72.Fluoroquinolones藥物第七碳位置上的piperazine環，其作用在於增加：
(A)血中脂溶性
(B)尿中水溶性
(C)抗革蘭氏陽性菌活性
(D)抗革蘭氏陰性菌活性

Figure 37.1 shows the basic quinolone structure.

The carboxyl group at position 3 and the ketone at position 4 are necessary for the antibacterial activity.

The fluorine at position 6 differentiates the quinolones from the fluoroquinolones and accounts for the improved gram-negative and gram-positive activity over the nonfluorinated quinolones, increased potency, and increased entry into bacteria.

At position 1, addition of a cyclopropyl (as for enrofloxacin and ciprofloxacin), an ethyl or a fluorophenyl improve the spectrum of activity against gram-positive and gram-negative bacteria.

Addition of a piperazine at position 7, as demonstrated for ciprofloxacin, marbofloxacin, and enrofloxacin, improves the spectrum of activity to include pseudomonads, among other gram-negative bacteria.

The change from a nitrogen (nalidixic acid) to a carbon at position 8 decreased some of the adverse central nervous system effects and increased act increased activity against staphylococci.  

Reference: Veterinary Pharmacology and Therapeutics, 10th Edition, page 954.