LD Model Research Articles

Polynucleotide base-pair orientation in solution: linear dichroism and molecular mechanical studies of poly[d(A)]-poly[d(T)

AbstractModel helices for poly[d(A)]‐poly[d(T)] in solution were constructed based on the base orientations determined by linear dichroism [LD; S. P. Edmondson & W. C. Johnson, Jr. (1985) Biopolymers 24, 824–841] and refined by molecular mechanical energy minimization. The results demonstrate that poly[d(A)]‐poly[d(T)] can form energetically stable conformations with the base orientations measured by LD. Further, only one of the four possible base‐pair orientations that are consistent with the LD results is feasible. Models with negative base tilts had large potential energies, indicating that the LD results do not reflect base motions. The LD models of poly[d(A)]‐poly[d(T)] are stabilized mainly by intrastrand base‐stacking interactions, particularly for the adenine strand.

The determination of dynamic characteristics of ion-exchange columns by the application of one-dimensional mathematical models

The one-dimensional mathematical models describing the processes in ion-exchange columns with packed bed and counter-current phase motion are compared. The exchange of Li + with H +, Na + with H + and K + with H + was studied on a sulphonated cation exchanger KU-2 × 8. The parameters of the models were determined by the method of random search from the experimental curves showing the responses to pulse perturbations of the hydrogen-ion concentration. Under the experimental conditions, the diffusion model (ED) and the plug-flow (PF) model are equally adequate. A more complicated model (LD) which takes into account the interphase non-equilibrium and axial dispersion in the liquid phase was also studied. The ranges of parameters for which the LD model reduces to the ED and PF models are established. It is shown that the LD model has no advantages over the ED and PF models, because axial dispersion and interphase non-equilibrium are additive.

Serine and β‐turns

AbstractSerine is one of the most frequent residues included within β‐turns in proteins. Conformational investigations have been carried out on the LL and LD model dipeptides ButCO‐Pro‐Ser‐NHMe in the solute state by ir and nmr spectroscopies and in the solid state by x‐ray diffraction. LL and LD sequences are folded in the βI‐ and βII‐types, respectively, and in both cases a weak intramolecular interaction takes place between the NH and Oγ sites of the serine residue.

Limitations of the College Entry Ld Model

Limitations of the College Entry Ld Model