Side-chain participation

The riL value of racemic PBG-ytL is several hundred ms and decreases gradually as temperature increases. These results show that the side chains participating in the stacking structure are not rigid. Tx at the y position below room temperature is governed by the rapid and small-amplitude libration as shown in Section 4.3. The libration is expected to exist in the stacked state as well as the free state. riL is considered to be governed by libration in the stacked state. In order to estimate the amplitude of the libration in the stacked state, we extracted the Tu component of the spectrum in the same way as that used for racemic PBG- d2. The spectra of the T1L and T, s components are shown in Fig. 37. The vq value of the r1L component is... 

Degenerate Ring Transformations Involving Side-Chain Participation... 

The transformation shown below is a desymmetrization that provides product I in modest % ee.220 Propose a mechanism-based explanation for the reaction assuming that only one B-alkyl side chain participates in the first step. What is the purpose of the second step ... 

What do these results indicate As shown earlier, pepsinogen contains an appreciable number of basic amino acid residues all clustered within a relatively small part of the molecule, whereas the acidic groups are distributed over the pepsin moiety. At neutral pH a significant number of acidic side chains are neutralized by the proximity of the -amino groups. Thus these charged side chains participate in some mutual interaction,... 

Radical cyclizations. When heated with tin hydride in benzene, thioesters and selenoesters bearing an alkenyl side chain give cyclic products. The side chain participates... 

The two basic AAs (Figure 15.5) have R groups with a net positive charge and a nentral pH. ARG and lysine side chains participate in electrostatic interactions in proteins. 

Fig. 10. Schematic conformation of sperm whale myoglobin after Kendtew et al. (48). The sausage is the protein chain, which is mostly in the a-hdical conformation. The heme group is held in a cleft by apolar contacts, and by two histidine side chmns. Three or four other histidine side chains participate in the fcxmaticm the folded protein structure. Since the side chain of histidine contains an imidazole ring, a free side chain can be protonated at low pH (the pK is ca. 6.5). However, the five or six buried side chains cannot be protonated, and hence the overall equilibrium constant for unfolding is strongly pH dependent in first approximation (34)...

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