Entry channel

A complete theoretical treatment of the + He collision should therefore take into account, first the single-electron capture process from the ground state entry channel... 

The potential energy curves of the Z" and 11 states are presented in Fig. 4. They show four avoided crossings in the range 15.0-10.0 a.u.] between the entry channel, the state corresponding to N +(ls2p3s) -t- He )and the three states of single-electron capture N +(ls2s3/) L-i-He+). 

While the molecular identity of the capacitative Ca2+ entry channel is not known, a candidate is a homolog of the Drosophila mutant trp. This photoreceptor mutant is incapable of maintaining a sustained photoreceptor potential. This phenotype could be mimicked by the calcium entry blocker lanthanum, and it was suggested that the related defect is a failure of Ca2+ entry. 

Now described as a trp superfamily, more than 20 vertebrate trp homologs have been identified and these can be divided phylogenetically into three subfamilies trpC (termed canonical as they are most closely related to Drosophila trp), trpV and trpM. Some of these proteins are potential contenders for the capacitative calcium ion entry channel. In particular, expression of members of the TRPC and TRPV families in mammalian cells has been shown to augment capacitative calcium ion entry, but their role is still a matter for debate [17]. 

The evidence that empty intracellular Ca2+ stores can stimulate Ca2+ entry into cells via the CCE pathway is unassailable (Putney 1997). However, neither the signal responsible for communicating the state of the stores to the Ca2+ entry channel,... 

The potential energy profile and also the location of saddle point appears different for exo- and endothermic reactions. In general, the saddle point is located in the entry channel for an exothermic reaction as shown in Fig. 9.16. For example, reaction F + H2 —> FH + H. The shift in location of the saddle point from symmetric position has also been related to the magnitude of the exo (endo) thermicity. 

Fig. 9.26 Effect of reactant translation and vibration on collisions, when saddle point (x) is in the entry channel.

Pgl and the 584-A/N2 photoionization electron spectra.81 For the three different electronic final states, energetically accessible in the Pgl transition from the single entry channel potential curve, rather narrow unshifted individual distributions and vibrational populations very similar to those for 584-A photoionization are observed. In Fig. 31 the population factors —differently normalized bt v)—for Pgl and photoionization are compared for some systems with well-resolved vibrational lines.48,74... 

The three curves into which the entry channel V (R) branches as the distance decreases dissociate into He+ (/i = 2) + Hg+(5distance between the charge at Hg+ and He+ (n = 2), the nearly degenerate helium states are split by the linear Stark effect, which leads to... 

The mechanism of deposition and mobilization of iron in ferritin has been much discussed.1106,1112,1113 Ions and neutral molecules can all pass through the protein shell, due probably to the presence of entry channels of different character. The Cd2+ ions found in the hydrophilic channel in the crystal structure could be metal ions entering the shell. 

A complete treatment has thus been undertaken, taking into account all the possible radial and rotational transitions occuring from [B2 (ls22s)2s + H(ls)]1,3 + entry channels. The adiabatic states involved in such a process are shown below... 

The process is mainly driven by the 1, E adiabatic states Their potential energy curves are presented in Figure 1. The states show clearly two avoided crossings between the entry channel and the [B+(2s2p)1P + H+] level, a very dia-batic one at about R = 13 a.u. and a wider one at about R = 3.7 a.u. which interferes with the avoided crossing between the entry channel and the [B+(2s2) S +... 

H+]II+ state. The states exhtbit only an avoided erasing at about R = 5.4 a.u. between the entry channel and the [B+(2s2p)3P + H+] state. 

This work provides accurate potential energy curves as well as coupling matrix elements for the B2+/H and B4+/ He systems. From the molecular point of view, it appears important to involve all levels correlated to the entry channels in the collision dynamics and, in particular, to take into account rotational effects, which might be quite important. The results concerning the double electron capture process in the (B4+ + He) collision point out the limitations ofthe potential approach model, especially to account for open shell levels, for which more elaborate calculations are necessary. 

No obvious evidence concerning substrate entry to the diiron cluster have been revealed indicating that the entry channel may be opened due to the proteins spontaneous flexibility or may be created by binding MMOB or MMOR (Wallar and Lipscomb, 1996). Recent data on crystal structure of MMOH from M. capsulatus demonstrate the geometric variability of the enzyme active site (Whittington et al., 2001). It is shown, that ferrous atoms, adjacent a-helix, and the Asn214 group have a certain pliability, which enables small molecules to penetrate into the active site. 

The collision system appears relatively complex, with consideration of the S2+(3s23p3d)3F°+H+ level accessible from the1,3 n and1,3 S+ S3+ (3s23p)2P° + H(ls)2S] entry channel. A great number of states have been taken into account in both the singlet and triplet manifolds, including one-electron capture processes as well as re-actions of capture and excitation of the 3s core electron ... 

The levels are not correlated to the entry channel and have not been calculated. 

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