Vacuum-ultraviolet circular dichroism study of saccharides by synchrotron radiation spectrophotometry

Vacuum-ultraviolet circular dichroism (VUVCD) spectra of five monosaccharides (D-glucose, D-mannose, D-galactose, D-xylose, and D-lyxose) and five disaccharides (maltose, isomaltose, cellobiose, gentiobiose, and lactose) were measured to 160 nm using a synchrotron-radiation VUVCD spectrophotometer in aqueous solution under high vacuum at 25 degrees C. Most of the saccharides show a positive peak with some shoulders at around 170 nm, except for D-galactose and lactose, which show two distinct negative peaks at around 165 and 177 nm. These spectra are influenced by such structural factors as alpha and beta anomers at C-1, axial and equatorial hydroxyl groups at C-2 and C-4, trans (T) and gauche (G) conformations of the hydroxymethyl group at C-5, and the type of glycosidic linkage. Deconvolution of the VUVCD spectra of D-glucose, D-mannose, and D-galactose into six independent Gaussian components for alpha-GG, alpha-GT, alpha-TG, beta-GG, beta-GT, and beta-TG conformations suggests that the alpha anomer has red-shifted spectra relative to the beta anomer, and that GG and GT conformations have positive and negative circular dichroism signs, respectively, while the sign for TG conformation is anomer dependent. These speculations from the deconvolution analyses are also supported by the VUVCD spectra of disaccharides. These results give new insight into the equilibrium conformations of saccharides, demonstrating the usefulness of synchrotron-radiation VUVCD spectroscopy.     

Improved estimation of the secondary structures of proteins by vacuum-ultraviolet circular dichroism spectroscopy

The vacuum-ultraviolet circular dichroism (VUVCD) spectra of 16 globular proteins (insulin, lactate dehydrogenase, glucose isomerase, lipase, conalbumin, transferrin, catalase, subtilisin A, alpha-amylase, staphylococcal nuclease, papain, thioredoxin, carbonic anhydrase, elastase, avidin, and xylanase) were successfully measured in aqueous solutions at 25 degrees C from 260 to 160 nm under a high vacuum using a synchrotron-radiation VUVCD spectrophotometer. These proteins exhibited characteristic CD spectra below 190 nm that were related to their different secondary structures, which could not be detected with a conventional CD spectrophotometer. The component spectra of alpha-helices, beta-strands, turns, and unordered structures were obtained by deconvolution analysis of the VUVCD spectra of 31 reference proteins including the 15 proteins reported in our previous paper [Matsuo, K. et al. (2004) J. Biochem. 135, 405-411]. Prediction of the secondary-structure contents using the SELCON3 program was greatly improved, especially for alpha-helices, by extending the short-wavelength limit of CD spectra to 160 nm and by increasing the number of reference proteins. The numbers of alpha-helix and beta-strand segments, which were calculated from the distorted alpha-helix and beta-strand contents, were close to those obtained on X-ray crystallography. These results demonstrate the usefulness of synchrotron-radiation VUVCD spectroscopy for the secondary structure analysis of proteins.     

Secondary-structure analysis of proteins by vacuum-ultraviolet circular dichroism spectroscopy

The vacuum ultraviolet circular dichroism (VUVCD) spectra of 15 globular proteins (myoglobin, hemoglobin, human serum albumin, cytochrome c, peroxidase, alpha-lactalbumin, lysozyme, ovalbumin, ribonuclease A, beta-lactoglobulin, pepsin, trypsinogen, alpha-chymotrypsinogen, soybean trypsin inhibitor, and concanavalin A) were measured in aqueous solutions at 25 degrees C in the wavelength region from 260 to 160 nm under a high vacuum, using a synchrotron-radiation VUVCD spectrophotometer. The VUVCD spectra below 190 nm revealed some characteristic bands corresponding to different secondary structures. The contents of alpha-helices, beta-strands, turns, and unordered structures were estimated using the SELCON3 program with VUVCD spectra data on the 15 proteins. Prediction of the secondary-structure contents was greatly improved by extending the circular dichroism spectra to 165 nm. The numbers of alpha-helix and beta-strand segments calculated from the distorted alpha-helix and beta-strand contents did not differ greatly from those obtained from X-ray crystal structures. These results demonstrate that synchrotron-radiation VUVCD spectroscopy is a powerful tool for analyzing the secondary structures of proteins.     

Secondary-structure analysis of denatured proteins by vacuum-ultraviolet circular dichroism spectroscopy

To elucidate the structure of denatured proteins, we measured the vacuum-ultraviolet circular dichroism (VUVCD) spectra from 260 to 172 nm of three proteins (metmyoglobin, staphylococcal nuclease, and thioredoxin) in the native and the acid-, cold-, and heat-denatured states, using a synchrotron-radiation VUVCD spectrophotometer. The circular dichroism spectra of proteins fully unfolded by guanidine hydrochloride (GdnHCl) were also measured down to 197 nm for comparison. These denatured proteins exhibited characteristic VUVCD spectra that reflected a considerable amount of residual secondary structures. The contents of alpha-helices, beta-strands, turns, poly-L-proline type II (PPII), and unordered structures were estimated for each denatured state of the three proteins using the SELCON3 program with Protein Data Bank data and the VUVCD spectra of 31 reference proteins reported in our previous study. Based on these contents, the characteristics of the four types of denaturation were discussed for each protein. In all types of denaturation, a decrease in alpha-helices was accompanied by increases in beta-strands, PPII, and unordered structures. About 20% beta-strands were present even in the proteins fully unfolded by GdnHCl in which beta-sheets should be broken. From these results, we propose that denatured proteins constitute an ensemble of residual alpha-helices and beta-sheets, partly unfolded (or distorted) alpha-helices and beta-strands, PPII, and unordered structures.     

Secondary-structure analysis of alcohol-denatured proteins by vacuum-ultraviolet circular dichroism spectroscopy

To elucidate the structural characteristics of alcohol-denatured proteins, we measured the vacuum-ultraviolet circular dichroism (VUVCD) spectra of six proteins-myoglobin, human serum albumin, α-lactalbumin, thioredoxin, β-lactoglobulin, and α-chymotrypsinogen A-down to 170 nm in trifluoroethanol solutions (TFE: 0-50%) and down to 175 nm in methanol solutions (MeOH: 0-70%) at pH 2.0 and 25°C, using a synchrotron-radiation VUVCD spectrophotometer. The contents of α-helices, β-strands, turns, poly-L-proline type II helices (PPIIs), and unordered structures of these proteins were estimated using the SELCON3 program, including the numbers of α-helix and β-strand segments. Furthermore, the positions of α-helices and β-strands on amino acid sequences were predicted by combining these secondary-structure data with a neural-network method. All alcohol-denatured proteins showed higher α-helix contents (up to ~ 90%) compared with the native states, and they consisted of several long helical segments. The helix-forming ability was higher in TFE than in MeOH, whereas small amounts of β-strands without sheets were formed in the MeOH solution. The produced α-helices were transformed dominantly from the β-strands and unordered structures, and slightly from the turns. The content and mean length of α-helix segments decreased as the number of disulfide bonds in the proteins increased, suggesting that disulfide bonds suppress helix formation by alcohols. These results demonstrate that alcohol-denatured proteins constitute an ensemble of many long α-helices, a few β-strands and PPIIs, turns, and unordered structures, depending on the types of proteins and alcohols involved.     

Experimental and theoretical studies of vacuum-ultraviolet electronic circular dichroism of hydroxy acids in aqueous solution

The electronic circular dichroism (ECD) spectra of three L-hydroxy acids (L-lactic acid, (+)-(S)-2-hydroxy-3-methylbutyric acid, and (-)-(S)-2-hydroxyisocaproic acid) were measured down to 160 nm in aqueous solution using a vacuum-ultraviolet ECD spectrophotometer. To assign the two positive peaks around 210 and 175 nm and the one negative peak around 190 nm in the observed spectra, the ECD spectrum of L-lactic acid was calculated using time-dependent density functional theory (DFT) for the optimized structures by DFT and a continuum model. The observed ECD spectrum was successfully reproduced as the average spectrum for four optimized structures with seven water molecules that localized around the COO(-) and OH groups of L-lactic acid. The positive peak around 210 nm and the negative peak around 185 nm in the calculated spectrum were attributable to the nπ* transition of the carboxyl group, with the latter peak also being influenced by the ππ* transition of the carboxyl group; however, the positive peak around 165 nm involved unassignable higher energy transitions. The comparison of the calculated ECD spectra for L-lactic acid and L-alanine revealed that the network with loose hydrogen bonding around the COO(-) and OH groups is responsible for the flexible conformation of hydroxy acids and complicated side-chain dependence of ECD spectra relative to amino acids.     

Comprehensive secondary‐structure analysis of disulfide variants of lysozyme by synchrotron‐radiation vacuum‐ultraviolet circular dichroism

To elucidate the effects of specific disulfide bridges (Cys6‐Cys127, Cys30‐Cys115, Cys64‐Cys80, and Cys76‐Cys94) on the secondary structure of hen lysozyme, the vacuum‐ultraviolet circular dichroism (VUVCD) spectra of 13 species of disulfide‐deficient variants in which Cys residues were replaced with Ala or Ser residues were measured down to 170 nm at pH 2.9 and 25°C using a synchrotron‐radiation VUVCD spectrophotometer. Each variant exhibited a VUVCD spectrum characteristic of a considerable amount of residual secondary structures depending on the positions and numbers of deleted disulfide bridges. The contents of α‐helices, β‐strands, turns, and unordered structures were estimated with the SELCON3 program using the VUVCD spectra and PDB data of 31 reference proteins. The numbers of α‐helix and β‐strand segments were also estimated from the VUVCD data. In general, the secondary structures were more effectively stabilized through entropic forces as the number of disulfide bridges increased and as they were formed over larger distances in the primary structure. The structures of three‐disulfide variants were similar to that of the wild type, but other variants exhibited diminished α‐helices with a border between the ordered and disordered structures around the two‐disulfide variants. The sequences of the secondary structures were predicted for all the variants by combining VUVCD data with a neural‐network method. These results revealed the characteristic role of each disulfide bridge in the formation of secondary structures. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Improved sequence-based prediction of protein secondary structures by combining vacuum-ultraviolet circular dichroism spectroscopy with neural network

Synchrotron-radiation vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy can significantly improve the predictive accuracy of the contents and segment numbers of protein secondary structures by extending the short-wavelength limit of the spectra. In the present study, we combined VUVCD spectra down to 160 nm with neural-network (NN) method to improve the sequence-based prediction of protein secondary structures. The secondary structures of 30 target proteins (test set) were assigned into alpha-helices, beta-strands, and others by the DSSP program based on their X-ray crystal structures. Combining the alpha-helix and beta-strand contents estimated from the VUVCD spectra of the target proteins improved the overall sequence-based predictive accuracy Q(3) for three secondary-structure components from 59.5 to 60.7%. Incorporating the position-specific scoring matrix in the NN method improved the predictive accuracy from 70.9 to 72.1% when combining the secondary-structure contents, to 72.5% when combining the numbers of segments, and finally to 74.9% when filtering the VUVCD data. Improvement in the sequence-based prediction of secondary structures was also apparent in two other indices of the overall performance: the correlation coefficient (C) and the segment overlap value (SOV). These results suggest that VUVCD data could enhance the predictive accuracy to over 80% when combined with the currently best sequence-prediction algorithms, greatly expanding the applicability of VUVCD spectroscopy to protein structural biology.     

Heme-linked spectral changes of the protein moiety of hemoproteins in the near ultraviolet region

Spectral changes of hemoproteins in the near ultraviolet region on binding to a ligand and on oxidation-reduction of the heme-iron were studied by computer-controlled spectrophotometry. Near ultraviolet difference spectra between the low spin and high spin forms of ferric hemoproteins were classified into three groups: Those showing two absorption peaks having maxima at around 285 and 295 nm, those showing a peak at around 275 nm, and those showing a peak at around 300 nm. No corresponding absorption peak was observed with model heme complexes of low molecular weight. The intensity of the peak in cyanide difference spectra of catalase and horseradish peroxidase in the near ultraviolet region was dependent on the concentration of added cyanide and paralleled the intensity of the spectral changes in the Soret region. The spectral changes in both the near ultraviolet and Soret regions developed within 6 ms after the addition of cyanide. Difference spectra between the reduced and oxidized forms of cytochrome c, cytochrome oxidase-cyanide complex, hemoglobin, and lactoperoxidase-cyanide complex showed a characteristic peak at around 285-290 nm. Various difference spectra of hemoglobin in the near ultraviolet region were also measured. The observed positions, shapes, combinations, and relative intensities of the peaks were compared with those of solvent perturbation difference spectra and pH difference spectra of proteins and aromatic amino acids and also with the diacetylchitobiose-induced difference spectrum of lysozyme. The kinds of aromatic amino acid residues possibly responsible for the observed difference peaks were discussed on the basis of the results of the comparison. Based on the results obtained, the common occurrence of a heme-linked functional response of the hemoprotein conformation was suggested.     

Circular dichroism studies on coat proteins of some strains and mutants of tobacco mosaic virus

1. Tobacco mosaic virus (TMV) protein has in near ultraviolet a complex but well resolved circular dichroism (CD) spectrum at room temperature. There are seven positive bands at 248, 252, 257, 265, 274, 281 and 291 nm, and a negative one at 296 nm. The CD spectrum is pH-dependent. The shape of the pH-dependence curves and the comparison with CD spectra of model compounds suggest that the bands at 248, 252 and 257 nm are mainly caused by phenylalanyl, those at 265, 274 and 281 nm by tyrosyl, and those at 291 and 296 nm by tryptophanyl side chains. 2. Only insignificant changes of the tertiary structure seem to occur between pH 6.5 and 8.5. Changes in ellipticity of TMV protein during the pH-induced polymerization reaction suggest that: (1) tyrosyl residues are involved in the binding of subunits, (2) phenylalanyl residues seem to be transferred to a less rigid environment, and (3) tryptophanyl residues are not essential for the reaction. 3. The proteins of several TMV strains and mutants studied have similar far ultraviolet CD spectra and apparently do not differ significantly in their structure. Their near ultraviolet CD spectra are, however, different. Replacements involving aliphatic amino acids do not change considerably the near ultraviolet CD spectra. On the other hand, replacements involving aromatic amino acids have a great effect on the spectra rendering possible identification of CD bands and recognition of the aromatic amino acid residues responsible for optical activity.     

Interpretation of the absorption and circular dichroic spectra of oriented purple membrane films.

The absorption and circular dichroic (CD) spectra of purple membrane films in which the plane of the membranes is oriented perpendicular to the incident beam are compared with the solution spectra. This enables one to relate structural features of the purple membrane to a coordinate system as defined by a normal to the membrane plane and two mutually perpendicular in-plane axes. The film and solution absorption spectra were similar except for a relative depression in the 200 - 225-nm region of the film spectrum. However, the CD spectra showed significant differences in the visible region, where the biphasic band in the solution spectrum was replaced by a single positive band at 555 nm in the film spectrum and in the far ultraviolet region, where the 208-nm band was deleted from the film spectra of the native and regenerated membranes. Moreover, a small shoulder occurred at 208 nm in the film spectrum of the bleached membrane. The near ultraviolet spectra also showed differences, whereas the 317-nm band remained essentially the same for both spectra. Based on excitonic interpretations of the visible and far ultraviolet spectra the following conclusions were reached: (a) a relatively strong in-plane monomeric interaction occurs between te retinyl chromophore and apoprotein; (b) the helical axes of the native and regenerated membrane proteins are oriented primarily normal to the membrane plane; and (c) the helical axes of the bleached membrane proteins are tilted more in-plane than the axes of the native or regenerated membrane. Additional conclusions were that an interaction occurs between an in-plane magnetic dipole moment of the retinyl chromophore and probably an in-plane electric dipole moment of a nearby aromatic amino acid(s), and that although the membrane is anisotropic with respect to coupling between electric and magnetic moments of the aromatic amino acids, the transition dipole moments of the aromatic amino acids are not preferentially oriented in either direction.     

The Relationship Between the Metal Clusters and the Conformation of Molybdenum-Iron Protein from Azotobacter vinelandii

The ultraviolet CD spectrum of nitrogenase MoFe protein from Azotobacter vinelandii had a negative trough with double peaks at 208 nm and 222 nm, respectively, and the shape of the trough was similar to those of other proteins with a-helix structure. After treatment with o-phenanthroline under an aerobic or anaerobic condition, the height of the peak at 222 nm (h222 nm) decreased with the decrease of the C2H2-reduction activity, Fe content and CD spectra at both 450 nm and 660 nm, or at 450 nm of the treated proteins. However, after reconstituting with a reconstituent solution containing Na2MoO4, Na2S, dithiothreitol and either ferric homocitrate or ferric citrate, the h222 nm Of the reconstituted proteins could be restored as well as the activity, Fe content and CD spectra at both of 450 nm and 660 nm. The results show that there is a significant relationship between the metal clusters (FeMoco and P-cluster) and the conformation of MoFe protein.     

Conformation and aggregation of bovine myelin proteins

CD and PMR spectra were obtained on three major protein fractions of bovine CNS myelin: the basic A-1 protein, the Folch-Lees proteolipid apoprotein (APL), and the Wolfgram proteolipid protein (WPP). Most PMR peaks of the A-1 broadened on going from D₂O to salt solutions or to 100% 2-Chloroethanol (2-CE). CD spectra showed no α-helix in water or salt solutions, but showed 42% in 2-CE. The APL showed no PMR in D₂O, but did show aromatic amino acid peaks in 1.5% SDS. CD spectra showed 37% α-helix in both cases. The PMR of the WPP in 1.5% SDS showed aromatic amino acids, and the CD showed <20% α-helix. All three proteins showed sharp PMR spectra in trifluoroacetic acid with α-CH chemical shifts characteristic of random coils. It was concluded that the A-1 and the APL aggregate.     

Characterization of the mechanism of interaction between α1-acid glycoprotein and lipid membranes by vacuum-ultraviolet circular-dichroism spectroscopy

α₁-Acid glycoprotein (AGP) interacts with lipid membranes as a peripheral membrane protein so as to decrease the drug-binding capacity accompanying the β→α conformational change that is considered a protein-mediated uptake mechanism for releasing drugs into membranes or cells. This study characterized the mechanism of interaction between AGP and lipid membranes by measuring the vacuum-ultraviolet circular-dichroism (VUVCD) spectra of AGP down to 170 nm using synchrotron radiation in the presence of five types of liposomes whose constituent phospholipid molecules have different molecular characteristics in the head groups (e.g., different net charges). The VUVCD analysis showed that the α-helix and β-strand contents and the numbers of segments of AGP varied with the constituent phospholipid molecules of liposomes, while combining VUVCD data with a neural-network method predicted that these membrane-bound conformations comprised several common long helix and small strand segments. The amino-acid composition of each helical segment of the conformations indicated that amphiphilic and positively charged helices formed at the N- and C-terminal regions of AGP, respectively, were candidate sites for the membrane interaction. The addition of 1 M sodium chloride shortened the C-terminal helix while having no effect on the length of the N-terminal one. These results suggest that the N- and C-terminal helices can interact with the membrane via hydrophobic and electrostatic interactions, respectively, demonstrating that the liposome-dependent conformations of AGP analyzed using VUVCD spectroscopy provide useful information for characterizing the mechanism of interaction between AGP and lipid membranes.     

Expression, purification, and physicochemical characterization of a recombinant Yersinia protein tyrosine phosphatase.

The Yersinia protein tyrosine phosphatase (PTPase) Yop51, a C235R point mutation (Yop51*), and a protein lacking the first 162 amino acids at the NH2 terminus (Yop51*delta 162) have been overexpressed in Escherichia coli and purified to homogeneity through the use of CM Sephadex C25 cation exchange chromatography followed by Sephadex G-100 gel filtration. Greater than 50 mg of homogeneous Yop51* and Yop51*delta 162 can be obtained from a single liter of bacterial culture, whereas the same procedure yields only 5 mg of pure Yop51. Large, diffraction-quality crystals have been obtained for Yop51*delta 162. Size exclusion chromatography, sedimentation equilibrium, and enzyme concentration dependence experiments have established that the Yersinia PTPases exist and function as monomers in solution. Yop51 and Yop51* display identical UV, CD, and fluorescence spectra and have identical kinetic and structural stability properties. These full-length Yersinia PTPases have 31% alpha-helix, an emission maximum of 342 nm, a turn-over number of 1200 s-1 at pH 5.0, 30 degrees C, and an unfolding delta G value of 6 kcal/mol at 25 degrees C. Yop51*delta 162 has very similar kinetic and fluorescence characteristics to the full-length molecules, whereas its CD and UV spectra show noticeable differences due to the elimination of 162 NH2-terminal residues. The Yersinia PTPases are by far the most active PTPases known, and their kinetic parameters are extremely sensitive to the ionic strength of reaction medium.     

Comparison of CD spectra in the aromatic region on a series of variant proteins substituted at a unique position of tryptophan synthase alpha-subunit

CD spectra in the aromatic region of a series of the mutant alpha-subunits of tryptophan synthase from Escherichia coli, substituted at position 49 buried in the interior of the molecule, were measured at pH 7.0 and 25 degrees C. These measurements were taken to gain information on conformational change produced by single amino acid substitutions. The CD spectra of the mutant proteins, substituted by Tyr or Trp residue in place of Glu residue at position 49, showed more intense positive bands due to one additional Tyr or Trp residue at position 49. The CD spectra of other mutant proteins also differed from that of the wild-type protein, despite the fact that the substituted residues at position 49 were not aromatic. Using the spectrum of the wild-type protein (Glu49) as a standard, the spectra of the other mutants were classified into three major groups. For 10 mutant proteins substituted by Ile, Ala, Leu, Met, Val, Cys, Pro, Ser, His, or Gly, their CD values of bands (due to Tyr residues) decreased in comparison with those of the wild-type protein. The mutant protein substituted by Phe also belonged to this group. These substituted amino acid residues are more hydrophobic than the original residue, Glu. In the second group, three mutant proteins were substituted by Lys, Gln, or Asn, and the CD values of tyrosyl bands increased compared to those of the wild-type proteins. These residues are polar. In the third group, the CD values of tyrosyl bands of two mutant proteins substituted by Asp or Thr were similar to those of the wild-type protein, except for one band at 276.5 nm. These results suggested that the changes in the CD spectra for the mutant proteins were affected by the hydrophobicity of the residues at position 49.     

Identification of peptides containing tryptophan, tyrosine, and phenylalanine using photodiode-array spectrophotometry

The characteristic absorption spectra of aromatic amino acids between 240 and 310 nm were used to identify tryptophan, tyrosine, and phenylalanine-containing peptides. In acidic solution, the absorption spectra of these amino acids exhibit minima or maxima at 255, 270, and 286 nm. Based on these characteristics, the content of the aromatic amino acid in peptide can be estimated. For this study, 2 nmol of tryptic peptides from human apolipoprotein A-1 was separated by high-performance liquid chromatography using a reverse-phase column. The peptide fragments were monitored by a photodiode-array spectrophotometer. This new approach offers a rapid, simple, sensitive, and direct identification of peptides containing aromatic amino acids. Those containing Trp, which may be of interest for DNA sequencing and important in sequence analysis of proteins, can be selectively purified using this technique.     

Protochlorophyll forms with different molecular arrangements

Spectral properties of protochlorophyll (PChl) forms were investigated in solid-film model systems by absorption. fluorescence and circular dichroism (CD) spectroscopy. The solid films were prepared from diethyl ether solution of PChl on a cover glass surface by evaporation of the solvent. After preparation the films usually showed an absorption maximum at 635 nm or in some cases at 640 nm. The PChl form with 635 nm absorption maximum had no CD signal, whilst the films with absorption maximum at 640 nm gave an intense negative CD band at about 640 nm and a positive one at 668 nm. The treatment of the films with ammonia or acetone vapour resulted in a red shift of the absorption maximum from 635 nm or 640 nm to 650 nm. The study of the CD spectra of the films with different PChl forms showed that, depending on the treatment, forms of PChl with similar absorption and fluorescence spectra, but with opposite CD signals, can exist. It is suggested that the differences of the CD spectra are mainly due to different arrangements of the aggregates.     

A study of the interaction between D-amino acid oxidase and quasi-substrates.

To study the interaction between D-amino acid oxidase [EC 1.4.3.3] and quasi-substrates such as benzoate and o-, m-, and p-aminobenzoate, visible circular dichroism spectra (CD spectra) were measured and the binding rate and affinity of o-aminobenzoate to the enzyme were observed by following the absorption changes at various wavelengths. We found a new CD band around 560 nm, corresponding to the charge-transfer complexes which result from the formation of aminobenzoate complexes with the enzyme. The ellipticity of this band was positive for the p-aminobenzoate complex, but negative for the o- and m-aminobenzoate complexes. Crossover points in CD spectra were observed at 470 nm for the m-aminobenzoate complex and at 475 nm for the o-aminobenzoate complex. They probably resulted from overlapping of the positive CD band of FAD bound with the enzyme and the negative CD band of the charge-transfer complex. We propose that the amino group in aminobenzoate, not the pi-electrons of the benzene ring, is the electron donor in the charge-transfer complex and that the position of the amino group is very important for the charge-transfer interaction. The binding rate and affinity of o-aminobenzoate to the enzyme were determined using the absorption changes at 370 nm (380 nm), caused by the modification of electronic states of FAD bound with the enzyme, and at 550 nm (565 nm), caused by the formation of the charge-transfer complex of o-aminobenzoate with the enzyme. No differences between these parameters with wavelength were observed. This independence of wavelength simplifies discussion of the experimental data obtained from absorption changes.     

Emission spectra from copper proteins containing type-3 centres

Aqueous solutions of copper-proteins containing type-3 centres (ceruloplasmin, tyrosinase, haemocyanin), excited within their absorption bands at 325-345 nm, show typical luminescence spectra. The emission bands peak at 415-445 nm and their decay time is no longer than 10 ns. A strong analogous fluorescence is obtained also by excitation of concentrated solutions of carboxylic acids and amino acids, which show again absorption bands around 330 nm. Such a fluorescence, although less intense, is also observed in copper(II) carboxylate solutions. In contrast, no fluorescence has been recorded in solutions of acetic anhydride and of polypeptides (valinomycin, gramicidin D), which do not have free carboxyl groups. We tentatively attribute this novel fluorescence in the investigated copper proteins to interactions between carboxyl groups of amino acids at, or near, the active site.