Determination of Protonation and Deprotonation Forms and Tautomeric States of Histidine Residues in Large Proteins Using Nitrogen−Carbon J Couplings in Imidazole Ring

Determination of Histidine Protonation States in Proteins by Fast Magic Angle Spinning NMR

Histidine residues play important structural and functional roles in proteins, such as serving as metal-binding ligands, mediating enzyme catalysis, and modulating proton channel activity. Many of these activities are modulated by the ionization state of the imidazole ring. Here we present a fast MAS NMR approach for the determination of protonation and tautomeric states of His at frequencies of 40-62 kHz. The experiments combine 1H detection with selective magnetization inversion techniques and transferred echo double resonance (TEDOR)-based filters, in 2D heteronuclear correlation experiments. We illustrate this approach using microcrystalline assemblies of HIV-1 CACTD-SP1 protein.

Novel Regeneration Approach for Creating Reusable FO-SPR Probes with NTA Surface Chemistry

To date, surface plasmon resonance (SPR) biosensors have been exploited in numerous different contexts while continuously pushing boundaries in terms of improved sensitivity, specificity, portability and reusability. The latter has attracted attention as a viable alternative to disposable biosensors, also offering prospects for rapid screening of biomolecules or biomolecular interactions. In this context here, we developed an approach to successfully regenerate a fiber-optic (FO)-SPR surface when utilizing cobalt (II)-nitrilotriacetic acid (NTA) surface chemistry. To achieve this, we tested multiple regeneration conditions that can disrupt the NTA chelate on a surface fully saturated with His6-tagged antibody fragments (scFv-33H1F7) over ten regeneration cycles. The best surface regeneration was obtained when combining 100 mM EDTA, 500 mM imidazole and 0.5% SDS at pH 8.0 for 1 min with shaking at 150 rpm followed by washing with 0.5 M NaOH for 3 min. The true versatility of the established approach was proven by regenerating the NTA surface for ten cycles with three other model system bioreceptors, different in their size and structure: His6-tagged SARS-CoV-2 spike fragment (receptor binding domain, RBD), a red fluorescent protein (RFP) and protein origami carrying 4 RFPs (Tet12SN-RRRR). Enabling the removal of His6-tagged bioreceptors from NTA surfaces in a fast and cost-effective manner can have broad applications, spanning from the development of biosensors and various biopharmaceutical analyses to the synthesis of novel biomaterials.

Expanding a Portfolio of (FO-) SPR Surface Chemistries with the Co(III)-NTA Oriented Immobilization of His6-Tagged Bioreceptors for Applications in Complex Matrices

Cobalt-nitrilotriacetic acid (Co(III)-NTA) chemistry is a recognized approach for oriented patterning of His₆-tagged bioreceptors. We have applied the matching strategy for the first time on a surface plasmon resonance (SPR) platform, namely, the commercialized fiber optic (FO)-SPR. To accomplish this, His₆-tagged bioreceptor (scFv-33H1F7) and its target PAI-1 were used as a model system, after scrutinizing the specificity of their interaction. When benchmarked to traditional carboxyl-based self-assembled monolayers (SAM), NTA allowed (1) more efficient FO-SPR surface coverage with bioreceptors compared with the former and (2) realization of thus far difficult-to-attain label-free bioassays on the FO-SPR platform in both buffer and 20-fold diluted human plasma. Moreover, Co(III)-NTA surface proved to be compatible with traditional gold nanoparticle-mediated signal amplification in the buffer as well as in 10-fold diluted human plasma, thus expanding the dynamic detection range to low ng/mL. Both types of bioassays revealed that scFv-33H1F7 immobilized on the FO-SPR surface using different concentrations (20, 10, or 5 μg/mL) had no impact on the bioassay sensitivity, accuracy, or reproducibility despite the lowest concentration effectively resulting in close to 20% fewer bioreceptors. Collectively, these results highlight the importance of Co(III)-NTA promoting the oriented patterning of bioreceptors on the FO-SPR sensor surface for securing robust and sensitive bioassays in complex matrices, both in label-free and labeled formats.

13C Chemical Shifts in Proteins: A Rich Source of Encoded Structural Information

Despite the formidable progress in Nuclear Magnetic Resonance (NMR) spectroscopy, quality assessment of NMR-derived structures remains as an important problem. Thus, validation of protein structures is essential for the spectroscopists, since it could enable them to detect structural flaws and potentially guide their efforts in further refinement. Moreover, availability of accurate and efficient validation tools would help molecular biologists and computational chemists to evaluate quality of available experimental structures and to select a protein model which is the most suitable for a given scientific problem. The ¹³C^α nuclei are ubiquitous in proteins, moreover, their shieldings are easily obtainable from NMR experiments and represent a rich source of encoded structural information that makes ¹³C^α chemical shifts an attractive candidate for use in computational methods aimed at determination and validation of protein structures. In this chapter, the basis of a novel methodology of computing, at the quantum chemical level of theory, the ¹³C^α shielding for the amino acid residues in proteins is described. We also identify and examine the main factors affecting the ¹³C^α-shielding computation. Finally, we illustrate how the information encoded in the ¹³C chemical shifts can be used for a number of applications, viz., from protein structure prediction of both α-helical and β-sheet conformations, to determination of the fraction of the tautomeric forms of the imidazole ring of histidine in proteins as a function of pH or to accurate detection of structural flaws, at a residue-level, in NMR-determined protein models.

The Biophysical Probes 2-fluorohistidine and 4-fluorohistidine: Spectroscopic Signatures and Molecular Properties

Fluorinated amino acids serve as valuable biological probes, by reporting on local protein structure and dynamics through ¹⁹F NMR chemical shifts. 2-fluorohistidine and 4-fluorohistidine, studied here with DFT methods, have even more capabilities for biophysical studies, as their altered pK_a values, relative to histidine, allow for studies of the role of proton transfer and tautomeric state in enzymatic mechanisms. Considering the two tautomeric forms of histidine, it was found that 2-fluorohistidine primarily forms the common (for histidine) τ-tautomer at neutral pH, while 4-fluorohistidine exclusively forms the less common π-tautomer. This suggests the two isomers of fluorohistidine can also serve as probes of tautomeric form within biomolecules, both by monitoring NMR chemical shifts and by potential perturbation of the tautomeric equilibrium within biomolecules. Fluorine also enables assignment of tautomeric states in crystal structures. The differences in experimental pK_a values between the isomers was found to arise from solvation effects, providing insight into the polarization and molecular properties of each isomer. Results also encompass ¹³C and ¹⁹F NMR chemical shifts, from both tautomers of 2-fluorohistidine and 4-fluorohistidine in a number of different environments. This work can serve as a guide for interpretation of spectroscopic results in biophysical studies employing 2-fluorohistidine and 4-fluorohistidine.

NMR studies of active-site properties of human carbonic anhydrase II by using (15) N-labeled 4-methylimidazole as a local probe and histidine hydrogen-bond correlations

By using a combination of liquid and solid-state NMR spectroscopy, (15) N-labeled 4-methylimidazole (4-MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF3 /CDF2 Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active-site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4-MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy. The possible hydrogen-bond clusters formed and the mechanism of the tautomerism are discussed. Information about the imidazole hydrogen-bond geometries is obtained by establishing a correlation between published (1) H and (15) N chemical shifts of the imidazole rings of histidines in proteins. This correlation is useful to distinguish histidines embedded in the interior of proteins and those at the surface, embedded in water. Moreover, evidence is obtained that the hydrogen-bond geometries of His64 in the active site of HCA II and of 4-MI in H64A HCA II are similar. Finally, the degeneracy of the rapid tautomerism of the neutral imidazole ring His64 reported by Shimahara et al. (J. Biol. Chem.- 2007, 282, 9646) can be explained with a wet, polar, nonaqueous active-site conformation in the inward conformation, similar to the properties of 4-MI in the Freon solution. The biological implications for the enzyme mechanism are discussed.

pH-dependent random coil (1)H, (13)C, and (15)N chemical shifts of the ionizable amino acids: a guide for protein pK a measurements

The pK a values and charge states of ionizable residues in polypeptides and proteins are frequently determined via NMR-monitored pH titrations. To aid the interpretation of the resulting titration data, we have measured the pH-dependent chemical shifts of nearly all the (1)H, (13)C, and (15)N nuclei in the seven common ionizable amino acids (X = Asp, Glu, His, Cys, Tyr, Lys, and Arg) within the context of a blocked tripeptide, acetyl-Gly-X-Gly-amide. Alanine amide and N-acetyl alanine were used as models of the N- and C-termini, respectively. Together, this study provides an essentially complete set of pH-dependent intra-residue and nearest-neighbor reference chemical shifts to help guide protein pK a measurements. These data should also facilitate pH-dependent corrections in algorithms used to predict the chemical shifts of random coil polypeptides. In parallel, deuterium isotope shifts for the side chain (15)N nuclei of His, Lys, and Arg in their positively-charged and neutral states were also measured. Along with previously published results for Asp, Glu, Cys, and Tyr, these deuterium isotope shifts can provide complementary experimental evidence for defining the ionization states of protein residues.

2D IR spectroscopy of histidine: probing side-chain structure and dynamics via backbone amide vibrations

It is well known that histidine is involved in many biological functions due to the structural versatility of its side chain. However, probing the conformational transitions of histidine in proteins, especially those occurring on an ultrafast time scale, is difficult. Herein we show, using a histidine dipeptide as a model, that it is possible to probe the tautomer and protonation status of a histidine residue by measuring the two-dimensional infrared (2D IR) spectrum of its amide I vibrational transition. Specifically, for the histidine dipeptide studied, the amide unit of the histidine gives rise to three spectrally resolvable amide I features at approximately 1630, 1644, and 1656 cm(-1), respectively, which, based on measurements at different pH values and frequency calculations, are assigned to a τ tautomer (1630 cm(-1) component) and a π tautomer with a hydrated (1644 cm(-1) component) or dehydrated (1656 cm(-1) component) amide. Because of the intrinsic ultrafast time resolution of 2D IR spectroscopy, we believe that the current approach, when combined with the isotope editing techniques, will be useful in revealing the structural dynamics of key histidine residues in proteins that are important for function.

13C Chemical Shifts in Proteins: A Rich Source of Encoded Structural Information

Despite the formidable progress in Nuclear Magnetic Resonance (NMR) spectroscopy, quality assessment of NMR-derived structures remains as an important problem. Thus, validation of protein structures is essential for the spectroscopists, since it could enable them to detect structural flaws and potentially guide their efforts in further refinement. Moreover, availability of accurate and efficient validation tools would help molecular biologists and computational chemists to evaluate quality of available experimental structures and to select a protein model which is the most suitable for a given scientific problem. The ¹³C^α nuclei are ubiquitous in proteins, moreover, their shieldings are easily obtainable from NMR experiments and represent a rich source of encoded structural information that makes ¹³C^α chemical shifts an attractive candidate for use in computational methods aimed at determination and validation of protein structures. In this chapter, the basis of a novel methodology of computing, at the quantum chemical level of theory, the ¹³C^α shielding for the amino acid residues in proteins is described. We also identify and examine the main factors affecting the ¹³C^α-shielding computation. Finally, we illustrate how the information encoded in the ¹³C chemical shifts can be used for a number of applications, viz., from protein structure prediction of both α-helical and β-sheet conformations, to determination of the fraction of the tautomeric forms of the imidazole ring of histidine in proteins as a function of pH or to accurate detection of structural flaws, at a residue-level, in NMR-determined protein models.

Limiting values of the 15N chemical shift of the imidazole ring of histidine at high pH

Tautomeric identification by direct observation of (15)N chemical shifts of the imidazole ring of histidine (His) has become a common practice in NMR spectroscopy. However, such applications require knowledge of the "canonical" limiting values of the (15)N chemical shift of the imidazole ring of His in which each form of His, namely, the protonated (H(+)) and the tautomeric N(ε2)-H and N(δ1)-H forms, respectively, is present to the extent of 100%. So far, the adopted canonical limiting values of the (15)N chemical shift have been those available from model compounds. Whether these canonical values reflect those of the individual pure forms of His is investigated here by carrying out an analysis of the second-order shielding differences, ΔΔ = |Δ(ε) - Δ(δ) |, with Δ(ξ) (ξ = ε or δ) being the density functional theory (DFT)-computed average shielding differences between the two nitrogens of the imidazole ring of His in each pure tautomeric form. In the high-pH limit, the results indicate that (i) the ΔΔ values from the DFT-computed shielding, but not from the commonly used canonical limiting values, are in closer agreement with those obtained with experimental chemical shift data from model compounds in solution and solid-state NMR; and (ii) the commonly used canonical limiting values of the (15)N chemical shifts lead to an average tautomeric equilibrium constant that differs by a factor of ∼2.6 from the one computed by using DFT-based (15)N limiting values, raising concern about the practice of using canonical limiting (15)N values. This can be avoided by reporting tautomeric equilibrium constants computed by using only limiting (15)N values for the N(ε2)-H tautomer.

Assessing the fractions of tautomeric forms of the imidazole ring of histidine in proteins as a function of pH

A method is proposed to determine the fraction of the tautomeric forms of the imidazole ring of histidine in proteins as a function of pH, provided that the observed and chemical shifts and the protein structure, or the fraction of H(+) form, are known. This method is based on the use of quantum chemical methods to compute the (13)C NMR shieldings of all the imidazole ring carbons ((13)C(γ), , and ) for each of the two tautomers, N(δ1)-H and N(ε2)-H, and the protonated form, H(+), of histidine. This methodology enabled us (i) to determine the fraction of all the tautomeric forms of histidine for eight proteins for which the and chemical shifts had been determined in solution in the pH range of 3.2 to 7.5 and (ii) to estimate the fraction of tautomeric forms of eight histidine-containing dipeptide crystals for which the chemical shifts had been determined by solid-state (13)C NMR. Our results for proteins indicate that the protonated form is the most populated one, whereas the distribution of the tautomeric forms for the imidazole ring varies significantly among different histidines in the same protein, reflecting the importance of the environment of the histidines in determining the tautomeric forms. In addition, for ∼70% of the neutral histidine-containing dipeptides, the method leads to fairly good agreement between the calculated and the experimental tautomeric form. Coexistence of different tautomeric forms in the same crystal structure may explain the remaining 30% of disagreement.

Solution structure of the parvulin-type PPIase domain of Staphylococcus aureus PrsA--implications for the catalytic mechanism of parvulins

Background

Staphylococcus aureus is a Gram-positive pathogenic bacterium causing many kinds of infections from mild respiratory tract infections to life-threatening states as sepsis. Recent emergence of S. aureus strains resistant to numerous antibiotics has created a need for new antimicrobial agents and novel drug targets. S. aureus PrsA is a membrane associated extra-cytoplasmic lipoprotein which contains a parvulin-type peptidyl-prolyl cis-trans isomerase domain. PrsA is known to act as an essential folding factor for secreted proteins in Gram-positive bacteria and thus it is a potential target for antimicrobial drugs against S. aureus.

Results

We have solved a high-resolution solution structure of the parvulin-type peptidyl-prolyl cis-trans isomerase domain of S. aureus PrsA (PrsA-PPIase). The results of substrate peptide titrations pinpoint the active site and demonstrate the substrate preference of the enzyme. With detailed NMR spectroscopic investigation of the orientation and tautomeric state of the active site histidines we are able to give further insight into the structure of the catalytic site. NMR relaxation analysis gives information on the dynamic behaviour of PrsA-PPIase.

Conclusion

Detailed structural description of the S. aureus PrsA-PPIase lays the foundation for structure-based design of enzyme inhibitors. The structure resembles hPin1-type parvulins both structurally and regarding substrate preference. Even though a wealth of structural data is available on parvulins, the catalytic mechanism has yet to be resolved. The structure of S. aureus PrsA-PPIase and our findings on the role of the conserved active site histidines help in designing further experiments to solve the detailed catalytic mechanism.

The low energy tautomers and conformers of the dipeptides HisGly and GlyHis and of their sodium ion complexes in the gas phase

The low-lying conformers of the dipeptides HisGly and GlyHis, and of their sodium cation complexes, have been studied with a combination of Monte Carlo search with the Amber force field and local geometry optimization at the ab initio HF/6-31G(d) level, completed with MP2(full)/6-311+G(2d,2p) energetics at the HF/6-31G(d) geometries. For each dipeptide, both the N(delta)-H and N(epsilon)-H tautomers of the imidazole side chain of His were considered. For each of the four isomeric dipeptides, 20-30 conformers were fully characterized at the ab initio level. All low energy structures are found to involve H-bonding at the N(delta) position of imidazole, either as a N-H donor or a N acceptor, depending upon the tautomer. In three out of the four species, the most stable conformer involves a C-terminus carboxylic acid in its less favorable trans conformation, in order to maximize intramolecular H bonding. It turns out that the lowest energy tautomer of HisGly is N(epsilon)-H, while that of GlyHis is N(delta)-H. This result argues in favor of the diversity of His tautomeric states in peptides and proteins. The sodium cation complexes of both GlyHis and HisGly have been studied as well, again considering both tautomers in each case. In three out of the four species, the most stable structure involves chelation of sodium by the two carbonyl oxygens and the imidazole ring. On the contrary, the sodium complex of the N(delta)-H tautomer of HisGly favors chelation to the peptidic carbonyl oxygen, the imidazole ring and the amino terminus. In the N(epsilon)-H tautomers of both peptides, the most favorable binding site of imidazole is the N(delta) nitrogen, while in the N(delta)-H tautomers, it is the pi cloud which provides side chain interaction. As a result, both GlyHisNa+ and HisGlyNa+ favor the N(epsilon)-H tautomer of His, in contrast to what was found for the free peptides.

A Solid State 13C NMR, Crystallographic, and Quantum Chemical Investigation of Chemical Shifts and Hydrogen Bonding in Histidine Dipeptides

We report the first solid-state NMR, crystallographic, and quantum chemical investigation of the origins of the 13C NMR chemical shifts of the imidazole group in histidine-containing dipeptides. The chemical shift ranges for Cgamma and Cdelta2 seen in eight crystalline dipeptides were very large (12.7-13.8 ppm); the shifts were highly correlated (R2= 0.90) and were dominated by ring tautomer effects and intermolecular interactions. A similar correlation was found in proteins, but only for buried residues. The imidazole 13C NMR chemical shifts were predicted with an overall rms error of 1.6-1.9 ppm over a 26 ppm range, by using quantum chemical methods. Incorporation of hydrogen bond partner molecules was found to be essential in order to reproduce the chemical shifts seen experimentally. Using AIM (atoms in molecules) theory we found that essentially all interactions were of a closed shell nature and the hydrogen bond critical point properties were highly correlated with the N...H...O (average R2= 0.93) and Nepsilon2...H...N (average R2= 0.98) hydrogen bond lengths. For Cepsilon1, the 13C chemical shifts were also highly correlated with each of these properties (at the Nepsilon2 site), indicating the dominance of intermolecular interactions for Cepsilon1. These results open up the way to analyzing 13C NMR chemical shifts, tautomer states (from Cdelta2, Cepsilon1 shifts), and hydrogen bond properties (from Cepsilon1 shifts) of histidine residue in proteins and should be applicable to imidazole-containing drug molecules bound to proteins, as well.

Triple-resonance methods for complete resonance assignment of aromatic protons and directly bound heteronuclei in histidine and tryptophan residues

A set of three experiments is described which correlate aromatic resonances of histidine and tryptophan residues with amide resonances in 13C/15N-labelled proteins. Provided that backbone 1H and 15N positions of the sequentially following residues are known, this results in sequence-specific assignment of histidine 1H(delta2)/13C(delta2) and 1H(epsilon1)/13C(epsilon1) as well as tryptophan 1H(delta1)/13C(delta1), 1H(zeta2)/13C(zeta2), 1H(eta2)/13C(eta2), 1H(epsilon3)/13C(epsilon3), 1H(zeta3)/13C(zeta3) and 1H(epsilon1)/15N(epsilon1) chemical shifts. In the reverse situation, these residues can be located in the 1H-(15)N correlation map to facilitate backbone assignments. It may be chosen between selective versions for either of the two amino acid types or simultaneous detection of both with complete discrimination against phenylalanine or tyrosine residues in each case. The linkages between delta-proton/carbon and the remaining aromatic as well as backbone resonances do not rely on through-space interactions, which may be ambiguous, but exclusively employ one-bond scalar couplings for magnetization transfer instead. Knowledge of these aromatic chemical shifts is the prerequisite for the analysis of NOESY spectra, the study of protein-ligand interactions involving histidine and tryptophan residues and the monitoring of imidazole protonation states during pH titrations. The new methods are demonstrated with five different proteins with molecular weights ranging from 11 to 28 kDa.

Solution Structure of a CCHHC Domain of Neural Zinc Finger Factor-1 and Its Implications for DNA Binding

The structure of a CCHHC zinc-binding domain from neural zinc finger factor-1 (NZF-1) has been determined in solution though the use of NMR methods. This domain is a member of a family of domains that have the Cys-X(4)-Cys-X(4)-His-X(7)-His-X(5)-Cys consensus sequence. The structure determination reveals a novel fold based around a zinc(II) ion coordinated to three Cys residues and the second of the two conserved His residues. The other His residue is stacked between the metal-coordinated His residue and a relatively conserved aromatic residue. Analysis of His to Gln sequence variants reveals that both His residues are required for the formation of a well-defined structure, but neither is required for high-affinity metal binding at a tetrahedral site. The structure suggests that a two-domain protein fragment and a double-stranded DNA binding site may interact with a common two-fold axis relating the two domains and the two half-sites of the DNA-inverted repeat.

Cu(II)Gly2HisGly oxidation by H2O2/ascorbic acid to the CuIII complex and its subsequent decay to alkene peptides

Protonation and stability constants for Gly2HisGly and its Cu(II) complexes (beta(mhl)), determined at 25.0 degrees C and mu = 0.10 M (NaClO(4)), have values of log beta(011) = 7.90, log beta(021) = 14.51, log beta(031) = 17.55, log beta(101) = 7.82, log beta(1-21) = -0.80, and log beta(1-31) = -12.7 (where the subscripts refer to the number of metal ions, protons, and ligands, respectively). The reaction of CuII(H(-2)Gly(2)HisGly)- with l-ascorbic acid and H(2)O(2) at p[H(+)] 6.6 rapidly generates Cu(III)(H(-2)Gly(2)HisGly) with lambda(max) at 260 and 396 nm, which is separated chromatographically. The Cu(III) complex decomposes to give alkene peptide isomers of glycylglycyl-alpha,beta-dehydrohistidylglycine that are separated chromatographically and characterized. These alkene peptide species are present as geometric isomers with imidazole tautomers that have distinct spectral and chemical characteristics. The proposed major isomeric form (94%), Z-N(tau)-H, has a hydrogen on the tau nitrogen of the imidazole ring and three conjugated double bonds. It has absorption bands at 295 and 360 nm at p[H+] 6.6 in the absence of copper. The proposed minor isomeric form (6%) is E-N(pi)-H with a hydrogen on the pi nitrogen of the imidazole ring. This isomer has four conjugated double bonds and strongly binds Cu(II) to give a complex with intense absorption bands at 434 and 460 nm.

Quantitative identification of the protonation state of histidines in vitro and in vivo

The C[bond]N coupling constants centered at the C(epsilon 1) and C(delta 2) carbons in histidine residues depend on the protonation state and tautomeric form of the imidazole ring, making them excellent indicators of pH or pK(a), and the ratio of the tautomeric states. In this paper, we demonstrate that the intensity ratios for the C(epsilon 1)-H and C(delta 2)-H cross-peaks measured with a constant time HSQC experiment without and with J(C[bond]N) amplitude modulation are determined by the ratios of the protonated and deprotonated forms and tautomeric states. This allows one to investigate the tautomeric state of histidines as well as their pK(a) in situations where changing the pH value by titration is difficult, for example, for in-cell NMR experiments. We apply this technique to the investigation of the bacterial protein NmerA and determine that the intracellular pH in the Escherichia coli cytoplasm is 7.1 +/- 0.1.

pH and pK determinations by high-resolution solid-state 13C NMR: acid-base and tautomeric equilibria of lyophilized L-histidine

Acid-base properties of lyophilized powders of L-histidine have been systematically investigated using parent solutions at pH varying from 1.8 to 10. For the first time, high-resolution solid-state 13C NMR was shown to allow separate observation of all three acid-base pairs in the successive deprotonations of the carboxylic end, the imidazolium cation, and the terminal ammonio group of histidine. 1H CRAMPS NMR spectra directly visualize the absence of the N3-H(pi) tautomer in neutral and anionic species. Solid-state titration shifts are enlarged by approximately 1-4 ppm with respect to those measured in solution, permitting unambiguous observation of conjugate acid-base pairs. Calculated pK's from solid-state acid-to-base ratios r are found equal to those classically measured in solution at 0 degrees C with a similar ionic strength of 0.1 mol x dm(-3). This proves that natural-abundance 13C solid-state determinations of r can be used to measure pK's in parent solutions without recourse to full titration curves and subsequent curve-fitting procedures. Such an approach also leads to noninvasive characterizations of the acidity of lyophilized powders, i.e., to the prediction of in situ pH of products obtained after rehydration and solubilization of powders. These results show the possibility of measuring the pK of nonvolatile acidic substrates dissolved in any sublimable solvent through lyophilization of the investigated solutions; this leads the way to pH and pK determinations when electrochemical or spectrophotometric measurements are impossible or ambiguous, e.g., for concentrated solutions, polyacids, or mixtures of acidic solutes, and possibly to the establishment of pK scales in nonaqueous solvents and in melts.