Dipolar ion structure

Problem 18.32 How does the dipolar ion structure of sulfanilic acid account for its (a) high melting point, (b) insolubility in H.O and organic solvents, (c) solubility in aqueous NaOH, (d) insolubility in aqueous HCI ... 

How would you expect the proton nmr spectrum of cyclopropanone in the cyclic ketone and dipolar ion structures (Section 17-11) to differ Show your reasoning. 

Review Sec. 17.7 if necessary, and use Table 17.1 for the structures of the amino acids. Write the structures in neutral form, recognizing that dipolar ion structures are possible and that the exact form and degree of ionization depend on the pH of the solution. 

All these properties are quite consistent with a dipolar ion structure for the amino acids (1). 

In further studies relevant to the mechanism of the Mitsunobu reaction, the reaction of diphenylphosphine (40) with di-isopropyl azodicarboxylate (41) was found to give a mixture of products one of which was assigned structure (44) formed via (42) and (43). The reaction of trico-ordinate phosphorus compounds with ortho-quinones,p-ketoalkenes and p-ketoimines is also a well-established source of pentaco-ordinate phosphorus compounds but in some cases the dipolar ion structure is more stable. Thus the reaction of (4Sab) with (46) produced the dipolar ion structures (47ab) but on the other hand, reaction of (45b) with the quinone imine (48) produced what appeared to be an equilibrium mixture of (49) and the pentaco-ordinate structure (50). The 1,3,2-X o -diazaphospholenes (Slab) have proved to be useful synthetic reagents in that on hydrolysis they yield the bis-hydroxylamine (52) which reacts with benzaldehyde to form the imidazoline (53). ... 

Amino acids with one amino group and one carboxyl group are better represented by a dipolar ion structure. ... 

No similar meta substituent effect exists for ions since resonance between a singlet and triplet (diradical) state is prohibited. The dipolar ion theory would suggest that suitably disubstituted radicals should be especially stable, particularly in polar solvents, because of structures like III. 

When proteins undergo hydrolysis, you wind up with 22 a amino acids, 20 of which are regulcir amino acids and 2 of which are derived amino acids. Amino acids are amphoteric (they possess the characteristics of both acids and bases and can react as either) because both acidic and basic groups are present. An internal acid-base reaction produces a dipolar ion known as a zwitterion (you can see the general structure of one in Figure 16-33). 

This chapter deals with the very important a-amino acids, the building blocks of the proteins that are necessary for the function and structure of living ceils. Enzymes, the highly specific biochemical catalysts are proteins. or-Amino acids are dipolar ions (zwitterions), RCH(N" H,)COO , as is indicated by their crystallinity, high melting point, and solubility in water rather than in nonpolar solvents. The standard (naturally occurring) amino acids are listed in Table 21-1 those marked with an asterisk are essential amino acids that cannot be synthesized in the body and so must be in the diet. They have 1° NHj s except for proline and hydroxyproline (2°). They have different R groups. 

Like amino acids, this tripeptide is a dipolar ion. The same structure can be abbreviated Ala-Val-Met or, using one-letter abbreviations, AVM. It is customary in describing amino acid sequences to place the amino-terminal (N-terminal) residue at the left end and the carboxyl-terminal (C-terminal) residue at the right end. Residues are numbered sequentially with the N-terminal residue as 1. An example is shown in Fig. 2-6. 

We see that the essence of the action of thiamin diphosphate as a coenzyme is to convert the substrate into a form in which electron flow can occur from the bond to be broken into the structure of the coenzyme. Because of this alteration in structure, a bond breaking reaction that would not otherwise have been possible occurs readily. To complete the catalytic cycle, the electron flow has to be reversed again. The thiamin-bound cleavage product (an enamine) from either of the adducts in Eq. 14-20 can be reconverted to the thiazolium dipolar ion and an aldehyde as shown in step b of Eq. 14-21 for decarboxylation of pyruvate to acetaldehyde. 

When pyridoxamine with a dipolar ionic ring structure (Fig. 14-9) and an absorption peak at 30,700 cm-1 (326 ran) is irradiated, fluorescence emission is observed at 25,000 cm 1 (400 ran). When basic pyridoxamine with an anionic ring structure and an absorption peak at 32,500 cm 1 (308 nm) is irradiated, fluorescence is observed at 27,000 cm-1 (370 nm), again shifted 5500 cm 1 from the absorption peak. However, when the same molecule is irradiated in acidic solution, where the absorption peak is at 34,000 cm 1 (294 nm), the luminescent emission at 25,000 cm 1 is the same as from the neutral dipolar ionic form and abnormally far shifted (9000 cm ) from the 34,000 cm-1 absorption peak.185186 The phenomenon, which is observed for most phenols, results from rapid dissociation of a proton from the phenolic group in the photoexcited state. A phenolic group is more acidic in the excited state than in the ground state, and the excited pyridoxamine cation in acid solution is rapidly converted to a dipolar ion. 

The first processing event (Eq. 28-6) for most of the pre-mRNA and snRNA transcripts made by RNA polymerase II is addition to the 5 end of a "cap," a terminal structure containing 7-methylguanosine from which a proton has dissociated to form a dipolar ion.563 565 The cap structure may be abbreviated 5 -m7G(5 )pppNm —. The 5 terminal ribose is often methylated on 02 , as shown below. More complex caps are methylated at additional sites, e.g., the guanine may be dimethylated on the 2-NH2 group.551 Most snRNAs, including the U1-U5 and U7-U13 snRNAs, have such 2,2,7-trimethylguanosine... 

All solid amino acids exist as dipolar ions known as zwitterions (Figure 1.2(a)). In aqueous solution the structure of amino acids are dependent on the pH of the solution (Figure 1.2(b)). The pH at which an aqueous solution of an amino acid is electrically neutral is known as the isoelectric point (pi) of the amino acid (Table 1.1). Isoelectric point values vary with temperature. They are used in the design of electrophoretic and chromatographic analytical methods for amino acids. 

The complete hydrolysis of proteins produces 20 a-amino acids that also occur as free metabolic intermediates. As free acids, they exist mostly as dipolar ions (zwitterions). Except for glycine, they contain chiral a-carbons and therefore exist in the d- and L-enantiomeric pair, of which the L-isomers are the monomeric units of proteins. They are differentiated structurally by their side-chain groups with varying chemical reactivities that determine many of the chemical and physical properties of proteins. These side-chain groups include ... 

Amino acids, from which proteins are formed, exist as dipolar ions. The structure of the dipolar ion of the amino acid alanine is... 

At a pH of 10.8, the amino acid arginine exists primarily as the following dipolar ion. Show the resonance structures for the cationic part of arginine and discuss their relative contributions to the resonance hybrid. 

Although we commonly write amino acids with an intact carboxyl (—COOH) group and amino (—NH2) group, their actual structure is ionic and depends on the pH. The carboxyl group loses a proton, giving a carboxylate ion, and the amino group is proto-nated to an ammonium ion. This structure is called a dipolar ion or a zwitterion (German for dipolar ion ). 

Structure 134 was proved by IR spectra. The existence of a dipolar ion CH2—C=C- -<-> CH2—C=C which is more readily attacked by nucleophilic reagents at the acetylenic carbon might account for the somewhat unusual course of the reaction. ... 

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