Degenerate pathways

Examples of the role of the lattice in generating radicals in a given conformation, and in selecting one among many nominally degenerate pathways, have been described (246). 

The confirmation that neurogenesis occurs in the adult brain and that NSCs reside in the adult CNS opens new opportunities for cellular therapy. Cell therapy would involve the stimulation of endogenous neural progenitor or stem cells, or the transplantation of adult-derived neural progenitor and stem cells to repair the degenerated pathways. 

As shown in Table I, a number of the pathways are degenerate in the sense that they lead to identical labeling patterns Consequently, our analysis will give only the sum of relative fluxes through degenerate pathways For convenience in... 

In N02, both the NO and O fragments have several low-lying electronic states that are accessible in the dissociation process. The NO fragment has two spin-orbit states (2n3/2 and 2ni/2), while the O atom can have three possible spin-orbit states (3P2, 3P3, and 2P0). A total of 18 doubly degenerate pathways are possible for N02 dissociation [101], Ion imaging... 

Actually, although hydroxy-assisted delivery of the peracid to the indicated C C double bond of 38 ensured the formation of 41, addition of oxygen to the other olefin would afford a degenerate pathway to the same produet. 

Using more precise language, the rates for catalyst decomposition pathways are unaffected by the rates of productive and degenerate pathways, assuming that catalyst decomposition reactions are zero-order in substrate, but first-order in catalyst. A similar conclusion can be drawn from the op posite assumption that catalyst... 

Isotope effect between the HH, HD, DH, and DD isotopomers was used as an important tool to determine the mechanism of the double-proton transfer. For concerted degenerate double-proton transfers in the absence of tunneling, the rule of the geometrical mean (RGM) should hold in good approximation, which states that /chh/ hd = /cdh/ dd-Tunneling may lead to a breakdown of this rule but the relation /chh > hd = dh > dd should remain valid. In the absence of secondary isotope effects the relation /chh HD = DH = 2 /cdd sliould liold for a stepwise pathway, even if tunneling is involved. 

The nigrostriatal tract is one of the four main dopaminergic pathways in the central nervous system. About 75% of the dopamine in the brain occurs in the nigrostriatal pathway with its cell bodies in the substantia nigra, whose axons project in the corpus striatum. Degeneration of the dopaminergic neurons in the nigrostriatal system results in Parkinsons disease. 

People with Parkinson s disease show a specific degeneration of the nigrostriatal tract so DA must be linked in some way to the control of motor function. It is also known that an imbalance of DA function on the two sides of the rat brain, either by stimulation or lesion of one SN, causes off-line or rotational movement (Ungerstadt and Arbuthnott 1970). This is best shown some days after 6-OHDA lesion of one substantia nigra and its nigrostriatal pathway when systemic apomorphine (DA agonist) causes animals to turn away from the lesioned side (contraversive), presumably... 

Parkinsonism is unique among diseases of the CNS, in that it results from the known loss of a particular NT, i.e. DA, resulting from the degeneration of a particular pathway, the nigrostriatal. Dopamine also has a relatively limited distribution in the brain and few peripheral effects. It should therefore be amenable to therapy based on augmenting its function. Also since the role of DA appears to be to maintain a tonic inhibitory control on GABA output pathways from the striatum, possibly in part by an extra synaptic action (Chapter 6), it may not be necessary for it to be released physiologically from nerve terminals. Thus it may be adequate to just provide DA extracellularly. 

References 29-33 introduce the notion of coherence spectroscopy in the context of two-pathway excitation coherent control. Within the energy domain, two-pathway approach to coherent control [25, 34—36], a material system is simultaneously subjected to two laser fields of equal energy and controllable relative phase, to produce a degenerate continuum state in which the relative phase of the laser fields is imprinted. The probability of the continuum state to evolve into a given product, labeled S, is readily shown (vide infra) to vary sinusoidally with the relative phase of the two laser fields < ),... 

The trans compound cannot be formed by a concerted reaction from the 33t-7t state because the predominant level perturbation is the almost degenerate interaction of K(jr) and 0(jr), that would give cis geometry of product. Since the trans adduct must be formed via biradical intermediate (the only other possibility), it has usually been convenient to suppose that the cis adduct is also formed from the same biradical intermediate. Another choice of mechanism is that cis compound is formed via a concerted reaction, and the trans compound arises from a biradical pathway. In this case, the spin prohibition could be outweighed by two factors, the favorable geometry and the stabilizing first-order perturbation. 

From examination of Fig. 11, it is inferred that the zn-n state is less reactive, and a biradical mechanism should be the major reaction pathway. The degenerate stabilizing perturbation of the bonding levels is missing, and concerted pathways are not likely if stabilized only by much smaller secondary interactions. If the hi-n singlet state could be intercepted in some way, the all-suprafacial concerted mechanism would be favored [K(ji ) -0(jr )] relative to the suprafacial-antarafacial mechanism [O(tt) - -K( i )]. 

---