Hydrogen proton deficient

Due to proton deficiency, crystal structures of amino acids display a much higher proportion of three-center hydrogen bonds. Their geometries, given in Th-bles 8.6 and 8.7, are based on neutron diffraction data, of which a relatively large number is available for this class of biological molecules. 

The three-center bonds represent — 70% of the total number of hydrogen bonds in the crystal structures surveyed (Thble 2.3). This is a significantly higher proportion than in the other biological molecules, and was attributed to proton deficiency, which occurs because the amino acids form zwitterionic crystal structures where the predominant hydrogen bonding is between the -NH3 and the... 

Proteins contain, on average, more acceptor than donor sites [596]. Similar proton deficiency in the amino acid zwitterion crystal structures results in the formation of three-center hydrogen bonds rather than in unsatisfied acceptor sites [74] (Part I A, Chap. 2.6). There is less flexibility in the orientation of hydrogen-bond donor and acceptor groups in proteins which would lead to a relatively larger number of unsatisfied acceptor sites. Some of the more unsymmetrical three-center bonds which might have been missed in the survey [596] because of the 3.5 A X - A cut-off limit will also contribute to reduce the number of unsatisfied acceptors in side-chains. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

The metal halide thus functions in similar manner to the proton and may be considered to be an acidic catalyst (cf. Luder and Zuffanti, 19). The catalyst-olefin complex differs in one significant respect from the product formed by the addition of the proton (or the corresponding acid) to the olefin the halide catalyst is a neutral but electronically deficient molecule and combines with the pi electrons of the double bond to form a coordinate bond between the carbon atom and the aluminum or boron. On the other hand, the addition of the positive proton to the double bond results in the formation of a true (covalent) link between carbon and hydrogen. In other words, the complex, while it contains an electron-deficient (hence, positive) carbon atom, is in itself electronically neutral the product of the addition of a proton to the alkene contains a similar carbon atom but is itself electrically positive. It has been suggested (Whitmore and Meunier, 20) that this difference is related to the fact that metal halide catalysts tend to yield much higher polymers than do the acid (proton) catalysts. 

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