The fractionation of iron-phosphorus compounds in sediments studied by Mössbauer spectroscopy

Iron compounds of phosphorus form a large part of the phosphorus bound in sediments. Mössbauer spectroscopy is a technique that enables us to study, directly, chemical forms of iron in solid samples. Mössbauer spectroscopy allowed us to check, directly, the selectivity of the extraction scheme for soil phosphorus proposed by Chang &; Jackson (1957), but only as far as the iron compounds are concerned. It appears that selectivity of the extraction method leaves much to be desired.

     

Conformational studies on the hypothalamic thyrotropin releasing factor and related compounds by 1-H nuclear magnetic resonance spectroscopy

Proton magnetic resonance studies at 220 MHz were carried out on the hypothalamic thyrotropin releasing factor (TRF) and a variety of synthetic analogs designed to yield specific information about side chain–side chain and side chain–backbone interactions. The results show that the very low field resonance position of one of the carboxamide protons observed for TRF dissolved in dimethylsulfoxide is neither caused by a seven-membered nor by a ten-membered hydrogen bonded ring, but rather is due to a short range interaction between the unprotonated histidyl side chain and the carboxamide residue. A systematic study of the preferred histidyl side chain conformation in various TRF analogs is in good agreement with this interpretation. It was demonstrated that this interaction is not strong enough to cause significant changes in the preferred back bone conformation of the hormone. The possibilities for typical dipole–dipole interaction are discussed. No such interaction has been detected in TRF dissolved in water. We conclude that the tertiary structure of TRF in polar solvents is determined primarily by the steric characteristics of the bulky side chains which maintain the molecule preferentially in an extended conformation.     

[4-Amino-D-phenylalanine6]luliberin. A biologically active abalog of the luteinizing hormone-releasing factor suitable for affinity labeling

The syntheis of [4-amino-D -phenylalanine⁶]luliberin by the solid-phase method is described. The analog was found to be ten times more potent than luliberin. Some possibilities for the easy derivatization of the 4-amino-phenylalanyl residue into addinity labels suitable for the study of hormone-receptor interations are considered. The preparation of N^α-t-butyloxycarbonyl-4-(benzyloxycarbonylamino)-D -phenylalanine, one of the key intermediates used in the synthesis of this luliberin analog, is also reported.     

Synthesis and conformational analysis of cyclo-TRI[l-valyl-d-hexahydromandelyl]

The synthesis of the cyclo-hexadepsipeptide [l-valyl-d-hexahydromandelyl]₃ is described. Examination of this macrocyclic compound by 220-MHz nuclear magnetic resonance spectroscopy shows that symmetrical conformations are stabilized in strongly polar solvents (trifluoroacetic acid, acetonitrile), whereas asymmetric conformations are preferred in nonpolar or slightly polar media such as carbon tetrachloride, chloroform, cyclohexane, and benzene.From analysis of the temperature dependence of the chemical shift and of the coupling constants, together with conformational energy calculations, a model is proposed for the preferred conformation of this molecule in nonpolar solvents.