E. Bunnenberg

Organic chemical applications of magnetic circular dichroism

Recent applications of magnetic circular dichroism spectroscopy (the Faraday effect) in organic chemistry are reviewed. Magnetic optical activity is a universal property of all matter and consequently can be applied to optically active as well as optically inactive molecules. It is emphasized, however, that natural and magnetic optical activity have separate physical origins and the information provided will necessarily be different. A brief theoretical background is given in order to explain the three types (A, B and C) of MCD curves which may be encountered in practice and the deductions that may be reached from their occurrence. Spectroscopic and analytical applications of MCD are illustrated for a wide variety of molecular systems such as metalloporphyrins, chlorins, corrins, aromatic hydrocarbons, carbonyl compounds, purine and pyrimidine bases and their nucleoside derivatives.

Magnetic circular dichroism studies: XIX. Determination of the tyrosine: Trytophan ratio in proteins

Abstract Recently (2), we reported the use of magnetic circular dichroism (MCD) for the determination of tryptophan in intact proteins (1). We pointed out that, in comparison to other methods currently in use, MCD has the unique advantage of giving unambiguous results even in cases in which the tyrosine: tryptophan ratio is high, that conformational and environmental effects do not interfere with the accuracy of the method, and that only small amounts of sample are required. Here, we wish to report an important refinement in the procedure that enables the technique to be extended to samples which, due to the presence of unknown quantities of water or inorganic salts, are not of analytical purity. This modification makes use of the fact that, in contradistinction to tryptophan, the quantitative determination of tyrosine by the usual chromatographic procedure subsequent to hydrolytic cleavage of the protein does not present serious difficulties. Consequently, the spectroscopic determination of the tyrosine: tryptophan ratio has the advantage that, given knowledge of the tyrosine content, accurate values for the tryptophan content can be obtained with only rough knowledge of the protein concentration. Thus, tyrosine serves as an internal standard in the determination of tryptophan by the MCD technique. In addition to enabling tryptophan determinations to be made with increased accuracy, this modification illustrates the advantages inherent in a spectroscopic method in which signals may occur with either positive or negative sign.

Calculation of vibronic magnetic rotational strengths in formaldehyde

The magnetic rotational strengths (MRS) associated with the non‐totally‐symmetric vibrations in formaldehyde are calculated with CNDO/2 wave functions. The effects of configuration interaction are examined. We have also identified a single term that dominates the MRS. This raises the possibility of an economy of thought in attempts at correlating MRS data with molecular structure in the case of magnetic dipole allowed transitions.

Magnetic circular dichroism spectroscopy: A definitive method for the determination of urinary porphyrins☆

Abstract The application of MCD for the determination of urinary porphyrins is assayed by specific application to the screening of workers exposed to lead arsenate. The definitive nature of this new spectroscopic method arises from: (1) its sensitivity to as little as 0.02 μg/ml of porphyrin dication; (2) its freedom from interference caused by other substances; (3) the highly characteristic shape of the MCD bands exhibited by porphyrin dications; and (4) the high information content of the MCD spectrum.

Partial reduction of aquomethemoglobin on a sephadex G-25 column as detected by magnetic circular dichroism spectroscopy and revised extinction coefficients for aquomethemoglobin

In a typical preparation of aquomethemoglobin, oxyhemoglobin is oxidized with potassium ferricyanide, and the resultant mixture of methemoglobin and potassium ferro- and ferricyanides is separated on a Sephadex G-25 column. We find that about 1% of the heme is reduced on the column and is eluted with the methemoglobin. Magnetic circular dichroism spectra show that the reduced species is oxyhemoglobin. Magnetic circular dichroism is more sensitive than is absorption spectroscopy to small amounts of oxyhemoglobin in such solutions; we can detect its presence at the 0.1% level. A redetermination of the extinction coefficients for methemoglobin gives a value of 0.80 for the absorbance ratio A570A630 at pH 6. This value lies clearly outside the currently accepted range of 0.83 to 0.87.

Magnetic circular dichroism studies—63. Sign variation in the magnetic circular dichroism spectra of some perimeter symmetric metallo porphyrins☆

Abstract It is known from previous work that porphine dication, porphine dianion and the ethanol adduct of magnesium porphine are exceptions to the “golden rule” of MCD spectroscopy that positive A terms are associated with the Q and B (Soret) electronic transitions of perimeter symmetric centrally substituted porphyrins since for them a negative A term is found for the Q 0 transition, giving rise to an overall electronic MCD band sign pattern of + − − +. The present work uncovers additional examples in a series of zinc tetrakis( ortho -halophenyl)porphyrins. Michls perimeter model is used to provide insight into structural effects which might lead to the condition of ΔHOMO ≲ ΔLUMO required to explain the sign inversions on a purely electronic basis. The possibility that the sign variations have a more general vibronic origin is also considered.

Magnetic circular dichroism studies XXXIV: Improved instrumentation for MCD measurements

Abstract The use of a 15-kG electromagnet in conjunction with a circular dichrometer of enhanced sensitivity affords MCD spectra of quality comparable to those obtained using a superconducting magnet and an instrument of lower sensitivity. To demonstrate this, the MCD spectra of zinc octaethylporphyrin, human serum albumin, a microsomal suspension containing cytochromes P -450 and b 5 , adenine, and benzene in the vapor phase are reported. The convenience, economy of operation, and availability of electromagnets should be a major incentive for the utilization of this spectroscopic technique in a variety of potentially interesting areas in chemistry and biochemistry.

Enhanced structure determination of beta-pyrrole and N-substituted porphyrins by desorption chemical ionization mass spectrometry.

Abstract The extensive macrocycle—but minimal substituent—fragmentation occurring in the chemical ionization mass spectra of porphyrins when ammonia, rather than hydrogen, is the reagent gas leads to more detailed structure determinations.

Magnetic circular dichroism studies. LXVI: Synthesis of demethyl monosubstituted porphyrins. The effect of substituent conformation on the magnetic circular dichroism spectra of ethoxycarbonyl porphyrins

Synthese et etude de dichroisme de trimethyl-2,12,18 porphyrinetripropionates-3,13,17 de trialkyle substitues en 8 par acetyl, vinyl, formyl, cyano ou ethoxycarbonyl

Search for porphyrins in lunar dust.

Evidence for porphyrins was obtained in the Apollo 11 bulk sample of lunar dust by fluorescence spectrometry and analytical demetallation. The indicated porphyrins showed major fluorescence excitation at 390 nanometers. Abundance was about 10-10 gram of porphyrin per gram of dust. Similar pigments were found in exhaust products from tests of a lunar descent engine. The similarity of results suggests that most if not all of the indicated porphyrin aggregate of the lunar sample probably was synthesized from rocket fuel during the landing of the lunar module. These compounds may be the product of a novel high-temperature synthesis of cosmochemical interest.