Polynucleation

For the purposes of the following discussion small molecules are considered to have fewer than 200 nonhydrogen atoms. Macromolecules, primarily proteins, polynucleic acids, and vimses, can have many thousands of atoms. 

The procedures for solving and refining macromolecular stmctures are similar to those for smaH molecules. The class of macromolecular stmctures solved consists mostly of proteins, some polynucleic acids, and some viral stmctures. These stmctures usuaHy contain thousands of atoms. 

This neglects the time taken for nucleation, which will be a good approximation if g is sufficiently small. The rate at which new layers form,, is proportional to (ig)1/2, hence the growth rate in the polynucleation regime for stems of thickness b is ... 

The second quantity is the kinetic length, Lk, and measures the average separation between growth patches in the polynucleation regime. From Eq. (3.14) ... 

Ribonuclease II [EC 3.1.13.1], also called exoribo-nuclease II, catalyzes the exonucleolytic cleavage of the polynucleic acid, preferring single-stranded RNA, in the 3 - to 5 -direction to yield 5 -phosphomononucleotides. The enzyme processes 3 -terminal extra-nucleotides of monomeric tRNA precursors, following the action of ribonuclease P. Similar enzymes include RNase Q, RNase BN, RNase PHI, and RNase Y. Ribonuclease T2 [EC 3.1.27.1] is also known as ribonuclease II. 

Innocenti et al. have studied the kinetics [101] of two-dimensional phase transitions of sulfide and halide ions, as well as electrosorption valency [102] of these ions adsorbed on Ag(lll). The electrode potential was stepped up from the value negative enough to exclude anionic adsorption to the potential range providing stability of either the first or the second, more compressed, ordered overlayer of the anions. The kinetic behavior was interpreted in terms of a model that accounts for diffusion-controlled random adsorption of the anions, followed by the progressive polynucleation and growth. 

M.L. Tedjamulia. Y. Tominaga, M. Sugiura, H. Kudo, M.L. Lee u. R.N. Castle, Polynucl. Aromal. Hydrocarbons, Int. Symp., 7th, 1161-1172 (1982) The Synthesis of Potentially Mutagenic Polycyclic Aromatic Amines". 

It is worth a pause, however, to consider how such models should best be used. Part of the motivation for developing linear scaling models has been to permit QM calculations to be carried out on biomolecules, e.g., proteins or polynucleic acids. However, one may legitimately ask whether there is any point in such a calculation, beyond demonstrating that... 

Virtually all types of metal ions have been complexed with macrocyclic ligands.2-7 Complexes of transition metal ions have been studied extensively with tetraaza macrocycles (Chapter 21.2). Porphyrin and porphyrin-related complexes are of course notoriously present in biological systems and have been receiving considerable investigative attention (Chapter 22).8 Macrocyclic ligands derived from the Schiffbase and template-assisted condensation reactions of Curtis and Busch also figure prominantly with transition metal ions.6,7 The chemistry of these ions has been more recently expanded into the realm of polyaza, polynucleating and polycyclic systems.9 Transition metal complexes with thioether and phosphorus donor macrocycles are also known.2... 

Hafez IM, Maurer N, Cullis PR (2001) On the mechanism whereby cationic lipids promote intracellular delivery of polynucleic acids. Gene Ther 8 1188-1196... 

According to the LH theory, depending on whether mononucleation (Regime I) or polynucleation (Regime II) is involved, is equal to 2 or 1, respectively [20]. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

Polynuclear PCMU can be synthesized by means of all the above described methods for the preparation of mononuclear PCMU, such as binding a few metals with polynucleating macromolecular ligands, immobilization of the ready-made and well characterized polynuclear complexes or clusters on polymers and assemblage of polymetallic sites on polymer supports. 

One of the most popular methods for the preparation of low-molecular weight polynuclear metallochelate is the interaction of MXn with polynucleating ligands... 

Undoubtly, cryptands remain the class of macrobicyclic and macropolycyclic receptors looking forward to a great future. The number of possible structures is practically unlimited and depends on a researcher imagination and creativity. Initially, the chemistry of cryptands had focused on the development of new synthetic strategies, either to improve the yields or to built up more complicated molecular architectures, like cylindrical, spherical or lateral macropolycycles, bi- or polynucleating, with hard and soft binding sites, etc. 

The above discussion dealt primarily with synthetic polyelectrolytes, which behave in solution more or less as flexible polymers. Another important group, the natural polyelectrolytes, will not be discussed here. They include polynucleic acids, proteins, carbohydrate derivates, etc. They generally behave as rigid-chain polymers, due to their helix conformation. 

Individual sequential covalent bond formation in a series of reaction-isolation steps. Bond formation usually involves only one or several connections per step. The method can be used to construct one-dimensional linear structures (e.g., polypeptides/polynucleic acids) where chronological sequencing can be used to introduce desired side groups or chirality by this linear covalent method. 

Only a handful of pseudotetrahedral or octahedral oxosulfido-Mo(VI) and -Mo(V) complexes are known, and their chemistry is described in Section 11. Mononuclear sulfido-Mo(V) complexes are unstable due to their susceptibility to redox, polynucleation, and hydrolysis reactions. Trispyrazolylborate derivatives such as sfructuraUy characterized Tp Mo SX2 (X = Cl, OPh derivative X2 = bdt, catecholate) are most commonly encountered. The Tp complexes exhibit short Mo=S bonds (ca. 2.13 A), intense S(ls)- - n (Mo=S) XAS transitions characteristic of terminal thio ligands, and EPR iso see g-Factor) values substantially lower than their 0x0 analogues. ... 

In some nodule areas, special facies of manganese nodules can be observed (the so-called polynucleated nodules. Fig. 9). They occur only on the submarine hills, and seem to be built of a number of small maganese nodules which grew together because they were lying close to each other. 

Fig. 9. Cross section of a typical polynucleated manganese nodule. Basalt fragments are coated and cemented together with oxyde layers.

---