Head-to-tail orientation

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

Semiempirical calculations show that the approach of the dipolarophile in a head-to-head manner is 5.91 kcal/mol more favorable than the head-to-tail orientation with electron-rich alkenes (93). This difference is manifest in the ground state of resulting cycloadducts where the head-to-head adduct is 6.6 kcal/mol more stable. Similarly, the head-to-head transition state is more favorable by 3.4 kcal/mol for electron-poor alkenes, and 7.4 kcal/mol for electron-rich alkenes as calculated with density functional theory (91). This preference is also carried over into the ground-state energies of the resulting cycloadducts. 

Figure 32. Head-head and head-to-tail orientation of photodimerizable molecules in hexatic BS phase. Head-head dimer results from reactions between molecules present within the same layer whereas head-tail dimer results from molecules present in different layers. [Reproduced with permission from V. Ramesh and R. G. Weiss, J. Org. Chem. 51, 2535 (1986).]...

Fig. 14). With mononucleotides in most cases head-to-tail orientations have been found with only rare exceptions131,158 with oligonucleotides, however, a head-to-head orientation - as it occurs in DNA - is more likely.

Photocycloaddition ofN-P -N and N-M -N. Irradiation of alkyl 2-naph-thoates is known to yield a cubanelike photocyclomer as the unique product (Fig. 7), although six isomeric cyclomers are formally possible [65-67,80-82], This selectivity originates from two restrictions. First, the photocycloaddition occurs only between the substituted rings. Second, in the cyclomer the substituents are in a head-to-tail orientation. It has been previously demonstrated that the yield of this... 

Fig. 3.2. Schematic of recombinant AAV dual-vector strategies for gene therapy for hemophilia A. (A) depicts the individual expression of the 5 and 3 ends of the factor VIII protein followed by heterodimerization to generate a function protein. (B) shows concatamerized 5 and 3 vectors in the correct head-to-tail orientation. Splicing of vectors to remove the intron and inverted terminal repeats leads to expression of the entire factor VIII protein from a single mature mRNA transcript. ITR, AAV inverted terminal repeat SD, splice donor SA, splice acceptor An, poly A.

Fig. 2. The structure of the cis-[Pt(NH3)2(5 -IMP)2]2 anion[14], showing the common head-to-tail orientation of the purine rings...

Recently, the group of Arpalahti [39] studied the cd-[(NH3)2Pt(Ado-N(7))i]2+ (Ado = adenosine) system. There are two independent complex cations in the unit cell, both exhibiting a head-to-tail orientation. The Pt-N(7) distances are within the range 2.00-2.05 A, and the dihedral angles between the bases are 83.5° and 86.6°. 

Electrochemical data indicate that self-assembled monolayers of 5 and 6 catalyze the two-electron reduction of O2 to H2O2. The monolayer from 6 is a more effective electrocatalyst for the reduction of O2 than that from 5 [300]. The different reactivity results from different interfacial architecture this is confirmed by infrared, X-ray photoelectron, and visible spectroscopic measurements [300] which revealed coplanar, inclined t -7z stacking of the porphyrin ring in the monolayer of 5 and head-to-tail orientation of the porphyrin ring in the monolayer of 6. Treatment of the monolayer of 8 with Co(OAc)2 in methanol resulted in electrocatalytic activity in the reduction of O2 [300]. In contrast, a monolayer of 7 treated similarly failed to catalyze dioxygen reduction [300], although treatment of a mixed monolayer of 7 and CH3(CH2)3SH with Co(OAc)2 results in electrocatalytic activity similar to that of 6. 

According to Figure 6.17, the height of the barrier between the excimer minimum and the pericyclic funnel depends both on their depths and on the relative placement of the two excited-state surfaces S and D. The depth of the diagonally distorted pericyclic funnel is determined by the nature of the biradical its dependence on molecular structure, on the head-to-head and head-to-tail orientation of the components, and on reaction medium can be discussed using the principles outlined in Section 4.4.1. 

The chiral bilayer effect. Micellar fibres are long-lived, if crystallization is connected with a slow rearrangement from tail-to-tail to head-to-tail oriented sheets Such rearrangements occur in fibres of pure enantiomers, but not in... 

Figure 7.5 The crystal structure of methyl stearate shows a tail-to-tail orientation, whereas the corresponding ethyl ester is head-to-tail oriented. ...

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