pKa shifts for protonation-dependent degrees of freedom

Raupp-Kossmann, Robert A.; Scharnagl, Christina

doi:10.1016/s0009-2614(01)00062-8

Cited by 34 publications

(9 citation statements)

References 11 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Calculated ground-state pK a 's are in excellent agreement with experiment and confirm the assignment of pK a ) 2.0 to the cation/neutral transition and pK a ) 8.3 to the neutral/anion transition. Together with the results from our previous studies, 21,37 we conclude that, besides electrostatic interactions, pK a shifts associated with entropy contributions due to selective restriction of internal rotational degrees of freedom are vital determinants to ensure functionality of chromophoric groups in proteins such as GFP, bacteriorhodopsin, photosynthetic bile pigments, or sensory pigments.…”

Section: Discussionsupporting

confidence: 83%

“…Table 4). Second, an extended thermodynamic scheme 37 has to account for entropy contributions to the dissociation free energies due to distinct conformational flexibility of the two ring-bridging bonds mediated by the protonation state. The presence of vanishingly small rotational barriers for ionization states with protonated phenolic hydroxyl implies that the harmonic oscillator approximation used in standard thermochemistry 34 will produce incorrect results for the entropy of the internal rotation modes.…”

Section: Reliability and Limits Of Methodsmentioning

confidence: 99%

“…The selective restriction of motion along these internal degrees of freedom upon binding of HBI to the apoprotein in GFP is therefore a dominant contribution to the adjustment of microscopic pK a 's in protein, as discussed in our previous contribution. 37 4.3. Excited-State: Deprotonation or Tautomerization Competing with Isomerization?…”

Section: Contributions To Ground-state Equilibriamentioning

confidence: 99%

“…The direction of the shift is in accord with the reduction of the rotational space upon deprotonation, leading to negative dissociation entropy. 37 The two transitions in the pH-titration of HBI correspond to the cation/neutral equilibrium in the acidic pH-region (pK R ) 2.0) and the neutral/anion equilibrium (pK β ) 8.3). These values calculated for the flexible chromophore are in excellent agreement with experimental pK a 's published recently (1.8 and 8.1).…”

Section: Microscopic Acid-base Equilibria Of Hbi In the Ground Statementioning

confidence: 99%

“…In contrast to the case of the test molecules, the presence of the two ring-bridging bonds of HBI requires us to take into account both dependence of the rotational barriers on the protonation state and geometric relaxation upon solvation. For the smooth torsional energy profiles of HBI, , the harmonic oscillator approximation used in standard thermochemistry will produce incorrect results for the entropy of the internal rotation modes. , Therefore, in the fourth step, the configuration integral was evaluated numerically for the coupled two-dimensional rotor profile and allowed the estimation of the protonation entropies related to these internal degrees of freedom. In the fifth step, a Förster cycle approach 17 was used to estimate p K a shifts upon transition to the first excited singlet (S1) state.…”

Section: Introductionmentioning

confidence: 99%

See 4 more Smart Citations

Solution pK_a Values of the Green Fluorescent Protein Chromophore from Hybrid Quantum-Classical Calculations

Scharnagl

Raupp-Kossmann

2003

J. Phys. Chem. B

View full text Add to dashboard Cite

We present a theoretical study of the four aqueous microscopic dissociation constants relating the relevant protonation forms (cation, neutral, anion, zwitterion) of the chromophore of the green fluorescent protein (GFP) in the ground and excited states. To take the protonation-state-dependent torsional flexibility around the ring-bridging bonds into account, configuration integrals in the torsion space were evaluated to yield the free energy differences. Conformational energies were calculated within a semiempirical quantum chemical scheme using a continuum solvation model. After establishing a linear regression of experimental aqueous pK a 's and calculated enthalpy differences for a series of reference molecules with phenolic hydroxyl or imino nitrogen, the applied method is able to reproduce the titration behavior of bifunctional groups within an average error of 0.8 pK-units. The calculated values for the GFP chromophore agree very well (deviation < 0.2 pK-units) with known ground-state aqueous pK a 's and confirm the equilibrium between neutral and anionic forms (pK a ) 8.3). Freezing torsional flexibility shifts this pK a to 6.6sin accordance with entropic contributions due to the enlarged configurational space of the protonated compoundsand is, therefore, a dominant contribution to the adjustment of pK a 's in the protein. Estimates for the excited-state pK a 's were derived from vertical excitation energies under the assumption that the protolytic equilibria are faster than conformational relaxation of solute and solventscorresponding to a recent experimental model of excited-state proton transfer in GFP. The calculations reveal that increase of both the acidity of the phenolic oxygen and the basicity of the heterocycle nitrogen works in synergy. The neutral form becomes a strong photoacid and transfers a proton with a pK a * ) 0.1; the imino-N becomes a strong photobase capable of accepting a proton with a pK a * ) 8.9. Provided an extended hydrogen-bonded network links the two functions, phototautomerization might be a possible decay pathway.

show abstract

Section: Discussionsupporting

confidence: 83%

Section: Reliability and Limits Of Methodsmentioning

confidence: 99%

Section: Contributions To Ground-state Equilibriamentioning

confidence: 99%

Section: Microscopic Acid-base Equilibria Of Hbi In the Ground Statementioning

confidence: 99%