X-ray diffraction experiments showed that the ability of the peptides to induce bilayer structures in palmitoyllysophosphatidylcholine and HII phase promotion in dielaidoylphosphatidylethanolamine systems follows the order: gramicidin A' (natural mixture) approximately equal to gramicidin A greater than gramicidin B approximately equal to gramicidin C greater than tryptophan-N-formylated gramicidin greater than P15. These results support the hypothesis that the shape of gramicidin and its aggregational behaviour, in which the tryptophan residues play an essential role, are major determinants in the unique lipid structure modulating activity of gramicidin.

Calorimetry, Differential Scanning↗
PubMed1986

Gramicidin-induced hexagonal HII phase formation in negatively charged phospholipids and the effect of N- and C-terminal modification of gramicidin on its interaction with zwitterionic phospholipids.

The effect of gramicidin on macroscopic structure of the negatively charged membrane phospholipids cardiolipin, dioleoylphosphatidylglycerol and dioleoylphosphatidylserine in aqueous dispersions was investigated and compared with the effect of gramicidin on dioleoylphosphatidylcholine. It was shown by small-angle X-ray diffraction, 31P nuclear magnetic resonance and freeze-fracture electron microscopy that in all these lipid systems gramicidin is able to induce the formation of a hexagonal HII phase. 31P-NMR measurements indicated that the extent of HII phase formation in the various lipids ranged from about 40% to 60% upon gramicidin incorporation in a molar ratio of peptide to lipid of 1 : 10. Next, the following charged analogues of gramicidin were prepared: desformylgramicidin, N-succinylgramicidin and O-succinylgramicidin. The synthesis was verified with 13C-NMR and the effect of these analogues on lipid structure was investigated. It was shown that, as with gramicidin itself, the analogues induce HII phase formation in dioleoylphosphatidylcholine, lower and broaden the bilayer-to-HII phase transition in dielaidoylphosphatidylethanolamine and form lamellar structures upon codispersion with palmitoyllysophosphatidylcholine. Differential scanning calorimetry measurements indicated that, again like gramicidin, in phosphatidylethanolamine the energy content of the gel-to-liquid-crystalline phase transition is not affected by incorporation of the analogues, whereas in phosphatidylcholine a reduction of the transition enthalpy is found. These observations were explained in terms of a similar tendency to self-associate for gramicidin and its charged analogues. The results are discussed in the light of the various factors which have been suggested to be of importance for the modulation of lipid structure by gramicidin.

Calorimetry, Differential Scanning↗
PubMed1985

Gramicidin K, a new linear channel-forming gramicidin from Bacillus brevis.

A new gramicidin has been isolated from a commercial mixture of gramicidins A, B, and C. This new molecule, designated gramicidin K, contains formyl and ethanolamine blocking groups, has a molecular weight approximately 20% higher than gramicidin A, and is strongly retained on reversed-phase liquid chromatographic columns. Gramicidin K can be resolved into two components, one of which contains tyrosine. In lipid bilayer membranes, both components form channels of considerably longer lifetime and somewhat lower conductance than gramicidin A. Gramicidin K appears to be a lipopeptide that consists of a fatty acyl chain attached to the ethanolamine of gramicidin A.

Amino Acids↗
PubMed1982

Orientation of gramicidin A transmembrane channel. Infrared dichroism study of gramicidin in vesicles.

Polarized infrared spectroscopy has been used to investigate the orientation of gramicidin A incorporated in dimyristoylphosphatidylcholine liposomes. Dichroism measurements of the major lipid (C = O ester, PO2-, CH2) and peptide (amide A, I, II) bands were performed on liposomes (with or without gramicidin) oriented by air-drying. The mean orientation of the lipid groups and of the pi LD helix chain in the gramicidin has been determined. It can be inferred from infrared frequencies of gramicidin that the dominant conformation of the peptide in liposomes cannot be identified to the antiparallel double-helical dimer found in organic solution. No shift in lipid frequencies was observed upon incorporation of gramicidin in the liposomes. However, a slight reorganization of the lipid hydrocarbon chains which become oriented more closely to the normal to the bilayer is evidenced by a change in the dichroism of the CH2 vibrations. The infrared dichroism results of gramicidin imply a perpendicular orientation of the gramicidin transmembrane channel with the pi LD helix axis at less than 15 degrees with respect to the normal to the bilayer.

Circular Dichroism↗
PubMed1992

Molecular and channel-forming characteristics of gramicidin K's: a family of naturally occurring acylated gramicidins.

The gramicidin K family is a set of naturally occurring acylated linear peptides in which a fatty acid is esterified to the ethanolamine hydroxyl of either gramicidin A or C, and possibly also to gramicidin B (Koeppe, R. E., II, Paczkowski, J. A., & Whaley, W. L. (1985) Biochemistry 24, 2822-2826). These acylated gramicidins form membrane-spanning channels in planar lipid bilayers and therefore constitute a model system with which to study the structural and functional consequences of acylation on membrane proteins. This paper serves to characterize further the channels formed by acylated gramicidins A and C and to demonstrate that these channels are structurally equivalent to the channels formed by the standard gramicidins. We also present additional evidence for the ester linkage in the natural acylated gramicidins A and C and identify the fatty acyl chains.

Acylation↗
PubMed1986

Deuterium nuclear magnetic resonance investigation of the exchangeable sites on gramicidin A and gramicidin S in multilamellar vesicles of dipalmitoylphosphatidylcholine.

Solid gramicidin A and S and their interaction with DPPC bilayers were examined by 2H NMR as well as 31P NMR and differential scanning calorimetry (DSC). The deuterium spectra arose from deuterons associated with the peptide through chemical exchange in 2H2O. The spectra from both peptides were characterized by a quadrupolar splitting parameter, omega Q/2 pi approximately 150 kHz, and an asymmetry parameter, eta approximately 0.17. An additional 33 kHz, eta = 0 component arising from deuterons on mobile ornithine side chains was present in gramicidin S. In the gel phase of dipalmitoylphosphatidylcholine liposomes the gramicidins gave spectra that had components identical with those obtained from the solids. In the liquid-crystalline phase gramicidin A containing samples gave multicomponent spectra with a maximum quadrupolar splitting value of 133 kHz, eta = 0. A minimum in the T2e was observed, coinciding with the onset of the broadened phase transition measured by DSC and 31P NMR, due to the onset of axial rotation of the peptide in the bilayer. The different powder patterns in the liquid-crystalline spectra from gramicidin A probably arise from different amide sites along the transmembrane channel. The broad component of the 2H NMR spectra from gramicidin S in liposome preparations was not affected by the lipid-phase transition. The T2e was also constant over this temperature range. The results are consistent with a location of gramicidin S at the membrane surface.

Calorimetry, Differential Scanning↗
PubMed1979

Biological role of gramicidin S in spore functions. Studies on gramicidin-S-negative mutants of Bacillus brevis ATCC9999.

Gramicidin-S-negative mutants of Bacillus brevis ATCC9999 have been isolated with a remarkly higher yield after ethidium bromide or acridine orange treatment, than after N-methyl-N'-nitro-N-nitrosoguanidine treatment. Four (MIV, Smr170, R5 and EB 16) of 38 isolated mutants were characterized with respect to the lesion in gramicidin-S-synthesizing activity. The mutants sporulate to the same extent as the parental strain except mutant Smr 170 which sporulates less. However, mutant spores were more heat-sensitive and possessed a reduced level of dipicolinic acid content. No significant difference was observed in the germination time of wild-type and mutant spores. All spores germinated after 80--110 min, but the outgrowth time was different: all gramicidin-S-negative mutants grew out immediately after germination whereas wild-type spores required a lag period of 9--10 h. When the mutants were allowed to sporulate in the presence of gramicidin S, the spores were found to be heat-resistant and their outgrowth postponed to the same period as the parent spores. The addition of gramicidin also eliminated the deficiency of dipicolinic acid. A new class of gramicidin-S-negative mutant, R5, which only activates L-valine and L-leucine, is described. A possible biological function of gramicidin S in the heat-resistance and in the timing of spore outgrowth is discussed.

Acridine Orange↗
PubMed1986

Single-channel studies on linear gramicidins with altered amino acid side chains. Effects of altering the polarity of the side chain at position 1 in gramicidin A.

The modulation of gramicidin A single-channel characteristics by the amino acid side chains was investigated using gramicidin A analogues in which the NH2 terminal valine was chemically replaced by other amino acids. The replacements were chosen such that pairs of analogues would have essentially isosteric side chains of different polarities at position 1 (valine vs. trifluorovaline or hexafluorovaline; norvaline vs. S-methyl-cysteine; and norleucine vs. methionine). Even though the side chains are not in direct contact with the permeating ions, the single-channel conductances for Na+ and Cs+ are markedly affected by the changes in the physico-chemical characteristics of the side chains. The maximum single-channel conductance for Na+ is decreased by as much as 10-fold in channels formed by analogues with polar side chains at position 1 compared with their counterparts with nonpolar side chains, while the Na+ affinity is fairly insensitive to these changes. The relative conductance changes seen with Cs+ were less than those seen with Na+; the ion selectivity of the channels with polar side chains at position 1 was increased. Hybrid channels could form between compounds with a polar side chain at position 1 and either valine gramicidin A or their counterparts with a nonpolar side chain at position 1. The structure of channels formed by the modified gramicidins is thus essentially identical to the structure of channels formed by valine gramicidin A. The polarity of the side chain at position 1 is an important determinant of the permeability characteristics of the gramicidin A channel. We discuss the importance of having structural information when interpreting the functional consequences of site-directed amino acid modifications.

Amino Acids↗
PubMed1995

Potassium flux through gramicidin ion channels is augmented in vesicles comprised of plasmenylcholine: correlations between gramicidin conformation and function in chemically distinct host bilayer matrices.

The functional role of distinct phospholipid subclasses and molecular species in modulating gramicidin-mediated K+ flux was characterized through quantification of changes in the fluorescence intensity of ion specific fluorescent probes encapsulated inside vesicles comprised of individual molecular species of plasmenylcholine and phosphatidylcholine. The rate constant of gramicidin-mediated K+ ion flux across bilayers comprised of 1-O-(Z)-hexadec-1'-enyl-2-octadec-9'-enoyl-sn-glycero-3-p hos phocholine (plasmenylcholine) was 18.9 +/- 1.7 s-1, while that present across bilayers comprised of 1-hexadecanoyl-2-octadec-9'-enoyl-sn-glycero-3-phosphocholine (phosphatidylcholine) was 12.3 +/- 1.5 s-1. The observed changes were not due to alterations in the nature of the sn-2 aliphatic chain or the net surface charge present at the membrane interface and were unaltered by the addition of several amphiphilic agents (including charged amphiphiles), suggesting that the observed alterations specifically reflect changes in channel function which result from the covalent alteration of host phospholipid in the proximal portion of the sn-1 aliphatic chain (i.e., phospholipid subclass-specific alterations). Addition of cholesterol to bilayer matrices comprised of plasmenylcholine resulted in dose-dependent attenuation of the rate of gramicidin-mediated K+ flux, but did not alter the rate of gramicidin-mediated K+ flux in membranes comprised of phosphatidylcholine. Gramicidin ion channels experience distinct environments in membranes comprised of phosphatidylcholine and plasmenylcholine host lipids demonstrated by both the different fluorescence anisotropies of endogenous tryptophan residues and the different C=O stretching frequencies of intramonomer carbonyls in gramicidin incorporated into these two choline glycerophospholipid subclasses.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol↗
PubMed1990

Deuterium NMR of 2HCO-Val1...gramicidin A and 2HCO-Val1-D-Leu2...gramicidin A in oriented DMPC bilayers.

Deuterium NMR is used to study the structure and dynamics of the formyl C-2H bond in selectively deuterated gramicidin molecules. Specifically, the functionally different analogues 2HCO-Val1...gramicidin A and 2HCO-Val1-D-Leu2...gramicidin A are studied by 2H NMR so that any conformational or dynamical differences between the two analogues can be correlated with their difference in lifetime. These analogues are first synthesized, purified, and characterized and then incorporated into oriented bilayers of dimyristoylphosphatidylcholine sandwiched between glass coverslips. Phosphorous NMR line shapes obtained from these samples are consistent with the presence of the bilayer phase and indicate that the disorder exhibited by the lipid matrix is approximately of the same type and degree for both analogues. Deuterium NMR line shapes obtained from these samples indicate that the motional axis of the formyl group of gramicidin is parallel to the coverslip normal, that the distribution of motional axis orientations has a width of 7-9 degrees, and that a similar, major conformational and dynamical state exists for the formyl C-2H bond of both analogues. In this state, if the only motion present is fast axial rotation, then the experimentally derived angle between the formyl C-2H bond and the motional axis is consistent with the presence of a right-handed, single-stranded, beta 6.3 helical dimer but is not consistent with the presence of a left-handed, single-stranded, beta 6.3 helical dimer. However, if fast axial rotation is not the only motion present, then the left-handed, single-stranded, beta 6.3 helical dimer cannot be absolutely excluded as a possibility. Also, a second, minor conformational and dynamical state appears to be present in the spectrum of 2HCO-Val1-D-Leu2...gramicidin A but is not observed in the spectrum of 2HCO-Val1...gramicidin A. This minor conformational and dynamical state may reflect the presence of monomers, while the major conformational and dynamical state may reflect the presence of dimers.

Amino Acid Sequence↗
PubMedDate not supplied

Effects of gramicidin and tryptophan-N-formylated gramicidin on the sodium and potassium content of human erythrocytes.

In order to get a better understanding in the mechanism by which tryptophan-N-formylated gramicidin (NFG) and gramicidin kill the malaria parasite Plasmodium falciparum in vitro, we studied the capacity of these peptides to change the potassium, as well as the sodium, composition of normal human erythrocytes, and their ability to cause cell lysis. It is shown that both peptides are able to induce potassium leakage from, and sodium flux into, erythrocytes in such a manner that it is most likely that they are able to form cation channels in the membrane of these cells. For both peptides, potassium efflux proceeds at a faster rate than sodium influx, but this difference is greater for NFG than for gramicidin. This explains the observation that gramicidin is more lytic than NFG is, even when comparing concentrations that show the same antimalarial activity. The finding that gramicidin is approximately 10 times more active than NFG in causing potassium efflux from normal erythrocytes, as well as in killing the malaria parasite, supports the hypothesis that peptide-induced parasite death is related to their capacity to induce potassium leakage from infected erythrocytes. Finally, the observation that erythrocytes are able to restore their normal ion contents after losing more than 50% of their potassium content by incubation with NFG or gramicidin, suggests that, in vivo, and upon treatment with drug concentrations that cause full inhibition of parasite growth, these cells would not be irreversibly damaged by action of the drugs.

Antimalarials↗
PubMed1991

Synthesis of acylated gramicidins and the influence of acylation on the interfacial properties and conformational behavior of gramicidin A.

Five gramicidin A analogs were synthesized in which various acyl chains, differing in length and unsaturation, were covalently coupled to the C-terminal ethanolamine group. The analogs were characterized by various spectroscopic techniques and their molecular properties were investigated using monolayer techniques and circular dichroism. It is demonstrated that neither the interfacial properties nor the conformational behavior of gramicidin A at the air/water interface are seriously affected upon acylation. It is proposed that at the limiting area the gramicidin molecule is oriented with its C-terminus towards the subphase with the covalently coupled acylchain located parallel to the helical axis in between the protruding tryptophans. Circular dichroism experiments, in which gramicidin-containing vesicles were prepared from different organic solvents, indicate that the presence of a covalently coupled fatty acylchain tends to stabilize the beta 6.3 helical conformation. It is demonstrated that, like for gramicidin A, also for the acylgramicidins the single-stranded beta 6.3 helical conformation, or channel conformation, is the preferred conformation upon incorporation in bilayers.

Acylation↗
PubMed2001

Comparison of gramicidin A and gramicidin M channel conductance dispersities.

To explore the possible role of Trp side chains in gramicidin channel conductance dispersity, we studied the dispersity of gramicidin M (gM), a gramicidin variant in which all four tryptophan residues are replaced with phenylalanine residues, and its enantiomer, gramicidin M(-) (gM(-)), and compared them to that of gramicidin A (gA). The conductances of highly purified gM and gM(-) were studied in alkali metal solutions at a variety of concentrations and voltages, in seven different types of lipid, and in the presence of detergent. Like gA channels, the most common gM channel conductance forms a narrow band. However, unlike gA channels, where the remaining 5-30% of channel conductances are broadly distributed below (and slightly above) the main band, in gM there is a narrow secondary band with <50% of the main peak conductance. This secondary peak was prominent in NaCl and KCl, but significantly diminished in CsCl and RbCl. Under some conditions, minor components can be observed with conductances yet lower than the secondary peak. Interconversions between the primary conductance state and these yet lower conductance states were observed. The current-voltage relations for both primary and secondary gM channel types have about the same curvature. The mean lifetime of the secondary channel type is below one third that of the primary type. The variants represent state deviations in the peptide or adjacent lipid structure.

Electric Conductivity↗
PubMed1992

Formation of non-beta 6.3-helical gramicidin channels between sequence-substituted gramicidin analogues.

Using the linear gramicidins as an example, we have previously shown how the statistical properties of heterodimeric (hybrid) channels (formed between the parent [Val1]gramicidin A (gA) and a sequence-altered analogue) can be used to assess whether the analogue forms channels that are structurally equivalent to the parent channels (Durkin, J. T., R. E. Koeppe II, and O. S. Andersen. 1990. J. Mol. Biol. 211:221-234). Generally, the gramicidins are tolerant of amino acid sequence alterations. We report here an exception. The optically reversed analogue, gramicidin M- (gM-) (Heitz, F., G. Spach, and Y. Trudelle. 1982. Biophys. J. 40:87-89), forms channels that are the mirror-image of [Val1]gA channels; gM- should thus form no hybrid channels with analogues having the same helix sense as [Val1]gA. Surprisingly, however, gM- forms hybrid channels with the shortened analogues des-Val1-[Ala2]gA and des-Val1-gC, but these channels differ fundamentally from the parent channels: (a) the appearance rate of these heterodimers is only approximately 1/10 of that predicted from the random assortment of monomers into conducting dimers, indicating the existence of an energy barrier to their formation (e.g., monomer refolding into a new channel-forming conformation); and (b), once formed, the hybrid channels are stabilized approximately 1,000-fold relative to the parent channels. The increased stability suggests a structure that is joined by many hydrogen bonds, such as one of the double-stranded helical dimers shown to be adopted by gramicidins in organic solvents (Veatch, W. R., E. T. Fossel, and E. R. Blout. 1974. Biochemistry. 13:5249-5256).

Amino Acid Sequence↗
PubMed2002

Stereochemistry of protected ornithine side chains of gramicidin S derivatives: X-ray crystal structure of the bis-Boc-tetra-N-methyl derivative of gramicidin S.

An X-ray crystallographic analysis of the bis-Ndelta-Boc-tetra-Nalpha-methyl derivative of gramicidin S, cyclo(-Val-MeOrn(Boc)-Leu-d-MePhe-Pro-)2, was undertaken successfully (R-factor = 0.088). As expected, the main chain adopts an antiparallel pleated beta-sheet conformation, but the pleated sheet is slightly twisted, and the sense of twisting is opposite to that found in the reported crystal structures of the gramicidin S-urea complex and the bis-Ndelta-(trichloroacetyl) and bis-Ndelta-(m-bromobenzoyl) derivatives of gramicidin S. In agreement with the observed resistance toward N-methylation, the urethane NH groups of the protected Orn side chains are hydrogen bonded to the carbonyl groups of the d-Phe residues. However, the side-chain-main-chain hydrogen bonding is in the i --> i - 3 mode, although hydrogen bonding in the i --> i + 2 mode was deduced from a 1H NMR study of protected gramicidin S derivatives and was actually found in the crystal structures of the diacylated gramicidin S.

Anti-Bacterial Agents↗