Interactions of
- and ß-avoparcin with bacterial cell-wall receptor-mimicking peptides studied by electrospray ionization mass spectrometry
Anca van de Kerk-van Hoof and
Albert J. R. Heck*
Department of Biomolecular Mass Spectrometry Bijvoet, Center for Biomolecular
Research, Department of Chemistry and Department of Pharmacy, Utrecht University,
Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands
 |
Abstract
|
---|
Solution phase affinity constants of the glycopeptide antibiotic
- and
ß-avoparcin, with a range of bacterial cell-wall receptor-mimicking model peptides, were
determined by a relatively new method: affinity electrospray ionization mass spectrometry
(ESI-MS). This method is relatively efficient and allows the parallel determination of several
affinity constants in mixtures of antibiotics and receptors. The determined binding constants for
- and ß-avoparcin were compared with those of the related glycopeptide antibiotic
vancomycin. The solution phase binding affinities of
- and ß-avoparcin on one hand,
and vancomycin on the other, were found to be in the same order, at least for the range of
receptor-mimicking peptides studied. However, ß-avoparcin displayed slightly higher
binding affinities than
-avoparcin, particularly for strong binding receptor-mimicking
peptides. The evidence that
- and ß-avoparcin and vancomycin are structurally similar,
combined with the present data revealing their similar affinity for bacterial cell-wall
receptor-mimicking peptides, supports the hypothesis that the appearance of
vancomycin-resistant enterococci (VRE) might be linked to the widespread use of avoparcin.
 |
Introduction
|
---|
Since many bacteria have become resistant to penicillin and a range of other antibiotics,
glycopeptide antibiotics now play an important role against Gram-positive bacterial infections.
This clinically important group of antibiotics act by binding to bacterial cell-wall precursors,
terminating in the sequence -Lys-D-Ala-D-Ala. This interaction inhibits
cross-linking of the growing cell wall, leading to bacterial cell death. Avoparcin is a glycopeptide
antibiotic of the vancomycin group and is structurally related to vancomycin (Figure 1).1,2 The
major
application of avoparcin is as an antibacterial, growth-promoting food additive used primarily in
animal feed.3 In Europe it was brought on to the market
under the tradename avotan. The recent appearance of vancomycin-resistant enterococci (VRE)
has been linked to the widespread use of avoparcin by farmers.4,56 In
contrast to Europe, environmental VRE isolates have not been observed in the USA, where these
antibiotics have never been licensed.7 As a consequence,
the use of avoparcin as an animal food additive has recently been banned by the European Union.
VRE are, however, emerging rapidly worldwide, not only due to the use of animal food additives,
but also due to the increased clinical use of vancomycin.8,9 Microbes may be able to adapt quite quickly when
vancomycin or other glycopeptide antibiotics are introduced into their environment, by
modifying their genes and becoming resistant.9 In addition,
there is the possibility that the conjugative transposons carrying resistance in one strain of
bacteria may be transferred to other bacteria, possibly even human enterococci.9,10 Consequently, there may be a
connection between human VRE, animal VRE and the use of farmyard antibiotics. A link
between avoparcin and human VRE is supported by the strong similarities between the chemical
structures of vancomycin and avoparcin (Figure 1).9,11 Prompted by the recent commotion
concerning VRE and
the possible role of animal-food antibiotic additives, the present study was performed to evaluate
the thermodynamic aspects of the molecular interactions of avoparcin with bacterial cell-wall
precursor, compared with vancomycin.
The commercial product used in our studies consists of a mixture of two antibiotics,
-
and ß-avoparcin, which differ only in the substitution of a Cl by an H on one of the aromatic
rings (Figure 1b). The MICs of
-avoparcin and vancomycin for
several strains of
staphylococci, streptococci and enterococci have been found to be mostly comparable, although
-avoparcin was generally less potent against staphylococci.12 In addition, an earlier report showed that the MIC values of
- and
ß-avoparcin differ by a factor of approximately two, ß-avoparcin being the more potent
antibiotic.2 The recent appearance of VRE infections is in
most cases caused by strains of bacteria in which peptidoglycan precursors terminate in D-Ala-D-Lac rather than D-Ala-D-Ala. 13 A different form of natural resistance has been reported, particularly
in the case of Enterococcus gallinarum and Enterococcus casseliflavus, where
peptidoglycan precursors terminating in D-Ala-D-Ser were found.9,14 Experimental
studies have shown that vancomycin binds approximately 10 times more weakly to
receptor-mimicking peptides such as N, N'-Ac2-L-Lys-D-Ala-D-Ser, and approximately 1000 times more weakly to N,
N'-Ac2-L-Lys-D-Ala-DLac,
although the mechanism of binding seems to be similar as for N, N'-Ac2-L-Lys-D-Ala-DAla.15,16 Binding constants of vancomycin and to a
lesser extent
- and ß-avoparcin with several bacterial cell-wall receptor-mimicking
precursor peptides have been measured by various methods, such as 1H nuclear magnetic resonance (NMR), capillary electrophoresis, UV difference
spectroscopy and microcalorimetry.17,18,19,20 Some of these
previously reported data are summarized in the last columns of Tables I
and II.
In recent years, relatively soft ionization methods have been developed in
mass spectrometry, which allow the detection of even weakly bound, intact, noncovalent
complexes.21,22,23,2425 In particular, nanoflow electrospray ionization mass spectrometry
(ESI-MS)26 has great potential in this important
biochemical area. With some success, nanospray ionization has even been used to assess
quantitatively the binding of proteinprotein and proteinligand interactions.27,28,29,30,31 In this study, nanoflow ESI-MS has been used to measure quantitatively the binding
of the antibacterial food additives
- and ß-avoparcin to peptidoglycan
receptor-mimicking peptides.
 |
Materials and methods
|
---|
Electrospray ionization mass spectra were recorded on a Finnigan/Thermoquest LC-Q (San
Jose, CA, USA) ion trap mass spectrometer, operating the mass spectrometer in positive ion
mode. All samples were introduced using a nanoflow electrospray source (Protana, Odense,
Denmark). Solutions of antibiotic and receptor peptides at concentrations of typically
15150 µM (depending on the strength of noncovalent binding) were made up in
aqueous 5 mM ammonium acetate, pH 5.1 (acidified using acetic acid), 298 K. Approximately 2
µL of these solutions were introduced into the electrospray needles. The applied
electrospray voltage between the needle and the capillary was typically 500 V. The capillary
temperature was 383 K. The offset between the capillary voltage and the tubelens
voltage was usually <5 V. For quantitative measurements, ion intensities were measured in
the selected-ion-monitoring mode, whereby only small mass (m/z) windows are
monitored. Peak intensities were obtained by integrating over the whole isotope envelope of the
ions. The general procedures for obtaining binding constants from the nanoflow electrospray
mass spectra have been reported previously.30 In short, the
equilibrium concentrations of the antibiotics, [
_Avop]eq. and [ß_Avop]eq.
can be derived from the peak intensities in the mass spectra using the following equations (square
brackets denote concentrations):
where
_Avop and
_AvopL are the integrated peak intensities of the antibiotic and its
complex with the ligand, respectively. [
_Avop]0 is the initial concentration of
the antibiotic. The concentration of the complex between the antibiotic and the ligand is
The concentration of the other antibiotic ( [ß_Avop]eq.) and its complex with
the ligand ( [ß_AvopL]eq.) can be derived in an analogous manner. For the
concentration of unbound ligand, the relation
holds, where [Ligand]0 is the total initial concentration of the ligand. The binding
constant of the antibiotic to the ligand can now simply be calculated from the equation
Similar equations can be used and derived for mixtures of two antibiotics and several
peptides. This mass spectrometric method is based on the assumption that the ionization
probability of the free antibiotics is identical to the ionization probability of the
antibioticligand complexes. This assumption was validated by performing quantitative
measurements at different concentrations of antibiotic(s) and ligand(s).30
The commercially available antibiotic avoparcin (Wyeth-Ayerst Research, Lederle
Laboratories, NY, USA) used in the present study consists of a 1:2 mixture of two products,
- and ß-avoparcin, which differ by the substitution of a Cl for an H on one of the
aromatic ring side chains (Figure 1b). The 1:2 ratio was confirmed by
capillary electrophoresis analysis using UV detection.
 |
Results and discussion
|
---|
The ionization probablities, using ESI, for both
- and ß-avoparcin were found
to be identical.
- and ß-avoparcin were detected, in a 1:2 ratio, almost exclusively as
doubly protonated ions. Figure 2 shows a nanoflow ESI mass spectrum of
a 15 µM 1:2 mixture of
- and ß-avoparcin, to which 30 µM of the
receptor-mimicking peptide N-Ac-D-Ala-D-Ala-D-Ala was added. The signals at m/z values of approximately 955 and 973 Th originate from the
doubly protonated
- and ß-avoparcin, respectively. The ion signals observed at 1093
and 1111 Th originate from the doubly protonated noncovalent complexes of N-Ac-D-Ala-D-Ala-D-Ala with protonated
- and
ß>-avoparcin, respectively. Besides these signals the spectrum is quite clean. At the start
of the experiment the concentrations of
-avoparcin, ß-avoparcin and N-Ac-D-Ala-D-Ala-D-Ala were 5, 10 and 30 µM,
respectively. Using the procedures outlined in the Materials and methods, it may be calculated
that according to the measured mass spectrum in Figure 2 the equilibrium
concentrations in solution were 2.35, 4.12 and 21.47 µM for
-avoparcin,
ß-avoparcin and N-Ac-D-Ala-D-Ala-D-Ala,
respectively. The noncovalent complexes of the antibiotic with the peptides had concentrations of
2.65 and 5.88 µM for the
- and ß-forms. Binding constants of
53 000/M and 66 000/M for
- and ß-avoparcin, respectively, were
determined from this particular spectrum. Table I summarizes the binding
constants obtained for a wide range of precursor peptides. The binding constants are averaged
over several measurements using typically different mixtures of receptor-mimicking peptides.
The absolute error in the binding constants is approximately 30%. The error in relative binding
constants is expected to be much smaller, i.e. within 10%.

View larger version (14K):
[in this window]
[in a new window]
|
Figure 2. Nanoflow ESI mass spectrum of a 5 mM aqueous ammonium acetate solution
containing a 15 µM 1:2 mixture of - and ß-avoparcin and 30 µM of the
receptor-mimicking peptide N-Ac-D-Ala-D-Ala-DAla.
|
|
Figure 3 shows selected ion monitoring mode ESI mass spectra
obtained from such a solution containing a complex mixture of
- and
ß-avoparcin (5 and 10 µM, respectively) and two equimolar (50 µM)
receptor-mimicking peptides, i.e. N, N'-Ac2-L-Lys-D-Ala-D-Ser and N-Ac-D-Ala-D-Ala.
-L-Lys-D-Ala-D-Ser termini have been observed in the
peptidoglycan structure of several VRE. The relative ion signal intensities observed in this
spectrum indicate immediately that the affinities of
- and ß-avoparcin for N-Ac-D-Ala-D-Ala are similar, whereas ß-avoparcin reveals a
relatively higher affinity for N, N'-Ac2-L-Lys-D-Ala-D-Ser than
-avoparcin. Additionally, the binding affinity of
-avoparcin for N-Ac-D-Ala-D-Ala is approximately equal
to the affinity for N, N'-Ac2-L-Lys-D-Ala-D-Ser. Therefore, from this single measurement, which takes, experimentally, only
seconds, one can derive several (in this case, four) binding constants simultaneously, revealing
one of the strengths of this mass spectrometrical approach. Binding constants for
-avoparcin, ß-avoparcin and vancomycin, all determined by ESI-MS, are summarized
in Tables I and II. The values determined by
ESI-MS for vancomycin were reported previously,30 except
for N, N'-Ac2-L-Lys-D-Ala-D-Ser. For comparison, several previously reported binding constants, obtained by alternative,
direct solution-phase methods are also given.

View larger version (16K):
[in this window]
[in a new window]
|
Figure 3. Selected ion monitoring mode nanoflow ESI mass spectra of a 5 mM aqueous
ammonium acetate solution containing a 15 µM 1:2 mixture of - and
ß-avoparcin and two equimolar (50µM ) receptor-mimicking peptides, N,N'-Ac2-L-Lys-D-Ala-D-Ser and N-Ac-D-Ala-D-Ala.
|
|
Most antibiotics of the vancomycin group self-associate and these dimers may be important
in promoting antibiotic activity.32,33 For a range of antibiotics, a large variation in the dimerization constants has
been observed. A mixture of two glycopeptide antibiotics might form hetero-dimers. In this
study, the self-association constants of
- and ß-avoparcin have been assessed in a
similar manner to that described previously for a range of related antibiotics.30 The determined dimerization constants were all rather low, around 2000/M,
for the 
-, ßß-homo-dimers and the
ß-avoparcin
hetero-dimer. As the dimer ion signals were quite small in the mass spectra measured, the error
in these self-association constants is approximately 50%. Reported dimerization constants of
vancomycin range from 50 to 4000/M.34,35,36 The tendency for self-association
of avoparcin was found to be rather poor, but comparable to that of vancomycin.
 |
Discussion
|
---|
The main aim of this study was to investigate whether the structurally related antibiotics of
-avoparcin, ß-avoparcin and vancomycin have (dis)similar affinities for peptides
that resemble the receptor sites of bacterial peptidoglycans. While an extensive amount of
affinity data was already available for vancomycin, few figures had been reported for avoparcin.
The data presented in this study reveal that
- and ß-avoparcin display similar
affinities to vancomycin for the whole range of investigated receptor-mimicking peptides. In
common with vancomycin,
- and ß-avoparcin exhibit strongly reduced affinities
for N, N'-Ac2-L-Lys-D-Ala-D-Lac, and to a lesser extent for N, N'-Ac2-L-Lys-D-Ala-D-Ser, two peptides that mimick the peptidoglycan receptors of VRE.
Subtle differences in molecular recognition could, however, be observed. For instance,
vancomycin displays a higher affinity for Ac-D-Ala-D-Ala than for
Ac-Gly-D-Ala by a factor of two, which is reflected by their binding constants of
19,000 and 11,000/M (see Table II). This difference in affinity is
markedly enhanced in the case of
- and ß-avoparcin. For example, the binding
constants for Ac-D-Ala-D-Ala and Ac-Gly-D-Ala differ by
a factor of 10, i.e. 15,000 and 2000/M, respectively (see Table I). The
effect that the methyl (Ala) to hydrogen (Gly) substitution has on the noncovalent association is
remarkable, maybe even more so than the fact that this effect of <2 kJ/mol in binding energy
can be monitored quantitatively by ESI-MS. Differences in affinity were also observed when
- and ß-avoparcin were compared. ß-avoparcin exhibited affinities up to twice as
high as those of
-avoparcin, especially for strong binding, receptor-mimicking peptides.
The tendency to self-associate is poor for
-avoparcin, ß-avoparcin and vancomycin.
In general, ESI-MS was observed to be a reliable method for screening the binding affinities
of several glycopeptide antibiotics with a range of receptor-mimicking peptides.
 |
Acknowledgments
|
---|
We would like to acknowledge the following people for generous gifts of antibiotics
and receptor-mimicking peptides: Dr Marshall Siegel (Wyett-Aherst, Pearl River, USA) for
-avoparcin, ß-avoparcin and vancomycin, Dr Thomas Staroske and Professor
Dudley H. Williams (Cambridge University, UK) for Ac2-L-Lys-D-Ala-D-Ser, and Dr Thomas J. D. Jørgensen and Professor Peter
Roepstorff (Odense University, Denmark) for Ac-Gly-D-Ala, Ac2-L-Lys-L-Ala-L-Ala and Ac-D-Ala-D-Ala-D-Ala.
 |
Notes
|
---|
* Corresponding author. Fax: +31-30-251-8219; E-mail: a.j.r.heck{at}chem.uu.ul 
 |
References
|
---|
1
.
Ellestad, G. A., Leese, R. A., Morton, G. O., Barbatschi, F., Gore, W. E., McGahren,
W. J. et al. (1982). Avoparcin and epi-avoparcin. Journal of the American Chemical Society
103, 65224.[ISI]
2
.
Fesik, S. W., Armitage, I. M., Ellestad, G. A.& McGahren, W. J. (1984). Nuclear magnetic resonance studies on the antibiotic avoparcin; conformational
properties in relation to antibacterial activity. Molecular Pharmacology 25, 27580.[Abstract]
3
.
Elwinger, K., Berndtson, E., Engstrom, B., Fossum, O. & Waldenstedt, L. (1998). Effect of antibiotic growth promoters and anticoccidials on growth of Clostridium perfringens in the caeca and on performance of broiler chickens. Acta Veterinaria Scandinavica
39,433
41.[ISI][Medline]
4
.
Donnelly, J. P., Voss, A., Witte, W. & Murray, B. E. (1996). Does
the use in animals of antimicrobial agents, including glycopeptide antibiotics, influence the
efficacy of antimicrobial therapy in humans? Journal of Antimicrobial Chemotherapy 37, 38992.[ISI][Medline]
5
.
Howarth, F. & Poulter, D. (1996). Vancomycin resistance: time to
ban avoparcin. Lancet 347, 1047.[ISI][Medline]
6
.
Bager, F., Madsen, M., Christensen, J. & Aarestrup, F. M. (1997).
Avoparcin used as a growth promoter is associated with the occurrence of vancomycin-resistant Enterococcus faecium on Danish poultry and pig farms. Preventive
Veterinary Medicine 31, 95112.[ISI][Medline]
7
.
Woodford, N. (1998). Glycopeptide-resistant enterococci: a decade of
experience. Journal of Medical Microbiology 47, 84962.[Abstract]
8
.
Van der Auwera, P., Pensart, N., Korten, V., Murray, B. E. & Leclercq, R. (1995). Influence of oral glycopeptides on the fecal flora of human volunteers:
selection of highly glycopeptide-resistant enterococci. Journal of Infectious Diseases 173, 112936.[ISI]
9
.
Murray, B. E. (1998). Diversity among multidrug-resistant enterococci.Emerging Infectious Diseases
4,37
47.[ISI][Medline]
10
.
Marcinek, H., Wirth, R., Muscholl-Silberhorn,A. & Gauer, M. (1998). Enterococcus faecalis gene transfer under natural conditions in municipal
sewage water treatment plants. Applied and Environmental Microbiology 64, 62632.[Abstract/Free Full Text]
11
.
Walsh, C. T. (1993). Vancomycin resistance: decoding the molecular
logic. Science 261, 3089.[ISI][Medline]
12
.
Cormican, M. G., Erwin, M. E. & Jones, R. N. (1997). Avoparcin,
a glycopeptide used in animal foods: antimicrobial spectrum and potency tested against human
isolates from the United States. Diagnostic Microbiology and Infectious Disease 29, 2418.[ISI][Medline]
13
.
Walsh, C. T., Fisher, S. L., Park, I.-S., Prahalad, M. & Wu, Z. (1996). Bacterial resistance to vancomycin: five genes and one missing hydrogen bond tell the
story. Chemistry and Biology 3,21
8.[ISI][Medline]
14
.
Reynolds, P. E., Snaith, H. A., Maguire, A. J., Dutka-Malen, S. & Courvalin, P.
(1994). Analysis of peptidoglycan precursors in vancomycin-resistantEnterococcus gallinarium BM4174. Biochemical Journal 301, 58.[ISI][Medline]
15
.
Billot-Klein, D., Blanot, D., Gutmann, L. & van Heijenoort, J. (1994). Association constants for the binding of vancomycin and teicoplanin to N-acetyl-D-alanyl-D-alanine and N-acetyl-D-alanyl-D-serine. Biochemical Journal 304,1021
2.[ISI][Medline]
16
.
Van Wageningen, A. M. A., Staroske, T. & Williams, D. H. (1998). Binding of D-serine-terminating cell wall analogues to glycopeptide antibiotics. Chemical Communications 4,1171
2.
17
.
Nieto, M. & Perkins, H. R. (1971). Modifications of the acyl-D-alanyl-D-alanine terminus affecting complex-formation with
vancomycin. Biochemical Journal 123, 789803.[ISI][Medline]
18
.
Fesik, S. W., Armitage, I. M., Ellestad, G. A. & McGahren, W. J. (1984). Nuclear magnetic resonance studies on the interaction of avoparcin with model
receptors of bacterial cell walls. Molecular Pharmacology 25, 2816.[Abstract]
19
.
Bardsley, B. & Williams, D. H. (1997). Measurement of the
different affinities of the two halves of glycopeptide dimers for acetate. Chemical
Communications 2, 104950.
20
.
Allen, N. E., LeTourneau, D. L. & Hobbs, J. N. (1997). Molecular
interactions of a semisynthetic glycopeptide antibiotic with D-alanyl- D-alanine and D-alanyl-D-lactate residues. Antimicrobial
Agents and Chemotherapy 41, 6671.
21
.
Henion, J., Li, Y. T., Hsieh, Y. L. & Ganem, B. (1993). Mass
spectrometric investigations of drug-receptor interactions. Therapeutic Drug
Monitoring 15, 5639.[ISI][Medline]
22
.
Lightwahl, K. J., Schwartz, B. L. & Smith, R. D. (1994).
Observation of the noncovalent quaternary associations of proteins by electrospray ionization
mass spectrometry. Journal of the American Chemical Society 116, 52718.[ISI]
23
.
Przybylski, M. & Glocker, M. O. (1996). Electrospray mass
spectrometry of biomacromolecular complexes with noncovalent interactionsnew
analytical perspectives for supramolecular chemistry and molecular recognition processes. Angewandte Chemie-International Edition in English
35, 80626.
24
.
Loo, J. A. (1997). Studying noncovalent protein complexes by
electrospray ionization mass spectrometry. Mass Spectrometry Reviews 16, 123.[ISI][Medline]
25
.
Smith, R. D., Bruce, J. E., Wu, Q. Y. & Lei, Q. P. (1997). New
mass spectrometric methods for the study of noncovalent associations of biopolymers. Chemical Society Reviews
26,191
202.[ISI]
26
.
Wilm, M. & Mann, M. (1996). Analytical properties of the
nanoelectrospray ion source. Analytical Chemistry 68, 18.[ISI][Medline]
27
.
Ganem, B., Li, Y. T. & Henion, J. D. (1991). Observation of
noncovalent enzyme-substrate and enzyme-product complexes by ion-spray mass spectrometry. Journal of the American Chemical Society 113, 78189.[ISI]
28
.
Lim, H.-K., Hsieh, Y. L., Ganem, B. & Henion, J. (1995).
Recognition of cell-wall peptide ligands by vancomycin group antibiotics: studies using ion spray
mass spectrometry. Journal of Mass Spectrometry 30, 70814.[ISI]
29
.
Robinson, C. V., Chung, E. W., Kragelund, B. B., Knudsen, J., Aplin, R. T. &
Poulsen, F. M. (1996). Probing the nature of noncovalent interactions by mass
spectrometry. A study of protein-CoA ligand binding and assembly. Journal of the
American Chemical Society 118, 864653.[ISI]
30
.
Jørgensen, T. J. D., Roepstorff, P. & Heck, A. J. R. (1998). Direct determination of solution binding constants for noncovalent complexes between
bacterial cell wall peptide analogues and vancomycin group antibiotics by electrospray ionization
mass spectrometry. Analytical Chemistry 70, 442732.[ISI]
31
.
Ayed, A., Krutchinsky, A. N., Ens, W., Standing, K. G. & Duckworth, H. W. (1998). Quantitative evaluation of protein-protein and ligand-protein equilibria of a
large allosteric enzyme by electrospray ionization time-of-flight mass spectrometry. Rapid Communications in Mass Spectrometry
12, 33944.[ISI][Medline]
32
.
Mackay, J. P., Gerhard, U., Beauregard, D. A., Westwell, M. S., Searle, M. S. &
Williams, D. H. (1994). Glycopeptide antibiotic activity and the possible role of
dimerization: a model for biological signaling. Journal of the American Chemical
Society 116, 458190.[ISI]
33
.
Beauregard, D. A., Williams, D. H., Gwynn, M. N. & Knowles, D. J. C. (1995). Dimerization and membrane anchors in extracellular targeting of vancomycin
group antibiotics. Antimicrobial Agents and Chemotherapy 39, 7815.[Abstract]
34
.
Allen, N. E., LeTourneau, D. L. & Hobbs, J. N. (1997). The role
of hydrophobic side chains as determinants of antibacterial activity of semisynthetic glycopeptide
antibiotics. Journal of Antibiotics 50,677
84.[ISI][Medline]
35
.
Gerhard, U., Mackay, J. P., Maplestone, R. A. & Williams, D. H. (1993). The role of the sugar and chlorine substituents in the dimerization of vancomycin
antibiotics. Journal of the American Chemical Society 115, 2327.[ISI]
36
.
Linsdell, H., Toiron, C., Bruix, M., Rivas, G. & Menendez, M. (1996). Dimerization of A82846B, vancomycin and ristocetininfluence on antibiotic
complexation with cell wall model peptides. Journal of Antibiotics 49, 18193.[ISI][Medline]
37
.
Popieniek, P. H. & Pratt, R. F. (1988). Rates of specific peptide
binding to the glycopeptide antibiotics vancomycin, ristocetin and avoparcin. Journal of the American Chemical Society
110, 12856.[ISI]
Received 22 February 1999;
returned 17 May 1999; revised 2 June 1999;
accepted 11 June 1999