Penicillin

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Introduction

Although many of the penicillins have assessable chromophores, the close structural similarity of the various forms makes identification difficult. However, penicillins do contain three chiral centers providing a good test of the selectivity of specific rotation.

This specificity is further tested by the fact that carbenicillin and ticarcillin each exist in two different epimeric forms. Epimers differ only in the arrangement of the functional groups at one of the chiral centers.

Carbenicillin and Ticarcillin

Here we show the polarimetric detection of a mixture of carbenicillin and ticarcillin after separation by reverse phase HPLC.

The peaks labeled (a) and (c) correspond to the 10R and 10S epimers of carbenicillin, respectively. Peaks (b) and (d) correspond to the 10R and 10S epimers of ticarcillin, respectively. The mobile phase was 3mM KH₂PO₄, 3 mM K₂HPO₄, 15 mM tetrabutylammonium iodide and methanol (2:2:2:5, v:v) at a flow rate of 0.7 mL/minute. The total amount of sample injected was 0.4 m g. The above separation of the carbenicillin and ticarcillin epimers demonstrates the sensitivity of polarimetric detection for these ubiquitous antibiotic substances. The detection limit for most penicillin analogs is at or below the 10 ng level.

Calculated Specific Rotations

The following table shows the calculated specific rotations for six penicillin analogs. The large differences in specific rotation measured for these six structural analogs are unprecedented.

Specific Rotation of Six Penicillin Analogs
Compound	Literature Value ^a,b	Direct Measure ^a,c,d	Calc. from Pk Ht ^a,c,d
Penicillin G	310	340 + 11	347 + 6
Ampicillin	281	314 + 11	321 + 6
Carbenicillin (10R)		192 + 8^e	180 + 1
Carbenicillin (10S)			213 + 1
Ticarcillin (10R)		179 + 8^e	166 + 1
Ticarcillin (10S)			210 + 2
^a deg dm^-1 g^-1 ml ^b From Merck Index, 9^th edition; 580 nm ^c 632.8 nm ^d Experimentally determined standard deviations ^e Epimeric mixture

Summary

The substitution of a carboxylic acid functionality (e.g. ticarcillin) for an amine (e.g. ampicillin) results in a change in specific rotation of over 100 deg dm^-1 g^-1 ml. Further, the differences noted for the epimers of ticarcillin and carbenicillin are extremely significant as these species differ by the arrangement of functionality at one chiral center out of three. In the separation of these four species, it is frequently difficult to identify the particular epimer producing a specific peak. However, specific rotation can now be used to identify the epimers of these two antibiotic species.