Dimer, initiation

Ursitti, J. A., Kotula, L., DeSilva, T. M., Curtis, P. J. and Speicher, D. W. Mapping the human erythrocyte P-spectrin dimer initiation site using recombinant peptides and correlation of its phasing with the a-actinin dimer site. /. Biol. Chem. 271 6636-6644,1996. 

Cyclic AMP diffuses away from the membrane and engages its own target which is an inactive protein kinase, called cAMP dependent protein kinase or protein kinase A (PKA). The inactive PKA is a tetramer of two catalytic subunits and two regulatory subunits. Binding of cAMP to the regulatory subunits causes structural changes and the two regulatory subunits dissociate from the two catalytic sub-units. The now activated protein kinase A, that is the C subunit dimer, initiates a downstream cascade by... 

Lodmell, J.S., Ehresmann, C., Ehresmann, B. and Marquet, R. (2000) Convergence of natural and artificial evolution an RNA loop-loop interaction The HIV-1 dimerization initiation site. RNA, 6, 1267-1276. 

Dimer initiators, (I), were also prepared by the author and are described. 

Zhou A, Carrell RW. Dimers initiate and propagate serine protease inhibitor polymerisation. J. Mol. Biol. 2008 375 36-42. 

Parallel to the photoinitiation processes (with hv) photoaddition processes are observed, as shown for example in the Figs. 4, 5, 11 to 13. After dimer initiation, trimer formation from the dimer is possible etc. The chain propagation within the DR or AC series is performed by photoaddition of monomer molecules M adjacent to the reaction centres, given by the dimer (DR2 or AC2), trimer (DR3 or AC3),... molecules. The molecules M are lowered in energy by the perturbation introduced by the reaction centres. They form a trap for the optical excitation energy. They may be excited directly (M + hv -> M ) or indirectly via nonperturbed monomer molecules (M -f- hv -> M ) and subsequent energy transfer (M + M M + M ). The chain propagation reaction therefore is in competition with the chain initiation reaction. 

Fig. 16. Photoreactions at low temperatures. The notation corresponds to the optical absorption lines of the reaction intermediates. An intermolecular chain termination reaction is assumed. The dimer initiation reaction requires two photons (hv). The photoaddition reaction is a one photon process (hv or hv ). The chain termination reactions are most effectively performed by resonant irradiation into the absorption of the DR or AC intermediates...

The thermal addition reactions following the photochemical dimer initiation reaction are described by the following reaction equations for the relevant, thermally reactive intermediates ... 

The dimer initiation is a multistep process with two essential steps, the creation of the reaction center DR/ with reactive electrons and the addition of an excited adjacent molecule M. As we know from the thermal addition reaction steps at low temperatures the nature of the second excitation in the bimolecular reaction must be extremely low in energy and, therefore, electronic excitations may be excluded. On the other hand molecular motions are likely to be more important in the dimer formation process. Therefore we have to inspect the topochemistry of the reaction. The most probable mechanism of the dimer formation is shown in Fig. 25. In the first step (1) (only by optical excitation) the metastable reaction center is generated. In the second step (2) the motion required in the reaction is performed by librational excitation of the adjacent molecules. This librational excitation may be thermally activated and therefore is present in the crystals at elevated temperatures only. At low temperatures (T < 80 K) this librational excitation is generated only by absorption of photons with subsequent radiationless relaxation processes, producing phonons and librons. A detailed analysis of the dimer initiation process has been given by Neumann et al. 

The low activation energy of the thermal addition polymerization reaction confirms the necessity of a (librational) motion of the molecules in the initiation process. The first addition process differs from all the following addition proccesses by the metastable monomer diradical structure, which — in contrast to the DR , AC , and DC structures with n > 2 — has a limited life-time given by the phosphorescence decay of the monomer triplet state. Therefore, the librational excitation must be performed during the life-time of the monomer reaction centre. In the case of the low temperature photopolymerization reaction the librational excitation has to be prepared optically via the decay of the electronic excitation. This is in contrast to the photopolymerization reaction at high temperatures, where numerous molecular motions are thermally and stationary present in the crystals. Due to this difference two photons (2hv) are required in every dimer initiation process at low temperatures and only one photon (hv -i- kT) is required at high temperatures. The two paths of the photoinitiation reaction are illustrated below by the arrows in Fig. 26. The respective pair states are characterized by M M and M M as discussed below. 

Fig. 25. Reaction scheme of the dimer initiation reaction. (1) Formation of the metastable monomer diradical M (2) Distortion of the adjacent monomer molecule M (3) Formation of the diradical dimer molecule Mj by 1,4 addition...

Owing to the bimolecularity of the initiation reaction the quantum yield of the dimer molecules (M2/Nji ) is proportional to the absorbed light quanta N bs and to the ratio kj/ko, characterizing the competition of the chemical dimer initiation process (kj) with the deactivation processes (ko) of the monomer excitation. A comparison of the dimer A absorption intensities of different diacetylene crystals shows that the ratio kj/ko is about a factor of 10 to 10 larger in the TS-6 crystals than in... 

Borane-amine complexes react similarly " . Kinetics of hydroboration of representative alkenes with well-characterized dialkylborane dimers show that the dimer initially dissociates into the monomer in an equilibrium ... 

Figure 18. Stereoscopic packing diagram of 7d top (010), space filling bottom (100) wire model with additional drawing of intended head-to-head photodimerization lines (d = 3.93oA) leading to a presumably unstable dimer initially.

Anionic dimerization initiated by addition of a cyclopropenyl anion to the double bond of the free cycloalkene [16]. 

Other dimerizations initiated by acetylene-allene interconversions have also been reported, and dimerization has also been applied to other cumulenes. The structure of the solid-state photodimer of tetraphenylbutatriene has now been reassigned, the new bis-allene structure (251) proposed being substantiated by ozonolysis to the known diketone (252), and by X-ray analysis. The solid-state photochemical reaction thus contrasts with the thermal dimerization, which gives the radialene (253). ... 

For atom-molecule and molecule-molecule reactions, the centrifugal barriers for low partial waves are typically a few mK or less. For barrierless reactions, therefore, Langevin behavior sets in at temperatures between 1 and 100 mK. Above this temperature, the details of the short-range potential become unimportant. An example of this is shown in Figure 1.7, which shows inelastic collision rates for Li + L12 for boson and fermion dimers initially in n = 1 and 2 [65]. The full quantum result approaches the Langevin value at collision energies above about 10 mK. 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

Chisholm s monomeric (BDI Mg(OtBu)(THF) (24) readily loses its coordinated THF molecule under polymerization cmiditions in CD2CI2. It polymerizes 100 equiv of d,l- or l-LA in dichloromethane in 2 or 10 min, respectively, the former leading essentially to atactic PLA [60, 61], Similarly, Coates dimeric initiator [ BDI Mg(0/Pr)]2 (25) converted quantitatively 100-500 equiv of d,l-LA to atactic PLA within 5 min at 20 °C the additicm of 1 equiv of iPrOH (vs. Mg) was required to obtain narrow molecular weight distributions (MJMn 1.20-1.35) as otherwise the nature of the polymer chains is less controlled (M /Mn = 1.59) [62]. These two studies revealed that in dichloromethane, the Mg-based 24 and 25 were... 

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