Circular Dichroism Measurements of the Tetracyclines IV. 5-Hydroxylated Derivatives

Abstract

From extensive circular dichroism measurements of a variety of tetracycline (TC) derivatives in the presence and absence of a variety of chelating ions, it is concluded that 5-hydroxytetracycline possesses an identical solution conformation to that possessed by all fermentation-derived, bioactive TC species in dilute aqueous solutions at pH 7.5 or below. These conditions resemble those encountered under normal physiological circumstances. In alkaline solutions, the conformation of of 5-hydroxytetracycline diverges from the non-5-hydroxylated subclass of TC species. This divergence is rationalized as due to a buttressing effect of colinear perisubstituents at positions 4, 5, and 6 and is assisted by the possibility of hydrogen bonding between the 5 and 12a hydroxyl groups. Chelation, in alkaline solutions, with Mg ions "locks" the molecule into a conformation which has been detected by others in nuclear magnetic resonance studies in concentrated non-aqueous solvents and in X-ray studies with the 5,12a-diacetoxy analogue. Parallel studies with a variety of model TC species and Ca ions provide strong support for the hypothesis that both Ca and Mg ions bind at the BCD juncture and that, above pH 7.5, the Ca ion binds to the A ring between 4-NMe2 and 12a-OH, whereas Mg ion does not do this. If a 5-OH group is present, the Ca ion still binds as before, whereas the Mg ion will bind to the AB rings between 5-OH and 12a-OH. These factors are discussed in conjunction with the various conformations potentially available to the molecule.Earlier papers in this series (9-11) have presented evidence consistent with the concept that all of the common fermentation-derived tetracyclines (TC) possess the same solution conformation in dilute aqueous acidic media. These findings are in agreement with evidence gathered from nuclear magnetic resonance (NMR) (12, 13)and X-ray (1, 2, 4-6, 14, 15) measurements. We have proposed that under neutral conditions the TC species undergo a subtle, but potentially very meaningful, conformational alteration resulting from hydrogen bonding between the 1, 3-diaxially disposed 12a-OH and 4-NMe2 groups. This change results in considerable flattening of the A-ring chromophore.bThere is evidence in the literature, first from NMR work (12, 13) and, quite recently, from X-ray measurements (6, 15), that 5-hydroxytetracycline (OTC) and some of its derivatives may exist in another conformation under cer-I Present address: Parke-Davis and Co., Detroit, Mich. 48232. tain conditions. We have carried out extensive circular dichroism (CD) measurements and conclude that, indeed, under certain conditions, this TC subgroup does behave in a manner distinct from the TC species lacking this function. In this paper, we would like to present results which confirm this X-ray-and NMR-based hypothesis and define some of the conditions under which this conformation is preferred. MATERIALS AND METHODSThe TC derivatives used in this study are the purest samples available and were obtained through the generosity of J. M. Boo...

Mechanism of Fluorometric Analysis of Tetracycline Involving Metal Complexation

“…The circular dichroism studies, in particular, suggest that the binding occurs at the A-ring binding site. These results are in agreement with other studies (19,20,22,25).…”

“…The BCD-ring chromophore is responsible for the bands at 360,320,285, and 225 nm; the A-ring chromophore contributes the 262-nm band (19). The circular dichroism spectrum is extremely sensitive to configuration change a t the C-4 position of the tetracycline molecule (22). At pH 7.5, only the bands associated with the BCD-ring chromophore are affected by calcium ion (Fig.…”

Chemical Modification of the Tetracyclines

“…Conformation has also been investigated under physiological conditions using the circular diehroism method (MITSCHER et al 1968(MITSCHER et al , 1969(MITSCHER et al , 1972. It can be concluded from the results obtained that the BDC-ring system is essentially planar and that the AB-ring system can undergo considerable conformational changes, whereby the conformation at physiological pH values is not necessarily the active conformation.…”

“…Recent X-ray crystallographic studies (STEZOWSKI 1976(STEZOWSKI , 1977PREWO and STEZ-OWSKI 1977;GLATZ et al 1979) and circular dichroism studies (MITSCHER et al 1968(MITSCHER et al , 1969(MITSCHER et al , 1972) have been applied in defining the conformations and hydrogen bonding of the tetracyclines under certain conditions. The most recent papers by Stezowski and co-workers KOL-LAT and STEZOWSKI 1982), already mentioned at various points in this chapter, have considerably broadened the outlook on structure-activity relationships in te-tracyclines.…”

Oxytetracycline in intensive aquaculture: water quality during and after its administration, environmental fate, toxicity and bacterial resistance

“…) is commonly proposed as the first site of coordination, although the exact position is often questioned (Mitscher et al . ; Newman & Frank ; Lambs et al . ; Wessels et al .…”