The synthesis of the mixture of the two diastereomers of guanosine 2 ',3 '-cyclophosphorothioate is described. Their chemical shifts in the 31P nuclear magnetic resonance (nmr) are identical with those of the corresponding isomers of uridine 2',3'-cyclophosphorothioate ( -75.22 ppm for the endo, -76.75 ppm for the exo isomer). From the unseparable mixture of isomers of guanosine 2 ',3 '-cyclophosphorothioate only the endo isomer is hydrolyzed by ribonuclease Ti to guanosine 3 '-phosphorothioate, without loss of sulfur.The resistant exo isomer is a competitive inhibitor ( = 2.5 X 10-4 m) for this enzyme. On enzymatic methanolysis of the mixture of isomers only the endo isomer reacts to r> X Vábonuclease Ti hydrolyzes guanosine 3 '-phosphate esters in a two-step mechanism to guanosine 3'-phosphate with guanosine 2',3'-cyclophosphate as intermediate. Apart from the specificity for guanosine instead of pyrimidine nucleosides it resembles pancreatic ribonuclease A in the overall mechanism. In contrast to pancreatic ribonuclease A, how-PHA-630 T, a glass electrode G-2222c, a calomel electrode K-401 and an autoburet ABU-la. The consumption of base was recorded on a titrigraph SBR-2c. The temperature was held at 30°. The enzyme was used as a 9.6 X 10-4 m solution in 0.2 m NaCl adjusted to pH 7.0. Substrate and inhibitor were also dissolved in 0.2 m NaCl. Microliter quantities of substrate and, where needed, inhibitor solutions were added to 2 ml of 0.2 m NaCl solution (pH 7.0), and the pH was kept at 7.0 with 0.01 n KOH for 1 hr. The reaction was then started by addibiochemistry, vol.

Individual haemoglobin I11 from Chironomus thummi thummi is a monomeric protein ( M , = 15500) which has recently been shown by us to be characterized by a simple Bohr effect. Three of the four histidines, the position of which in the polypeptide chain is known from sequence analysis, are distinguished by their titration characteristics, namely His-G2, His-GI9 and His-E5. The chemical shift of the C-2 proton resonances of the imidazole groups, which varies upon protonation of the histidine, has been used for the determination of the intrinsic p K values. His-F8, the proximally bound histidine, is not observable because its C-2 proton resonance is broadened.One of the three titratable histidines reveals a pK value which is varied in the range from 7.5 to 7.1, depending on the 6th ligand a t the haem iron. The relatively high pK value of this histidine and its abnormal value of the chemical shift of the C-2 proton resonance a t low pH indicate an interaction with a carboxylate group. This shift of the pK value caused by the binding of CO to reduced haemoglobin is in agreement with the interaction energy, calculated from the amplitude of the Bohr effect curve, and corresponds to 500 cal.I n this simple case the assignment of the C-2 proton resonances to the known histidines is possible considering the atomic model, described by Huber et al. [4]. The results indicate that His-G2 is the allosteric binding site; the remaining two detectable histidines are not involved in the allosteric interaction. The molecular mechanism of the Bohr effect in this haemoglobin is discussed.Contrary to the mammalian haemoglobins, the larval haemoglobins of the insect Chironomus thummi thummi are characterized by a simple, alkaline Bohr effect [I]. It is of special interest that two of these haemoglobins, namely individual haemoglobins I11 and IV, which are monomeric in the range of pH 5 to 10, also show this Bohr effect Recently we reported results of proton magnetic resonance studies of histidine residues of horse myoglobin [5]. I n the case of myoglobin nine of the eleven histidine C-2 proton magnetic resonances have been resolved and from the titration curves the intrinsic p K values of these histidines have been determined. I n this communication the results of a titration of the histidine residues of haemoglobin I11 are described. This haemoglobin has a molecular weight of 15800 [3] and a tertiary structure quite similar Abbreviations. NMR, nuclear magnetic resonance; Me,Si, tetramethylailane.to that of myoglobin [4]; however, only four histidines appear in the sequence : His-F8, His-G2, His-E5, and His-G19. This small number of histidines allows a more exact determination and assignment of the intrinsic pK values and of their possible variation in the case of conformational changes induced by ligand binding. Such a conformational change is re%ected by the Bohr effect.The Bohr effect describes a negative interaction between the 0,-binding site and proton-binding sites in the mammalian haemoglobins 16-81. The identification of the ...

In continuation of earlier work new results on the behaviour of certain proton resonances of pancreatic ribonuclease A and of nucleotides during complex formation are reported. By varying the structure of the base or the position of the phosphate group at the ribose moiety it can be shown that the main structural feature of the ribonuclease · pyrimidine‐nucleotide complexes is maintained. However, the microscopic binding constants of the interactions of the various parts of a nucleotide with the corresponding amino acid sidegroups are different for nearly all the investigated complexes. While the modified purine base of 8‐oxoadenosine 3′‐phosphate seems to be bound in the pyrimidine binding site of ribonuclease A also, the base of the other adenosine nucleotides studied occupy other sites at the enzyme. For adenosine 3′‐ and 5′‐monophosphate this might be the binding site shown to be specific for purine bases by X‐ray crystallography. But the results obtained for the ribonuclease · adenosine‐2′‐phosphate complex suggest that the adenosine nucleotides do not bind exclusively in this purine binding site.

SynopsisThe enthalpies of the helix-coil transitions of the ordered polynucleotide systems of poly(inosinic acid)-poly(cytidy1ic acid) [poly(I + C)] , (helical duplex), and of poly (inosinic acid) [poly(I + I + I)], (proposed secondary structure: a triple-stranded helical complex), were determined by using an adiabatic twin-vessel differential calorimeter. Measuring the temperature course of the heat. capacity of the aqueous polymer solutions, the enthalpy values for the dissociation of the helical duplex poly(1 + C) and the three-stranded helical complex poly(1 + I + I), respectively, were obtained by evaluat.ing the additional heat capacity involved in t,he conformat,ional change of the polynucleotide system in the transition range. The AH values of the helix-coil t.raiisition of poly(1 + C) result,irig from t,he analysis of the calorimetric measnremeiits vary between the limits 6.5 f 0.4 kcal/mole (I + C ) and 8.4 = ! I 0.4 kcal/mole (I + C), depending on the variation of the cation concentration rauging from 0.063 mole cations/ kg H20 to 1.003 mole cationsjkg H20. The calorimetric investigation of an aqueous poly I solution (cation concentration 1.0 mole/kg HIO) yielded the enthalpy value AH = 1.9 zk 0.4 kcal/mole (I), a result which has been interpreted qualitatively following current models of inter-and intramolecular forces of biologically significant macromolecules. Additional information on the transitiott behavior of poly(1 + C ) was obtained by ultraviolet and infrared absorption measiiremetits.

Using the fast Fourier transform technique it has been possible to get readable naturalabundance carbon-13 iiuclear magnetic resonance spectra of some oligopeptides, in particular the octapeptide angiotensin 11. Abbreviations. NMR, nuclear magnetic resonance; CD, circular dichroism.Thus all evidence points to the marked influence of the environment on angiotensin I1 conformations and the desirability to more accurately define the prevailing relationships.For this purpose carbon-I3 NMR studies have been initiated. Indeed, 13C data of some amino acids (as also presented in this work) have shown that pH-titration curves, which are obtained by plotting the chemical shift values of the carbon resonances against pH, may reflect the titration of somewhat distant groups, possibly because of weak interactions between the groups or simply by a change in the local electric field charge [11,12]. Thus similar effects could be expected in the titration behaviour of peptide and proteins.Extensive titration data for angiotensin I1 and model peptides were made practicable by use of the fast Fourier transform technique. Assignments of chemical shifts to specific carbons were made on the basis of the titration behaviour of the spectral peaks and on literature data for chemical shifts of the 13C resonances of the free amino acids [i 1,13,14].The drawback in relying upon the latter data is that chemical shifts of the aliphatic carbons are displaced when peptide bonds are formed. Thus model compounds of tripeptide length and larger are preferable in making peak assignments [IS]. It is hoped that these investigations will not only help to interpret the angiotensin structure but also be useful for model studies of larger peptides.