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Biomedical subjects

L A Mitscher

Publications and source records attributed to L A Mitscher.

At least 73 records · Page 4Linked to original sources
PubMed1975

Chemical-ionization mass spectrometry of beta-lactam antibiotics.

Chemical-ionization (CI) mass spectra are described for methyl esters of eight clinically significant penicillins and their breakdown products. The substances give spectra with very few fragment ions and contain easily discernible protonated molecule ions. The main cleavage reaction is postulated to involve a retro 2+2 Diels-Alder-type fragmentation of the beta-lactam ring liberating one fragment (m/e=174) that is characteristic of the penicillin nucleus and a second fragment that is molecule specific, as it contains the elements of the side chain. The other fragment ions, though interesting, are of minor intensity. The free acids, on the other hand, fragment more extensively because of their relative instability and lack of volatility. These spectra resemble electron impact spectra more closely and, though they encode more structural information, are less reproducible from run to run. The ease with which the esters can be made and the relative simplicity of their CI mass spectra make this method significant for the identification and characterization of beta-lactam antibiotics.

Esters↗
PubMed1973

Use of the 295- to 300-nanometer circular dichroism through of ribonucleic acid to study helix winding: effect of acridine orange.

Acridine orange decreases the amplitude of the 295-nm circular dichroism (CD) trough of ribosomal ribonucleic acid (rRNA) where the trough has been related to coil character. Since acridine orange is known from earlier work to intercalate between base pairs of nucleic acids, causing an unwinding of the coil, and our studies show a decrease in the 295-nm CD trough, it appears that CD measurements may be used to observe relative unwinding of rRNA. Under similar solution conditions, melting temperatures with acridine orange indicate no significant change in the stabilization of rRNA structure by acridine orange. Hypochromicity studies show no increase in the percent base pairing in rRNA when 0.1 M tris(hydroxymethyl)aminomethane (pH 7.6) with 1.35 M KCl is used. These results indicate that CD changes in the amplitude of the 295-nm trough of rRNA are related to helix winding in rRNA.

Acridines↗
PubMed1972

Circular dichroism measurements of the tetracyclines. IV. 5-Hydroxylated derivatives.

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 peri-substituents 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-NMe(2) 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.

Circular Dichroism↗