Spectrally silent transitions in the bacteriorhodopsin photocycle

The photocycle kinetics of bacteriorhodopsin were analyzed from 0 to 40 degrees C at 101 wavelengths (330-730 nm). The data can be satisfactorily approximated by eight exponents. The slowest component (half-time 20 ms at 20 degrees C) belongs to the 13-cis cycle. The residual seven exponentials that are sufficient to describe the all-trans photocycle indicate that at least seven intermediates of the all-trans cycle must exist, although only five spectrally distinct species (K, L, M, N, and O) have been identified. These seven exponentials and their spectra at different temperatures provide the basis for the discussion of various kinetic schemes of the relaxation. The simplest model of irreversible sequential transitions includes after the first K--> L step the quasiequilibria of L<-->M, M<-->N, and N<-->O intermediates. These quasiequilibria are controlled by rate-limiting dynamics of the protein and/or proton transfer steps outside the chromophore region. Thus there exists an apparent kinetic paradox (i.e., why is the number of exponents of relaxation (at least seven) higher than the number of distinct spectral intermediates (only five)), which can be explained by assuming that some of the transitions correspond to changes in the quasiequilibria between spectrally distinct intermediates (i.e., are spectrally silent).

Protonation changes during the photocycle of sensory rhodopsin II from Natronobacterium pharaonis

The Fourier Transform Infrared (FTIR) spectra of photocycle intermediates of sensory rhodopsin II (pSRII) from Natronobacterium pharaonis were measured. The results of the FTIR experiments indicate considerable conformational movements of pSRII already at the stage of the early K-like intermediate which persist at least during the lifetime of the long lived intermediate. These changes in the amide bond region are more intense than those described for sensory rhodopsin I (SRI) and are quite similar to those observed for rhodopsin. Concomitantly with the deprotonation of the Schiff base a carboxyl group located in a hydrophobic environment is protonated. In analogy to bacteriorhodopsin, this carboxyl group might arise from Asp-75 which probably serves as counter ion to the Schiff base. The protonation reaction differs from the situation observed in SRI where the protonation is pH independent over the range of pH 5-8.

Solid-state 13C-NMR of [(3-13C)Pro]bacteriorhodopsin and [(4-13C)Pro]bacteriorhodopsin: evidence for a flexible segment of the C-terminal tail

The configuration of an Xaa-Pro bond can be determined by solid-state magic-angle-sample-spinning (MASS)-13C-NMR spectroscopy since the chemical shifts of C beta and Cgamma of the proline ring are sensitive to the isomerization state of the preceding peptide bond. (3-13C)Pro and (4-13C)Pro have been chemically synthesized; the former by means of an asymmetric synthesis. The 13C-labeled Pro residues were biosynthetically incorporated into bacteriorhodopsin with a yield of 80%. The solid-state-MASS-13C-NMR spectra of [(3-13C)Pro]bacteriorhodopsin and [(4-13C)Pro]bacteriorhodopsin revealed isotropic chemical shifts at 29.8 ppm and 25.5 ppm, respectively. From the chemical-shift values we conclude that all Xaa Pro peptide bonds are in the trans configuration confirming previous results from solution-NMR studies on solubilized bacteriorhodopsin in organic solvents [Deber, M.C., Sorrell, B.J. & Xu, G.Y. (1990) Biochem. Biophys. Res. Commun. 172, 862-869]. Inversion-recovery experiments could differentiate between three classes of Pro residues distinguished by their relaxation time t1. Tentatively, these three distinct groups of Pro residues could be assigned to the helical, the loop, and the C-terminal parts of the protein. The resonances of the two C-terminal Pro could be identified by removing the C-terminus by proteolysis. Although they are separated by only one Glu they occupy different chemical environments and possess different flexibilities. These results indicate that the first part of the C-terminal tail is constrained. Pro238 marks the position where the tail becomes freely mobile. It is proposed that the C-terminus is fixed to the membrane via salt bridges between divalent cations and negative charges of the C-terminus as well as interhelical loops.

Experimental investigation of sidechain interactions in early folding intermediates

Kinetic studies of folding sometimes reveal very rapid spectroscopic changes that may indicate the population of intermediates, but it is difficult to elucidate in detail the nature of the interactions involved. In this review, we focus on one important aspect of this problem: how to probe the nature and extent of clustering of hydrophobic sidechains. As the information obtainable from different experimental approaches is outlined, it becomes clear that a combination of methods is likely to be necessary to build up a reasonable picture of early folding events.

Ligand binding and protein dynamics in cupredoxins

Type 1 copper sites bind nitric oxide (NO) in a photolabile complex. We have studied the NO binding properties of the type 1 copper sites in two cupredoxins, azurin and halocyanin, by measuring the temperature dependence of the ligand binding equilibria and the kinetics of the association reaction after photodissociation over a wide range of temperature (80-280 K) and time (10(-6)-10(2) s). In both proteins, we find nonexponential kinetics below 200 K that do not depend on the NO concentration. Consequently, this process is interpreted as geminate recombination. In azurin, the rebinding can be modeled with the Arrhenius law using a single pre-exponential factor of 10(8.3) s-1 and a Gaussian distribution of enthalpy barriers centered at 22 kJ/mol with a width [full width at half-maximum (FWHM)] of 11 kJ/mol. In halocyanin, a more complex behavior is observed. About 97% of the rebinding population can also be characterized by a Gaussian distribution of enthalpy barriers at 12 kJ/mol with a width of 6.0 kJ/mol (FWHM). The pre-exponential of this population is 1.6 x 10(12) s-1 at 100 K. After the majority population has rebound, a power-law phase that can be modeled with a gamma-distribution of enthalpy barriers is observed. Between 120 and 180 K, an additional feature that can be interpreted as a relaxation of the barrier distribution toward higher barriers shows up in the kinetics. Above 200 K, a slower, exponential rebinding appears in both cupredoxins. Since the kinetics depend on the NO concentration, this process is identified as bimolecular rebinding.(ABSTRACT TRUNCATED AT 250 WORDS)

Angioimmunoblastic lymphadenopathy (AILD)-type T-cell lymphoma: prognostic impact of clinical observations and laboratory findings at presentation. The Kiel Lymphoma Study Group

BACKGROUND

In order to establish the clinico-pathological properties of angioimmunoblastic lymphadenopathy (AILD)-type T-cell lymphoma, we evaluated the type, incidence and prognostic significance of clinical and laboratory symptoms.

PATIENTS AND METHODS

Sixty-two consecutive patients diagnosed at the Kiel lymph node registry participated in the study. The median patient age was 64 years (range 21-87 years) and the female to male ratio was 1:1.4. Ninety percent of the patients were in stage III and IV and B-symptoms were observed in 68%. At diagnosis patients presented with skin rash (49%), pruritus (32%), edema (38%), pleural effusion (37%), arthritis (18%) and ascites (23%). Furthermore, they exhibited autoimmune phenomena such as cold agglutinines, circulating immune complexes, a positive Coombs test, smooth muscle antibodies, rheumatoid factors, immune hemolysis, a paraprotein, antinuclear antibodies and cryoglobulins.

RESULTS

In univariate analysis, survival was significantly related to age (p=0.032), stage (p=0.037), B symptoms (p=0.007), rash/pruritus (p=0.038), edema (p=0.030), ascites (p=0.013), number of clinical symptoms including B symptoms (p=0.004) and excluding B symptoms (p=0.017), lactate dehydrogenase (p=0.007) and hemoglobin (p=0.020).

CONCLUSIONS

AILD type T-cell lymphoma characteristically differs from other non-Hodgkin's lymphomas in its clinical signs and laboratory symptoms.

Kinetics of interaction of partially folded proteins with a hydrophobic dye: evidence that molten globule character is maximal in early folding intermediates

Interaction with 8-anilino-1-naphthalenesulfonate (ANS) is widely used to detect molten globule states of proteins. We have found that even with stable partially folded states, the development of the fluorescence enhancements resulting from such interactions can be relatively slow and kinetically complex. This is probably because initial binding of the dye can induce subsequent changes in the protein structure, so that the ultimate resulting fluorescence enhancement is not necessarily a good, nonperturbing probe of the preexisting state of the protein. When ANS is used to study folding mechanisms the problem is compounded by the difficulty of distinguishing effects due to the development of dye interactions from those due to the changing populations of folding intermediates. Many of these complications can be avoided by experiments where the ANS is introduced only after folding has been allowed to proceed for a variable time. The initial fluorescence intensity after mixing, resulting only from rapid and therefore hopefully relatively nonperturbing interactions with the protein, can be monitored at different refolding times to provide a better reflection of the progress of the reaction, uncomplicated by dye interaction effects. Such studies of the folding of carbonic anhydrase and alpha-lactalbumin have been compared with conventional single-mix experiments and large discrepancies observed. When ANS was present throughout refolding, time-dependent changes attributed to the formation or reorganization of protein-ANS complexes were clearly superimposed on those associated with the actual progress of refolding, and the folding kinetics and population of intermediates were also substantially perturbed by the dye. Thus, it is clear that the pulse method, though cumbersome, should be used where refolding reactions are to be probed by dye binding. The results emphasize that fluorescence enhancement tends to be greatest in early intermediates, in contrast to what, for carbonic anhydrase at least, might appear to be the case from the more conventional experiments. Later intermediates in the folding of both of these proteins actually induce little fluorescence enhancement and therefore may be quite different in nature from equilibrium molten globule states.

New calibration method for the determination of the absolute density of CH radicals through laser-induced fluorescence

Laser-induced fluorescence (LIF) was applied at the B-X transition of the CH radical to measure the absolute densities of CH radicals in an electron-cyclotron resonance methane plasma. The absolute experimental uncertainty is only approximately 30% as a result of a new calibration procedure. The experimental setup was calibrated through the comparison of the LIF signal of N(2)(+) with that of CH. The absolute N(2)(+) density was derived from the spatially resolved N(2)(+) LIF signal and the line-averaged electron density as measured with microwave interferometry.

The primary structure of sensory rhodopsin II: a member of an additional retinal protein subgroup is coexpressed with its transducer, the halobacterial transducer of rhodopsin II

The blue-light receptor genes (sopII) of sensory rhodopsin (SR) II were cloned from two species, the halophilic bacteria Haloarcula vallismortis (vSR-II) and Natronobacterium pharaonis (pSR-II). Upstream of both sopII gene loci, sequences corresponding to the halobacterial transducer of rhodopsin (Htr) II were recognized. In N. pharaonis, psopII and phtrII are transcribed as a single transcript. Comparison of the amino acid sequences of vHtr-II and pHtr-II with Htr-I and the chemotactic methyl-accepting proteins from Escherichia coli revealed considerable identities in the signal domain and methyl-accepting sites. Similarities with Htr-I in Halobacterium salinarium suggest a common principle in the phototaxis of extreme halophiles. Alignment of all known retinal protein sequences from Archaea identifies both SR-IIs as an additional subgroup of the family. Positions defining the retinal binding site are usually identical with the exception of Met-118 (numbering is according to the bacteriorhodopsin sequence), which might explain the typical blue color shift of SR-II to approximately 490 nm. In archaeal retinal proteins, the function can be deduced from amino acids in positions 85 and 96. Proton pumps are characterized by Asp-85 and Asp-96; chloride pumps by Thr-85 and Ala-96; and sensors by Asp-85 and Tyr-96 or Phe-96.

Deep venous thrombosis and pulmonary artery embolism in high-grade non Hodgkin's lymphoma: incidence, causes and prognostic relevance

To analyse incidence, risk factors, causes and prognostic significance of venous thromboembolism (VTE) in high-grade non-Hodgkin's lymphoma (HG-NHL) a prospective clinical trial (N = 593), also undertaken to analyse other aspects of HG-NHL, a study of haemostasis (N = 25) and a post-mortem analysis (N = 70) were performed. Clinical analysis documented a 6.6% incidence of VTE, and 77% of all cases occurred before or within the first 3 months of chemotherapy. Ann Arbor stage IV and B-mediastinal clear cell histology were risk factors for VTE, while rapid changes in tumour load or application of consolidation chemotherapy were not. Vessel compression by HG-NHL was the leading cause of VTE, whereas a significant (paraneoplastic or chemotherapy-induced) thrombophilic state was not disclosed by haemostatic tests. While VTE-related fatality was found to be low in the clinical trial (1.7%) and at necropsy (8.5%), the occurrence of VTE was associated with an unsatisfactory response of HG-NHL to chemotherapy and a high incidence of treatment-related mortality due to diffuse alveolitis. Thus, fatal VTE in HG-NHL is rare, but VTE is associated with an unfavourable clinical course of HG-NHL.

Raman spectroscopic study of the blue copper protein halocyanin from Natronobacterium pharaonis

Halocyanin, a blue copper protein from Natronobacterium pharaonis was studied by Raman spectroscopic techniques. Near-infrared Fourier transform Raman spectra, which for the first time have been employed to study copper proteins in both oxidation states, display the Raman bands of the protein and, in the oxidized state, also the preresonance-enhanced bands of the copper center. The frequency of the amide I band at 1676 cm-1 indicates a predominant beta-sheet protein structure, which is typical for small blue copper proteins. The Raman spectra provide no evidence for major redox-linked changes of the secondary structure. Resonance Raman spectra of the oxidized protein obtained upon excitation close to the 600-nm absorption band were measured in the pH range between 7.7 and 4.5. The vibrational band pattern in the Cu-cysteine stretching region is closely related to that of azurin, indicating far-reaching similarities of the coordination geometry of the copper center in both proteins. Significantly lower frequencies, however, are noted for the Cu-histidine stretches, which appear as a closely spaced doublet at ca. 260 cm-1. Lowering the pH to 4.5 leads to an increase of this band splitting with one component shifting down to 247 cm-1. This downshift is attributed to the rupture of a hydrogen bond between one of the histidine ligands and a nearby carboxyl group, which becomes protonated at such a low pH. On the other hand, no major changes in the Cu-cysteine stretching region are noted at pH 4.5, implying that the coordination geometry remains largely unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral T-cell non-Hodgkin's lymphomas of low malignancy: prospective study of 25 patients with pleomorphic small cell lymphoma, lymphoepitheloid cell (Lennert's) lymphoma and T-zone lymphoma. The Kiel Lymphoma Study Group

Peripheral T-cell lymphomas comprise a heterogenous group of low- and high-grade malignancies differing in their histopathological appearance and also in clinical and prognostic aspects. We prospectively studied 25 patients with low-grade peripheral T-cell lymphomas: pleomorphic, small cell lymphoma (PSC) (n = 9), lymphoepitheloid (Lennert's) lymphoma (LEL) (n = 12) and T-zone lymphoma (TZL) (n = 4). The median patient age was 55 years (range 19-75 years); the male to female ratio was 1.5. 13 patients (52%) had limited stages (I+II), 12 patients (48%) had advanced disease (stage III+IV). 21 patients received the COPBLAM/IMVP-16 regimen. Two patients received more intensive treatments; two received less intensive therapy. Complete remissions were achieved in 16/25 patients (64%). The median observation time of surviving patients was 30 months (range 5-72 months). The actuarial overall survival and event-free survival at 2 years of 21 patients receiving COPBLAM/IMVP-16 were 69% and 35%, respectively. Intensive chemotherapy led to complete remissions in about 60% of the patients and to long-term disease-free survival for one-third. The observed clinical courses illustrate the aggressive nature of PSC, LEL and TZL.

Blue halorhodopsin from Natronobacterium pharaonis: wavelength regulation by anions

Halorhodopsin, the chloride pump from Natronobacterium pharaonis (pharaonis hR), was isolated under conditions of low ionic strength. The quotient between the optical densities of pharaonis hR in 4 M NaCl at 280 and 577 nm amounts to 1.1, indicating a high purity of the protein and integrity of the chromophore. Gel filtration chromatography of the purified pharaonos hR at neutral pH and in the absence of inorganic salts leads to a shift to the absorption maximum to 600 nm (pharaonis hRblue). The purple color can be regained by the addition of anions such as chloride, iodide, azide, nitrate, and also fluoride. The absorption maxima are dependent on the nature of the anion and the pH. At pH 7, sulfate does not influence the 600-nm absorption, while at pH 4.5 the absorption maximum is shifted to 581 nm. The blue form of halorhodopsin (pharaonis hRblue) was titrated with different sodium salts, such as chloride, azide, and nitrate. The half-maximal binding is in the millimolar range, with Br < Cl < NO3 < N3 < BrO3. Deprotonation of the Schiff base can be accomplished by treatment of pharaonis hR or pharaonis hRblue with base. The apparent pK of the Schiff base in pharaonis hRblue was determined to be 8.5. The pK shifted to 8.0 in the presence of 150 mM Na2SO4, whereas the pK of pharaonis hR in 150 mM NaCl is 9.6. In the photocycle of the chloride- and nitrate-containing pharaonis hR, a species similar to hR520 could be detected. Pharaonis hR reconstituted with azide shows photochemical reactions similar to the photocycle of bacteriorhodopsin.(ABSTRACT TRUNCATED AT 250 WORDS)

The primary structure of halocyanin, an archaeal blue copper protein, predicts a lipid anchor for membrane fixation

Halocyanin, a small blue copper protein, was isolated from the haloalkaliphilic archaeon Natronobacterium pharaonis. The NH2 terminus was not accessible to Edman degradation. About 70% of the amino acid sequence was determined by protein sequence analysis. The sequence information of two peptides was used for cloning and sequencing the halocyanin gene (hcy). The open reading frame codes for 489 base pairs, which account for a protein with 163 amino acids and a molecular mass of 17,223 Da. The discrepancy between this value and the molecular mass of 15,456 +/- 1.5 Da for the copper-free protein determined by electrospray mass spectrometry can be explained by a post-translational processing of the gene product. The NH2-terminal sequence of the open reading frame contains a motif that is characteristic for prokaryotic lipoproteins. Assuming a similar processing for halocyanin, Cys at position 25 of the primary transcript would be modified by a diphytanyl (glycerol)diether. Subsequently, the precursor is cleaved by a signal peptidase II-like protease and then acetylated at its NH2-terminal alpha-amino group. These modifications would yield a protein with a calculated molecular mass of 15,456 Da. A comparison of the primary structure of halocyanin with a number of other blue copper proteins places it into the plastocyanin-related group.

Cytokine efficiency in the treatment of high-grade malignant non-Hodgkin's lymphomas: results of a randomized double-blind placebo-controlled study with intensified COP-BLAM +/- rhGM-CSF

In high-grade malignant non-Hodgkin's lymphomas (hNHL) recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) was evaluated as support to chemotherapy. In a phase III trial, 172 patients (age 18-73 years, stage II-IV) were risk-stratified according to LDH levels and lymphoma size and randomized to receive rhGM-CSF (400 micrograms) (87 patients) or placebo (85 patients) subcutaneously days 8-14 of each cycle of an intensified COP-BLAM regimen. RhGM-CSF significantly reduced the length and nadir of neutropenia, the length of fever episodes, the frequency of all and of severe infections, and of hospitalization and antibiotic requirements. Complete response rates were 63% for all patients and 64% vs. 61% (n.s.) in the rhGM-CSF vs. the control group. Deviations from protocol in applied dosages of myelotoxic drugs and in cycle intervals maintained differed slightly in favor of the rhGM-CSF arm. However, there were no significant differences in overall survival between the GM-CSF treatment and control groups (21 vs. 23 months). Early relapse rates were markedly lower than in the standard-dose COP-BLAM/IMVP-16 regimen. Thus, GM-CSF abates toxic side effects of chemotherapy and may help to maintain dose intensity in high-risk hNHL.

Analysis of the redox reaction of an archaebacterial copper protein, halocyanin, by electrochemistry and FTIR difference spectroscopy

Halocyanin is a recently discovered archaebacterial copper protein classified as "type I" small blue copper protein (Scharf, B., Ph.D. Thesis, University of Bochum, Germany). Its redox properties were investigated by a combination of protein electrochemical and spectroscopic techniques. Using electrochemical reactions in an ultrathin-layer electrochemical cell developed for UV/vis and IR spectroscopy, halocyanin could be quantitatively and reversibly oxidized and reduced. The titration of the absorption band at 600 nm can be perfectly described by a Nernst curve with n = 1 electron transferred; a quantitative fit yields a midpoint potential, Em, of 183 mV (vs SHE) at a pH of 7.3. The midpoint potential falls constantly from +333 mV at pH 4 to +119 mV at pH 10, with three regions around pH 4.5, 6.5, and 8.5 where the pH dependence is ca. -60 mV/pH unit, indicating the uptake of a proton with the reduction. By analogy with other small type I copper proteins, the three pK values suggested by the pH dependency of Em might be associated with three histidines which interact with the redox site. Electrochemically induced reduced-minus-oxidized Fourier transform infrared difference spectra in the 1800-1000 cm-1 range at neutral pH show a number of strong difference bands between ca. 1700 and 1600 cm-1 as well as smaller difference structures between 1600 and 1200 cm-1. The maximum amplitude of the difference bands--only ca. 1% of the amide-I absorption at ca. 1639 cm-1--indicates that only small protein rearrangements occur upon the redox transition.(ABSTRACT TRUNCATED AT 250 WORDS)

Halocyanin, an archaebacterial blue copper protein (type I) from Natronobacterium pharaonis

A small blue copper protein with a molecular mass of about 15.5 kDa has been isolated from the haloalkaliphilic Natronobacterium pharaonis. This protein, which was named halocyanin, contains one Cu2+. The circular dichroism spectrum in the region of the absorption of the peptide bond reveals mainly beta-structural elements. The absorption spectrum in the visible range has three maxima with the main band at 600 nm. The circular dichroism spectrum in this region is characterized by four extrema with maxima at 413 and 590 nm and minima at 477 nm as well as 721 nm. The electron paramagnetic resonance spectrum displays a hyperfine structure which shows a close resemblance to those of plastocyanin. The cumulative spectral data agree well with those of other small blue copper proteins with axial symmetry of the Cu ligands. In analogy, one can deduce that halocyanin may adopt a type I copper binding site with two His, one Met, and one Cys as probable ligands. This conclusion is confirmed by the C-terminal sequence which contains, homologous to other known sequences of type I copper proteins, three of the four copper ligands. Halocyanin can only be removed from the membrane by mild treatment with detergents. This observation indicates that halocyanin is a peripheral membrane protein which serves as a mobile electron carrier. The presence of type I blue copper proteins is archaebacteria might lead to further insights into their phylogenetic origin.

Randomized, double-blind, placebo-controlled, phase III study of recombinant human granulocyte-macrophage colony-stimulating factor as adjunct to induction treatment of high-grade malignant non-Hodgkin's lymphomas

We evaluated recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; Sandoz Pharma [Basel, Switzerland]/Schering-Plough [Kenilworth, NJ]) as an adjunct to a modified (mainly cyclophosphamide and doxorubicin increased 1.5-fold) COP-BLAM regimen in the primary treatment of high-grade malignant non-Hodgkin's lymphomas (NHL). Patients (n = 182; stage II-IV; age, 15 to 73 years) were randomized to rhGM-CSF (400 micrograms) or placebo for 7 days subcutaneously after chemotherapy. Efficacy was analyzed for patients receiving at least 70% of study medication (n = 125). The frequency of clinically relevant infection was reduced by rhGM-CSF (28 v 69 infections, 16 v 30 patients, P = .02) with a cumulative probability of remaining infection free in 70% versus 48% (P = .05 log rank test at 190 days). Periods of neutropenia (P = .01 in 5 of 6 courses), days with fever (2.1 v 4.0, P = .04) and days of hospitalization for infection (3.5 v 8.0 days, P = .01) were significantly reduced. Complete response (CR) rates, assessed by prognostic risk, were 15 of 19 (79%) in treated versus 20 of 21 (95%) in controls in the low-risk group (P = .12). In the high-risk group, 31 of 45 (69%) treated patients achieved CR versus 25 of 52 (48%) of controls (P = .04). No difference in survival has been seen after 1 year. Only injection site reactions (45% treated v 7% controls) and rash (26% v 2%) occurred more frequently in treated patients (n = 176). These data show that rhGM-CSF is well tolerated in most patients with NHL, significantly reduces infection, and improves response.

Aspartic acid-212 of bacteriorhodopsin is ionized in the M and N photocycle intermediates: an FTIR study on specifically 13C-labeled reconstituted purple membranes

Purple membrane was regenerated from the denatured proteolytic (protease V8) fragments V-1 and V-2 of bacteriorhodopsin (BR), native membrane lipids, and all-trans-retinal. FTIR difference spectra of M and N intermediates of the reconstituted system are in close correspondence to those obtained from native BR. Asp-212 is the only internal aspartic acid in the V-2 fragment (helices F and G). Reconstituting a V-2 fragment from a [4-13C]Asp-labeled BR preparation with an unmodified V-1 fragment and vice versa have allowed us to assign IR bands to either Asp-212 or any of the remaining aspartic acids on V-1 (helices A-E). A carboxylate vibration at 1392 cm-1 has been identified in the M and N intermediates and assigned to Asp-212. Since no contribution of this residue to C = O stretches of protonated carboxyl groups was detected, Asp-212 must be ionized in light-adapted BR as well. The effect of [4-13C]Asp labeling of V-1 revealed a carboxylate vibration at 1385 cm-1 in light-adapted BR. Since Asp-96 and Asp-115 are protonated, this band is caused by Asp-85. All absorption changes of C = O stretches of protonated carboxyl groups are due to Asp residues on V-1. Correspondingly, the proton acceptor for Schiff base deprotonation in M is located on V-1, and must be Asp-85 (the only ionized Asp on V-1). The band assignments are compared with those reported for BR mutants, and the potential role of Asp-212 for proton translocation is discussed.