Arylsulphatase activity and cerebroside sulphates in the frog oviduct during the reproductive cycle

Summary The presence of arylsulphatase A and cerebroside sulphates in different tracts ofRana esculenta oviduct during different phases of the reproductive cycle were investigated by histochemical and biochemical procedures. The results indicate that enzyme activity shows seasonal fluctuations connected with the phase of the sexual cycle. The concentrations of cerebroside sulphates (the natural substrates of arylsulphatase A) is related to the activity of this hydrolytic enzyme. The role of arylsulphatase A activity in regulating the substrate concentration, and particularly that of sulphatides, is discussed.     

Fast and accurate computation of the 13C chemical shifts for an alanine-rich peptide

The purpose of this work is, first, to present a fast and accurate technique to compute Boltzmann-averaged values of the quantum-chemical 13C chemical shifts for each amino acid in oligopeptides, demonstrated here by an application to the peptide Ac-XXAAAAAAAOO-NH2 (where X denotes diaminobutyric acid, A is alanine, and O is ornithine) [XAO] and, second, to discuss the capability of the 13Calpha and 13Cbeta chemical shifts to distinguish the PP(II) conformation from the alpha-helix and statistical-coil conformations. Use is made of a combination of approaches, summarized as follows: (1) derivation of an ensemble of conformations by using a molecular mechanics technique; (2) use of a clustering procedure to form families and build a reduced set of conformations consisting of the lowest-energy conformations of each family, and (3) computation of the 13C chemical shifts for the lowest-energy conformations of each family, using a quantum-chemical approach that treats a selected residue, or group of residues, with a 6-311+G(2d,p) locally-dense basis set, while the remaining residues in the sequence are treated with a 3-21G basis set. The whole procedure is quite accurate and speeds up the computation of the Boltzmann-averaged values of the 13C-chemical shifts by several orders of magnitude. The present application sheds some light on the conformational preference for alanine and non-alanine residues to occupy the PP(II) helical region of the Ramachandran map.     

Unblocked statistical-coil tetrapeptides in aqueous solution: quantum-chemical computation of the carbon-13 NMR chemical shifts

We recently reported a theoretical characterization of representative ensembles of statistical-coil conformations for tetrapeptides with unblocked termini in aqueous solution, at pH 7. The results showed good agreement between the computed Boltzmann-averaged and experimentally-determined values for both the vicinal coupling constants ³J_NH and the -proton chemical shifts. Here, we carry out a cluster analysis of the ensembles of conformations generated in that study, and use them to compute the Boltzmann-averaged values of the quantum-chemical ¹³C chemical shifts for different amino acids in the unblocked tetrapeptides GGXA (where X stands for Phe, Arg, His, Glu, Ile, Lys, Gln, Tyr, Leu, Thr, Ala, Gly and Val). The values of the ¹³C chemical shifts in these thirteen amino acids (for which experimental data are available) were computed by using Density Functional Theory with a 6–311+G(2d,p) basis set. Good agreement is found in terms of both the correlation coefficient (R) and standard deviations of the difference between the computed Bolztmann-averaged and the NMR-determined values for the ¹³C chemical shifts. These results suggest that it may be possible to build a reliable theoretically-derived database of chemical shifts for statistical-coil residues. The results of the current study contribute to our understanding of the relations between chemical shifts, dihedral angles and vicinal coupling constants, ³J_NH. In addition, they can shed light as to how the statistical-coilconformation is related to the conformational preference of more structured states, such as the -helical conformation.