Properties of the Schiff base

The pKa of the protonated Schiff base in bacteriorhodopsin is shifted from around 7 in model compounds in solution to well above 10 [86,87]. In contrast, the halorhodopsin Schiff-base pK is not raised and a pH-dependent equilibrium at 7.4 between the 

In halorhodopsin the Schiff-base pKa is raised by as much as 1.5 units in the presence of chloride, and less with other anions, suggesting specific binding in the vicinity of the Schiff base [51,89], consistent with other information (cf. above). 

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Pyridoxal Derivatives. Various aldehydes of pyridoxal (Table 3) react with hemoglobin at sites that can be somewhat controlled by the state of oxygenation (36,59). It is thereby possible to achieve derivatives having a wide range of functional properties. The reaction, shown for PLP in Figure 3, involves first the formation of a Schiffs base between the amino groups of hemoglobin and the aldehyde(s) of the pyridoxal compound, followed by reduction of the Schiffs base with sodium borohydride, to yield a covalently-linked pyridoxyl derivative in the form of a secondary amine. 

Despite the considerable number of papers which have appeared on the structure of Schiff base metal complexes, a relatively small number of free ligands have been structurally characterized. In addition, only two SALAN derivatives are reported in the reviews collecting structure determinations published up to 1965.9 Therefore, no previous review on structural properties of free Schiff bases has been so far reported. Thus, these aspects will be fully described in the next subsections. However, it may be useful to report here the values quoted in ref. 9 of average C—N bond lengths for single, partial double and triple bonds (Table 2). 

The ability of monomeric Schiff base complexes or polymeric Schiff base ligands to absorb solvent molecules is well-known. Sawodny et al.27) found that the polymer Schiff base complexes exhibit this property much more markedly. The polymers were prepared by the polycondensation reaction of polymer ligands of the Schiff base (29) with metal salts (Eq. 8). The adsorption of solvent is reversible and the ability is dependent upon the nature of the central metal atom and of the ring size around the metal. This profile suggests that the above polymers can be used as a molecular sieve. 

Schiff Bases. The suggestion that a protonated Schiff base is the primary form of the retinal-opsin binding in natural pigments has stimulated considerable work on the spectroscopic properties of free Schiff bases of retinal (RSB) in solution, especially in their protonated forms (PRSB). Schiff base formation does not alter substantially the spectrum of retinal isomers, being associated with a 20-nm blue shift in the position of the main band, I. Except for the absence of the low-energy (n, tr ) transition, theory predicts only small changes in the location of all other states (75,121). This 1 s been confirmed by the extensive experimental study of Schaffer et al. (75). It appears from theoretical calculations (121) that Schiff bases lack Ag-lfiJ... 

Palladium, rhodium and ruthenium complexes of the Schiff base salen are synthesized in the supercages of zeolite Y. The existence of intracrystalline transition metal-salen complexes is verified by a detailed physicochemical characterization. The catalytic properties of the prepared host/guest inclusion compounds are explored in the hydrogenation of hexene-(l) or an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l). 

T. Inabe, I. Gautier-Luneau, N. Hoshino, K. Okaniwa, H. Okamoto, T. Mitani, U. Nagashima, and Y. Maruyama, Structure and optical properties of thermochromic Schiff bases. Charge transfer interaction and proton transfer in the V-tetrachlorosalicylideneaniline and JV-tetrachlorosalicyli-dene-l-pyrenylamine crystals, Bull. Chem. Soclpn. 64, 801-810 (1991). 

The solid state of the Schiff bases is of great interest because of their photochromic and thermochromic properties. A few studies of Schiff bases in the solid state exist. Salman and coworkers found for aniline Schiff bases of 2-hydroxy-1-naphthaldehyde that at... 

Irradiation into this band caused photoisomerization, appreciable quantities of the 11-cw-isomer being formed. Several papers189-191 report calculations related to the spectroscopic properties of retinylidene Schiff bases. The principal absorption axes of rhodopsin and prelumirhodopsin have been determined.192 Photoacoustic Spectroscopy. The photoacoustic spectra of visual pigments have been reported.198... 

Yost and Harrison (118) have reported that m-MDH is irreversibly inactivated by pyridoxal 5 -phosphate (PLP). In view of the usual reversibility of this reagent when reacted with the -amino group of lysine, and the unusual spectral properties of the derivative formed, it was concluded that the reaction of m-MDH with PLP proceeds via modification of lysine followed by the formation of a secondary stable complex. Such a complex might be a thiazolidine-like compound formed by the reaction of the Schiff base with a neighboring cysteinyl residue. The role of the lysine residue in the active site of m-MDH remains to be clarified. 

The IR spectra, chemical properties and visible spectra showed that the unidentate ligands are trans to each other, indicating square planar coordination of the Schiff base macrocycle. 

The presence of pyridoxal phosphate confers specific absorbance properties onto the holoenzyme. The absorption maximum is at 330 mp, a wavelength far removed from that of the Schiff base. Therefore, the combination of apoenzyme and coenzyme in the active phosphorylase molecule does not involve the formation of a Schiff base. Krebs and others studied the site of attachment of the phosphorus in the phosphorylase molecule by preparing P-labeled phosphorylase. After the enzyme was treated with trypsin, it became clear that the phosphorus was present in a basic hexa-peptide containing two basic amino acids. The hexa-peptide has the following amino acid sequence lysine, glutamine, isoleucine, serine phosphate, valine, and arginine. 

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