pH-induced structural changes in bacteriorhodopsin studied by Fourier transform infrared spectroscopy.

Previous C13-NMR studies showed that two of the four internal aspartic acid residues (Asp-96 and Asp-115) of bacteriorhodopsin (bR) are protonated up to pH = 10, but no accurate pKa of these residues has been determined. In this work, infrared spectroscopy with the attenuated total reflection technique was used to characterize pH-dependent structural changes of ground-state, dark-adapted wild-type bacteriorhodopsin and its mutant (D96N) with aspartic acid-96 replaced by asparagine. Data indicated deprotonation of Asp-96 at high pH (pKa = 11.4 +/- 0.1), but no Asp-115 titration was observed. The analysis of the whole spectral region characteristic to complex conformational changes in the protein showed a more complicated titration with an additional pKa value (pKa1 = 9.3 +/- 0.3 and pKa2 = 11.5 +/- 0.2). Comparison of results obtained for bR and the D96N mutant of bR shows that the pKa approximately 11.5 characterizes not a direct titration of Asp-96 but a protein conformational change that makes Asp-96 accessible to the external medium.     

Iron coordination in photosystem II: interaction between bicarbonate and the QB pocket studied by Fourier transform infrared spectroscopy

The non heme iron environment of photosystem II is studied by light-induced infrared spectroscopy. A conclusion of previous work [Hienerwadel, R., and Berthomieu, C. (1995) Biochemistry 34, 16288-16297] is that bicarbonate is a bidendate ligand of the reduced iron and a monodentate ligand in the Fe(3+) state. In this work, the effects of bicarbonate replacement with lactate, glycolate, and glyoxylate, and of o-phenanthroline binding are investigated to determine the specific interactions of bicarbonate with the protein. Fe(2+)/Fe(3+) FTIR spectra recorded with (12)C- and (13)C(1)-labeled lactate indicate that lactate displaces bicarbonate by direct binding to the iron through one carboxylate oxygen and the hydroxyl group in both the Fe(2+) and Fe(3+) states. This different binding mode with respect to bicarbonate could explain the lower midpoint of the iron couple observed in the presence of this anion [Deligiannakis, Y., Petrouleas, V., and Diner, B. A. (1994) Biochim. Biophys. Acta 1188, 260-270]. In agreement with the -60 mV/pH unit dependence of the iron midpoint potential in the presence of bicarbonate, the proton release upon iron oxidation by photosystem II is directly measured to 0.95 +/- 0.05 by the comparison of infrared signals of phosphate buffer and ferrocyanide modes. This accurate method may be applied to the study of other redox reactions in proteins. The pH dependence of the iron couple is proposed to reflect the deprotonation of D1His215, a putative iron ligand located at the Q(B) pocket, since the signal at 1094 cm(-1) assigned to the nu(C-N) mode of a histidinate ligand in the Fe(3+) state is not observed in the presence of o-phenanthroline. Specific regulation of the pK(a) of D1His215 by bicarbonate is inferred from the absence of the band at 1094 cm(-1) in Fe(2+)/Fe(3+) spectra recorded with glycolate, glyoxylate, or lactate. A broad positive continuum, maximum at approximately 2550 cm(-1), observed in the presence of bicarbonate, but absent with o-phenanthroline or lactate, glycolate, and glyoxylate, indicates a hydrogen bond network from the non heme iron toward the Q(B) pocket involving bicarbonate and His D1-215. Proton release of about 1, measured upon iron oxidation at pH 6 with the latter anions, points to a proton release mechanism different from that involved in the presence of bicarbonate.     

Protein secondary structure from Fourier transform infrared spectroscopy: a data base analysis

An infrared (ir) method to determine the secondary structure of proteins in solution using the amide I region of the spectrum has been devised. The method is based on the circular dichroism (CD) matrix method for secondary structure analysis given by Compton and Johnson (L. A. Compton and W. C. Johnson, 1986, Anal. Biochem. 155, 155-167). The infrared data matrix was constructed from the normalized Fourier transform infrared spectra from 1700 to 1600 cm-1 of 17 commercially available proteins. The secondary structure matrix was constructed from the X-ray data of the seventeen proteins with secondary structure elements of helix, beta-sheet, beta-turn, and other (random). The CD and ir methods were compared by analyzing the proteins of the CD and ir databases as unknowns. Both methods produce similar results compared to structures obtained by X-ray crystallographic means with the CD slightly better for helix conformation, and the ir slightly better for beta-sheet. The relatively good ir analysis for concanavalin A and alpha-chymotrypsin indicate that the ir method is less affected by the presence of aromatic groups. The concentration of the protein and the cell path length need not be known for the ir analysis since the spectra can be normalized to the total ir intensity in the amide I region. The ir spectra for helix, beta-sheet, beta-turn, and other, as extracted from the data-base, agree with the literature band assignments. The ir data matrix and the inverse matrix necessary to analyze unknown proteins are presented.     

Probing conformational changes in the nicotinic acetylcholine receptor by Fourier transform infrared difference spectroscopy.

We have developed a Fourier transform infrared (FTIR) difference method for probing conformational changes that occur upon the binding of ligands to the nicotinic acetylcholine receptor (nAChR). Our approach is to deposit reconstituted nAChR membranes in a thin film on the surface of a germanium internal reflection element, acquire FTIR spectra in the presence of bulk aqueous solution using attenuated total reflection, and then trigger conformational changes by sequentially flowing a buffer either with or without an agonist past the film surface. Using the fluorescent probe, ethidium bromide, it is demonstrated that the method of nAChR film deposition does not affect the ability of the receptor to undergo the resting-to-desensitized state transition. The difference of FTIR spectra of nAChR films recorded in the presence and absence of agonists reveal highly reproducible infrared bands that are not observed in the difference of spectra recorded with only buffer flowing past the film surface. Some of the bands are assigned to changes in protein secondary structure and to changes in the structure of individual amino acid residues. Bands arising from the vibrations of the agonist bound to the receptor are also observed. The results demonstrate that FTIR difference spectroscopy can detect structural changes in the nAChR that occur upon the binding of ligands. The technique will be an effective method for investigating nAChR structure and function as well as receptor-drug interactions.     

Protein structure by Fourier transform infrared spectroscopy: second derivative spectra

Second derivative Fourier transform infrared spectra of the proteins ribonuclease A, hemoglobin, and beta-lactoglobulin A (native and denatured) have been obtained in deuterium oxide solution from 1350 to 1800 cm-1. The relationship of the original spectra to their second derivatives is briefly discussed. In the second derivative spectra, clearly resolved peaks are observed which can be associated with the alpha-helix, beta-strands, and turns. No protein spectra with such resolution have heretofore been reported. Tentative assignments are proposed, and the observed peaks are related to the secondary structure of the proteins studied. The data appear to present the first direct spectroscopic evidence of turns in a native protein.     

Triplet formation on a monomeric chlorophyll in the photosystem II reaction center as studied by time-resolved infrared spectroscopy

The process of formation of the triplet state of chlorophyll in the photosystem II (PS II) reaction center complex was studied by means of time-resolved infrared (IR) spectroscopy. Using a dispersive-type IR spectrometer with a time resolution of approximately 55 ns, transient spectra in the C=O stretching region (1760--1600 cm(-1)) were measured at 77 K. The data were analyzed by singular-value decomposition and subsequent least-squares fitting. Two distinct spectral components having different kinetic behaviors were resolved. One had spectral features characterized by negative peaks at 1740 and 1680 cm(-1) and an overall positive background and was assigned to the P(680)(+)Phe(-)/P(680)Phe radical pair by static FTIR measurements of the P(680)(+)/P(680) and Phe(-)/Phe differences. The other had prominent negative and positive peaks at 1668 and 1628 cm(-1), respectively, which were previously assigned to the keto C==O change upon triplet formation of the monomeric chlorophyll denoted as Chl(T) [Noguchi, T., Tomo, T., and Inoue, Y. (1998) Biochemistry 37, 13614-13625]. The former component of P(680)(+)Phe(-)/P(680)Phe exhibited a multiphasic decay with time constants of 77 ns (75%), 640 ns (18%), 8.3 micros (4%), and 0.3 ms (3%), while the latter component of (3)Chl(T)/Chl(T) was formed with a single-exponential rise with a time constant of 57 ns and had a lifetime of 1.5 ms. From the observations that only the two spectral components were resolved without any other triplet intermediates and the time constant of (3)Chl(T) formation roughly agreed with or seemed even faster than that of the major phase of the P(680)(+)Phe(-) decay, two alternative mechanisms of triplet formation are proposed. (i) (3)Chl(T) is directly formed from P(680)(+)Phe(-) by charge recombination at Chl(T), and (ii) (3)P(680) is formed, and then the triplet is transferred to Chl(T) with a time constant of much less than 50 ns. The location of Chl(T) in the D1 subunit as the monomer chlorophyll corresponding to the accessory bacteriochlorophyll in the L subunit of purple bacteria is favored to explain the former mechanism as well as the triplet properties reported in the literature. The physiological role of the triplet formation on Chl(T) is also discussed.     

Molecular interactions of the redox-active accessory chlorophyll on the electron-donor side of photosystem II as studied by Fourier transform infrared spectroscopy

A Fourier transform infrared (FTIR) difference spectrum upon photooxidation of the accessory chlorophyll (Chl_z) of photosystem II (PS II) was obtained at 210 K with Mn-depleted PS II membranes in the presence of fericyanide and silicomolybdate. The observed Chl_z⁺/Chl_z spectrum showed two differential bands at 1747/1736 and 1714/1684 cm⁻. The former was assigned to the free carbomethoxy C = 0 and the latter to the keto C = 0 that is hydrogen-bonded or in a highly polar environment. Also, the negative 1614 cm⁻ band assignable to the macrocycle mode indicated 5-coordination of the central Mg. The negative 1660 cm⁻¹ band, possibly due to the strongly hydrogen-bonded keto C = 0, may suggest oxidation of one more Chl_z, although an alternative assignment, the amide I mode of proteins perturbed by Chl_z oxidation, is also possible.     

Photooxidation pathway of chlorophyll Z in photosystem II as Studied by Fourier transform infrared spectroscopy

The oxidation pathway of chlorophyll Z (ChlZ) in photosystem II (PSII) at cryogenic temperatures was studied by means of light-induced Fourier transform infrared (FTIR) difference spectroscopy. To examine the involvement of redox-active beta-carotene (Car) in the pathway, two Car molecules in Mn-depleted PSII membranes of spinach were selectively bleached by illumination at 250 K in the presence of ferricyanide and silicomolybdate. Successful bleaching of Car was demonstrated by disappearance of the light-induced FTIR signals of Car+ at 1465, 1440, and 1147 cm(-1) at 80 K under an oxidative condition. Even in the Car-bleached PSII, the ChlZ+/ChlZ signal at 1713/1687 cm(-1), which is attributed to the upshift of the 9-keto C=O band of ChlZ upon its oxidation, was induced by illumination at 80 K retaining about 80% of the intensity of the control PSII sample. The concomitant appearance of shoulders at 1727/1699 cm(-1) may indicate that both of the two ChlZ molecules on the D1 and D2 sides are photooxidized. The multiphasic kinetics of formation of the ChlZ+/ChlZ signal by continuous illumination at 80 K were mostly unchanged by Car depletion, while the formation rates at 210 K were appreciably reduced in Car-bleached PSII. These results indicate that there are electron-transfer pathways from ChlZ to P680+ that do not involve Car, and they are indeed dominant at 80 K. Although the pathways via Car are mostly blocked at this temperature, the contribution of such pathways to ChlZ oxidation becomes significant at higher temperatures.     

Perturbation of the structure of P680 and the charge distribution on its radical cation in isolated reaction center complexes of photosystem II as revealed by fourier transform infrared spectroscopy

The structure and the electronic properties of P680 and its radical cation in photosystem II (PSII) were studied by means of Fourier transform infrared spectroscopy (FTIR). Light-induced P680+/P680 FTIR difference spectra in the mid- and near-IR regions were measured using PSII membranes from spinach, core complexes from Thermosynechococcus elongatus, and reaction center (RC) complexes (D1-D2-Cytb559) from spinach. The spectral features of the former two preparations were very similar, indicating that the structures of P680 and its radical cation are virtually identical between membranes and cores and between plants and cyanobacteria. In sharp contrast, the spectrum of the RC complexes exhibited significantly different features. A positive doublet at approximately 1724 and approximately 1710 cm-1 due to the 131-keto C=O stretches of P680+ in the membrane and core preparations were changed to a prominent single peak at 1712 cm-1 in the RC complexes. This observation was interpreted to indicate that a positive charge on P680+ was extensively delocalized over the chlorophyll dimer in RC, whereas it was mostly localized on one chlorophyll molecule (70-80%) in intact P680. The significant change in the electronic structure of P680+ in RC was supported by a dramatic change in the characteristics of a broad intervalence band in the near-IR region and relatively large shifts of chlorin ring bands. It is proposed that the extensive charge delocalization in P680+ mainly causes the decrease in the redox potential of P680+/P680 in isolated RC complexes. This potential decrease explains the well-known phenomenon that YZ is not oxidized by P680+ in RC complexes.     

Fourier transform infrared analysis of bacteriorhodopsin secondary structure.

Fourier transform infrared spectroscopy at a resolution of 1 cm-1 has been used to study the conformation of dark-adapted bacteriorhodopsin in the native purple membrane, in H2O and D2O suspensions. A detailed analysis of the amide I bands was made using derivative and deconvolution techniques. Curve-fitting results of four independent experiments indicate, after estimation of the methodological errors, that native bacteriorhodopsin contains 52-73% alpha-helices, 13-19% reverse turns, 11-16% beta-sheets, and 3-7% unordered segments. Our analysis has enabled the identification of several components corresponding to alpha-helices, beta-sheets, and reverse turns. Besides the alpha I- and alpha II-helices (peaking at 1658 and 1665 cm-1), we propose that two more infrared bands arise from alpha-helical structures: one at 1650 cm-1 from alpha I and another one at 1642 cm-1 in H2O suspension, which could originate from type III beta-turns (i.e., one turn of 3(10)-helix). The relatively high content of reverse turns suggests the presence of one reverse turn per loop, plus another one in the C-terminal segment. On the other hand, several reasons argue that the calculated mean beta-sheet content of around 14% should be decreased somewhat. These beta-sheets could be located in the noncytoplasmatic links of the bacteriorhodopsin molecule.     

Herbicide effect on the hydrogen-bonding interaction of the primary quinone electron acceptor QA in photosystem II as studied by Fourier transform infrared spectroscopy

The redox potential of Q(A) in photosystem II (PSII) is known to be lower by approximately 100 mV in the presence of phenolic herbicides compared with the presence of DCMU-type herbicides. In this study, the structural basis underlying the herbicide effects on the Q(A) redox potential was studied using Fourier transform infrared (FTIR) spectroscopy. Light-induced Q(A)(-)/Q(A) FTIR difference spectra of Mn-depleted PSII membranes in the presence of DCMU, atrazine, terbutryn, and bromacil showed a strong CO stretching peak of Q(A)(-) at 1,479 cm(-1), while binding of phenolic herbicides, bromoxynil and ioxynil, induced a small but clear downshift by approximately 1 cm(-1). The CO peak positions and the small frequency difference were reproduced in the S(2)Q(A)(-)/S(1)Q(A) spectra of oxygen-evolving PSII membranes with DCMU and bromoxynil. The relationship of the CO frequency with herbicide species correlated well with that of the peak temperatures of thermoluminescence due to S(2)Q(A)(-) recombination. Density functional theory calculations of model hydrogen-bonded complexes of plastoquinone radical anion showed that the small shift of the CO frequency is consistent with a change in the hydrogen-bond structure most likely as a change in its strength. The Q(A)(-)/Q(A) spectra in the presence of bromoxynil, and ioxynil, which bear a nitrile group in the phenolic ring, also showed CN stretching bands around 2,210 cm(-1). Comparison with the CN frequencies of bromoxynil in solutions suggested that the phenolic herbicides take a phenotate anion form in the Q(B) pocket. It was proposed that interaction of the phenolic C-O(-) with D1-His215 changes the strength of the hydrogen bond between the CO of Q(A) with D2-His214 via the iron-histidine bridge, causing the decrease in the Q(A) redox potential.     

Difference in molecular structure of rod and cone visual pigments studied by Fourier transform infrared spectroscopy

To investigate the local structure that causes the differences in molecular properties between rod and cone visual pigments, we have measured the difference infrared spectra between chicken green and its photoproduct at 77 K and compared them with those from bovine and chicken rhodopsins. In contrast to the similarity of the vibrational bands of the chromophore, those of the protein part were notably different between chicken green and the rhodopsins. Like the rhodopsins, chicken green has an aspartic acid at position 83 (D83) but exhibited no signals due to the protonated carboxyl of D83 in the C=O stretching region, suggesting that the molecular contact between D83 and G120 through water molecule evidenced in bovine rhodopsin is absent in chicken green. A pair of positive and negative bands due to the peptide backbone (amide I) was prominent in chicken green, while the rhodopsins exhibited only small bands in this region. Furthermore, chicken green exhibited characteristic paired bands around 1480 cm(-1), which were identified as the imide bands of P189 using site-directed mutagenesis. P189, situated in the putative second extracellular loop, is conserved in all the known cone visual pigments but not in rhodopsins. Thus, some region of the second extracellular loop including P189 is situated near the chromophore and changes its environment upon formation of the batho-intermediate. The results noted above indicate that differences in the protein parts between chicken green and the rhodopsins alter the changes seen in the protein upon photoisomerization of the chromophore. Some of these changes appear to be the pathway from the chromophore to cytoplasmic surface of the pigment and thus could affect the activation process of transducin.     

Protein and chlorophyll in photosystem II probed by infrared spectroscopy

The infrared spectra of photosystem II (PS II) enriched submembrane fractions isolated from spinach are obtained in water and in heavy water suspension Other spectra are obtained after a photooxidation reaction was performed on PS II to bleach the pigments. The water bands are removed by computer subtraction and the amide bands (A, B, I, II, and III) of the protein are identified. Computer enhancement techniques are used to narrow the bandwidth of the bands that the weak chlorophyll bands, buried in the much stronger protein bands, can be observed. Comparing the spectra of native and photooxidized PS II pr in water and in heavy water, we determine that three polypeptide domains are present in the native material. The first domain, which contains 22% of th is situated in the peripheral region of the PS II system. The polypeptides in this region are unfolded and devoid of chlorophyll. The second domain con of the polypeptides, is more organized, and contains the chlorophylls. The third domain has an alpha-helix configuration, does not contain chlorophyll, a affected by the photooxidation reaction or by the proton/deuteron exchange. Three different types of chlorophyll organisation are identified: two have carbonyl groups non-bonded, differing from one another only in their hydrophobic milieux; the third is weakly bonded to another unidentified group. Other forms of chlorophyll organisation are present but could not be observed because their absorption is buried in the protein amide I band.     

pH dependence of the flash-induced S-state transitions in the oxygen-evolving center of photosystem II from Thermosynechoccocus elongatus as revealed by Fourier transform infrared spectroscopy

pH dependence of the efficiencies of the flash-induced S-state transitions in the oxygen-evolving center (OEC) was studied by means of Fourier transform infrared (FTIR) difference spectroscopy using photosystem II (PSII) core complexes from the thermophilic cyanobacterium Thermosynechoccocus elongatus. The PSII core complexes dark-adapted at different pHs in the presence of ferricyanide as an electron acceptor were excited by four consecutive saturating laser flashes, and FTIR difference spectra induced by each flash were recorded in the region of 1800-1200 cm(-1). Each difference spectrum was fitted with a linear combination of standard spectra measured at pH 6.0, which represent the spectra upon individual S-state transitions, and the transition efficiencies were estimated from the fitting parameters. It was found that the S1 --> S2 transition probability is independent of pH throughout the pH region of 3.5-9.5, while the S2 --> S3, S3 --> S0, and S0 --> S1 transition probabilities decrease at acidic pH with pK values of 3.6 +/- 0.2, 4.2 +/- 0.3, and 4.7 +/- 0.5, respectively. These findings, i.e., the pH-independent S1 --> S2 transition probability and the pK values for the inhibition in the acidic range of the other three transitions, were in good agreement with recent results obtained by electron paramagnetic resonance measurements for PSII-enriched membranes of spinach [Bernát, G., Morvaridi, F., Feyziyev, Y., and Styring, S. (2002) Biochemistry 41, 5830-5843]. On the basis of this correspondence for quite different types of PSII preparations exhibiting marked difference in the pH dependence of the apparent proton release pattern, it is concluded that the inhibition of the S2 --> S3, S3 --> S0, and S0 --> S1 transitions in the acidic region is an inherent property of the OEC. This feature probably reflects proton release from substrate water in these three transitions. On the other hand, all of the S-state transitions remained generally efficient up to pH 9.5 in the alkaline region, except for a slight decrease of the S3 --> S0 transition probability above pH 8 (pK approximately 10). This observation partly differs from the tendency reported for spinach preparations, suggesting that a mechanism different from that in the acidic region is responsible for the transition efficiencies in the alkaline region.     

Heat-induced secondary structure and conformation change of bovine serum albumin investigated by Fourier transform infrared spectroscopy

Fourier transform infrared (FT-IR) spectra were measured for an aqueous solution (pD = 5.40) of defatted monomer bovine serum albumin (BSA) over a temperature range of 25-90 degrees C to investigate temperature-induced secondary structure and conformation changes. The curve fitting method combined with the Fourier self-deconvolution technique allowed us to explore details of the secondary structure and conformation changes in defatted BSA. Particularly striking in the FT-IR spectra was an observation of the formation of an irreversible intermolecular beta-sheet of BSA on heating above 70 degrees C. A band at 1630 cm(-1) in the spectra was assigned to short-segment chains connecting alpha-helical segments. The transition temperature for the short-segment chains connecting alpha-helical segments is lower by 17-18 degrees C, when compared to those of the alpha-helix, turn, and intermolecular beta-sheet structures of BSA, suggesting that the alpha-helix and turn structures of BSA are cooperatively denatured on heating. Moreover, the results give an important feature in heat-induced denaturation of BSA that the conformation changes occur twice around both 57 and 75 degrees C. The appearance of two peaks is interpreted by the collapse of the N-terminal BSA domain due to the crevice in the vicinity between domains I and II at low-temperature transition and by the change in cooperative unit composed of the other two BSA domains at high-temperature transition.     

Changes in protein secondary structure during gluten deformation studied by dynamic fourier transform infrared spectroscopy

Fourier transform infrared (FT-IR) spectroscopy was used to monitor changes in the secondary structure of wheat prolamins, the main components of gluten, during mechanical deformation in a series of cycles of extension and relaxation. A sample derived from protein bodies isolated from developing grain showed a buildup of persistent beta-sheet structure. In gluten, the ratio of beta-sheet to random and beta-turn structures changed on extension. After the applied force was released, the sample recovered some of its original shape and structure, but the material became stiffer in consecutive extension cycles. The relationship between gluten structure and mechanical properties is discussed in terms of a model in which conversion of beta-turn to beta-sheet structure is a response to extension and a means by which elastic energy is stored in the system.     

Conformational changes in concanavalin A associated with demetallization and alpha-methylmannose binding studied by Fourier transform infrared spectroscopy

Infrared spectra of concanavalin A have been obtained both in the absence and in the presence of the metal ions, Mn2+ and Ca2+, and the saccharide, alpha-methylmannose. Second derivative calculations have been used to determine the frequencies of the different amide I and II components. In the demetallized protein dissolved in H2O buffer, absorptions in the amide I, II and III regions at 1695 and 1634, 1532 and 1237 cm-1, respectively, are assigned to beta-structure, while absorptions at 1563 and both 1318 and 1343 cm-1 are assigned to turns and bends. After deuterium exchange, the residual amide II maximum in the difference spectrum shifts from 1538 to 1563 cm-1, indicating that exchange is faster in the beta-structure than in the turns. In the presence of Mn2+ and Ca2+, the amide II band component at 1532 cm-1 shifts 4-6 cm-1 to higher wavenumbers, and the amide I band component at 1634 shifts 1 cm-1 in the same direction, both in H2O and 2H2O buffers, suggesting changes in the hydrogen-bonding network of a large portion of the protein, particularly in the beta-sheet regions. The addition of alpha-methylmannose increases the magnitude of exchange from 55% to above 90%. Comparison with existing X-ray crystallographic data has been made, and the usefulness of FT-IR to complement this technique is discussed.     

Structural analysis of the PsbQ protein of photosystem II by Fourier transform infrared and circular dichroic spectroscopy and by bioinformatic methods

The structure of PsbQ, one of the three main extrinsic proteins associated with the oxygen-evolving complex (OEC) of higher plants and green algae, is examined by Fourier transform infrared (FTIR) and circular dichroic (CD) spectroscopy and by computational structural prediction methods. This protein, together with two other lumenally bound extrinsic proteins, PsbO and PsbP, is essential for the stability and full activity of the OEC in plants. The FTIR spectra obtained in both H(2)O and D(2)O suggest a mainly alpha-helix structure on the basis of the relative areas of the constituents of the amide I and I' bands. The FTIR quantitative analyses indicate that PsbQ contains about 53% alpha-helix, 7% turns, 14% nonordered structure, and 24% beta-strand plus other beta-type extended structures. CD analyses indicate that PsbQ is a mainly alpha-helix protein (about 64%), presenting a small percentage assigned to beta-strand ( approximately 7%) and a larger amount assigned to turns and nonregular structures ( approximately 29%). Independent of the spectroscopic analyses, computational methods for protein structure prediction of PsbQ were utilized. First, a multiple alignment of 12 sequences of PsbQ was obtained after an extensive search in the public databases for protein and EST sequences. Based on this alignment, computational prediction of the secondary structure and the solvent accessibility suggest the presence of two different structural domains in PsbQ: a major C-terminal domain containing four alpha-helices and a minor N-terminal domain with a poorly defined secondary structure enriched in proline and glycine residues. The search for PsbQ analogues by fold recognition methods, not based on the secondary structure, also indicates that PsbQ is a four alpha-helix protein, most probably folding as an up-down bundle. The results obtained by both the spectroscopic and computational methods are in agreement, all indicating that PsbQ is mainly an alpha protein, and show the value of using both methodologies for protein structure investigation.     

Conformational changes of bacteriorhodopsin detected by Fourier transform infrared difference spectroscopy

We report the first Fourier transform infrared difference spectra of purple membrane. Evidence is presented that alterations in the vibrations of both the retinylidene chromophore and the protein groups of bacteriorhodopsin associated with photocycling can be detected. This method provides a new tool for probing the conformational changes occurring in bacteriorhodopsin during the proton pump cycle.     

Fourier transform infrared spectrum of the secondary quinone electron acceptor Q(B) in photosystem II

Fourier transform infrared difference spectra upon single reduction of the secondary quinone electron acceptor Q(B) in photosystem II (PSII), without a contribution from the electron donor-side signals, were obtained for the first time using Mn-depleted PSII core complexes of the thermophilic cyanobacterium Thermosynechococcus elongatus. The Q(B)(-)/Q(B) difference spectrum exhibited a strong C...O stretching band of the semiquinone anion at 1480 cm(-)(1), the frequency higher by 2 cm(-)(1) than that of the corresponding band of Q(A)(-), in agreement with the previous S(2)Q(B)(-)/S(1)Q(B) spectrum of the PSII membranes of spinach [Zhang, H., Fischer, G., and Wydrzynski, T. (1998) Biochemistry 37, 5511-5517]. Also, several peaks originating from the Fermi resonance of coupled His modes with its strongly H-bonded NH vibration were observed in the 2900-2600 cm(-)(1) region, where the peak frequencies were higher by 7-24 cm(-)(1) compared with those of the Q(A)(-)/Q(A) spectrum. These frequency differences suggest that H-bond interactions of the CO groups, especially with a His side chain, are different between Q(B)(-) and Q(A)(-). Furthermore, a prominent positive peak was observed at 1745 cm(-)(1) in the C=O stretching region of COOH or ester groups in the Q(B)(-)/Q(B) spectrum. The peak frequency was unaffected by D(2)O substitution, indicating that this peak does not arise from a COOH group but probably from the 10a-ester C=O group of the pheophytin molecule adjacent to Q(B). The absence of protonation of carboxylic amino acids upon Q(B)(-) formation in contrast to the previous observation in the purple bacterium Rhodobacter sphaeroides suggests that the protonation mechanism of Q(B) in PSII is different from that of bacterial reaction centers.