SEQUENCING OF PEPTIDES

Abstract: A peptide is sequenced from its carboxyl terminal by coupling with an isothiocyanate having a chemical formula R( x )Si(SCN) y where x=0-3 and y=l-4. R may be an alkyl or an aryl. The coupling reaction is preferably conducted at a temperature range of 50°C-90°C for a period to approximately sixty (60) minutes or less. A weak acid anhydride, preferably a carboxylic acid anhydride, or a weak acid chloride, preferably a carboxylic acid chloride, may be mixed with the isothiocyanate. An organic base catalyst selected from a group consisting of tertiary amines including aromatic amines such as pyridine may be utilized.

Description: BACKGROUND OF THE INVENTION A. Field of the Invention This invention relates to a method of sequencing a peptide to separate in a stepwise manner the amino acids at the carboxyl terminal of the peptide for individual identification. The invention also relates to the materials used in such sequencing. B. Description of the Prior Art It is often desirable to sequence a peptide by isolating and identifying sequentially derivatives of the amino acids forming the peptide. For example, such sequencing is desirable in research to identify the order in which the amino acids are arranged in the peptides so that the peptides can be subsequently synthesized. Such knowledge can also enable the design of suitable probes to obtain gene sequences which in turn will enable production of the peptide using recombinant DNA technology. Such sequencing can thus lead to the development of materials which will combat diseases. Methods are now in use for sequencing amino acids from the carboxyl terminal and the N-terminal of peptides. The methods of sequencing peptides from the N-terminal are relatively elegant in their ease of operation, reliability, sequencing speed, sensitivity and span of sequence determination. In contrast, the present methods of sequencing amino acids from the carboxyl terminals of peptides are not nearly as well developed. For example, not all of each amino acid in the sequence is cleaved from the peptide. As a result, the amount of peptide available for the cleavage of successive amino acids becomes progressively reduced. The identification of successive amino acids in the peptide sequence accordingly becomes increasingly difficult. This limits the number of amino acids capable of being sequenced and accurately identified in the peptide. There are other serious disadvantages with the present method of sequencing amino acids from the carboxyl terminal of the peptides. For example, ionic thiocyanates are generally used to convert the peptides into an intermediate form, such as peptidyl thiohydantoin derivative, which can be processed to cleave the amino acids. Such thiocyanate reactants tend to be self-reactive at ambient temperatures and quickly lose their ability to act upon the peptide. Furthermore, even when the thiocyanate is able to act upon the peptide, the reaction temperature has to be relatively high, such as in the order of 90°C, and the reaction time has to be relatively long. The problems of sequencing amino acids from the carboxyl terminal of the peptides have been known for some time. A considerable effort has been made to develop methods of, and chemicals for, overcoming such problems. In spite of such efforts, the problems have still remained. SUMMARY OF THE INVENTION This invention provides methods of, and chemicals for, overcoming the problems specified above. In producing the peptidyl thiohydantoin derivative, the invention uses a chemical which is stable at room temperatures. Thus, such chemical can be stored for relatively long periods of time without losing its effectiveness. The chemical is effective in each step in producing a substantially complete conversion of the peptide to a peptidyl thiohydantoin derivative, thereby providing for substantially complete cleavage of the thiohydantoin derivative of the amino acid at each carboxyl position of the peptide. This facilitates the identification of the amino acid in each of the thiohydantoin derivatives and also provides for an increase in the number of the amino acids sequentially capable of being sensitively and reliably identified in a peptide. The invention is also effective in reducing the reaction temperatures and times for producing the peptidyl thiohydantoin derivative, thus reducing the experimental time required to achieve the desired information. In the method constituting this invention, peptides at their carboxyl terminals are sequenced by applying to the peptides an isothiocyanate having a chemical formula R( x )Si(SCN) y where x=0-3 and y=l-4. R may constitute an alkyl or an aryl substituent. The application of the isothiocyanate to the peptide preferably occurs in a temperature range of approximately 50°-90°C for a period to approximately sixty (60) minutes or less. A weak acid anhydride, preferably a carboxylic acid anhydride, or a weak acid chloride, preferably a carboxylic acid chloride, may be mixed with the isothiocyanate. A catalyst may also be included. The catalyst is an organic base and may be selected from a group consisting of tertiary amines including aromatic amines such as pyridine. A thiohydantoin substituent is thereby produced from the amino acid at the carboxyl terminal of the peptide. Excess reagents may then be removed by extraction with a suitable solvent. This removal is facilitated if the peptide is initially immobilized by fixation to a solid suport or entrapment within a matrix or retentive material, such as polybrene. The peptidyl thiohydantoin derivative may then be cleaved to separately provide a thiohydantoin derivative of the amino acid previously at the carboxyl terminal of the original peptide and a peptidyl residue. The process specified above may then be repeated to yield a peptidyl thiohydantoin derivative of the amino acid at the new carboxyl terminal of the peptidyl residue, and this peptidyl thiohydantoin derivative may be cleaved to separate the next amino acid in the sequence for identification. The amino acids sequentially separated from the carboxyl terminal of the peptide may be individually identified. BRIEF DESCRIPTION OF EQUATIONS Equation l illustrates the formation of a peptidyl thiohydantoin derivative by reaction of a peptide with an isothiocyanate in acetic anhydride, in the presence of a catalyst, in one step of a method constituting this invention: Equation 2 illustrates a diagram of the cleavage of the peptidyl thiohydantoin derivative, as by hydrochloric acid, in another step of the method constituting this invention to yield a peptidyl residue and a thiohydantoin derivative of an amino acid previously at the carboxyl termination of the initial peptide: DETAILED DESCRIPTION OF THE INVENTION In one embodiment of the invention, a peptide is sequenced at its carboxyl terminal to separate the amino acids in the peptide so that the amino acids can be individually identified. The peptides can be quite large. For example, peptide with as many as fifty (50) amino acids or more can be sequenced to identify the amino acids sequentially. The peptides are treated with a silyl isothiocyanate. Preferably, the isothiocyanate is trimethylsilyl isothiocyanate. However, the isothiocyanate can be defined as R x SI(SCN) y where x=0-3 and y=l-4 and where R can be an alkyl, aryl, substituted alkyl, substituted aryl, or any combination of these. Trimethylsilyl isothiocyanate is advantageous because it is stable at ambient temperatures. As a result, it can be stored on a shelf for long periods of time without any significant deterioration. However, since trimethylsilyl isothiocyanate is reactive with water, it should be isolated from water or water vapor. The treatment of the peptide with the isothiocyanate specified in the previous paragraph is facilitated by the inclusion of a weak acid anhydride or a weak acid chloride in the mixture. Acetic anhydride is preferred, although the use of acetyl chloride is also desirable. The reaction of the peptide with the agent such as trimethylsilyl isothiocyanate is expedited by the inclusion of a catalyst such as pyridine in the mixture. The inclusion of the pyridine in the mixture helps to couple the peptide to the isothiocyanate. Preferably a concentration of pyridine of approximately two percent (2%) by volume in the mixture may be used. However, the amount of pyridine in the mixture may be used anywhere in the range to approximately eighty percent (80%) by volume in the mixture with beneficial results. Other catalysts such as dimethylaminopyridine, triethylamine and dimethyl ethylamine may also be used. All of these catalysts are organic bases. The catalyst may be selected from a group consisting of tertiary amines including aromatic amines such as pyridine. In a typical experiment, l0 µl of peptide solution containing 2 to 20 nmoles of peptide was mixed with 50 µl of acetic anhydride. The mixture was heated for approximately l0 minutes, and then a large excess of the coupling reagent (5 µl of trimethylsilyl isothiocyanate) was added. The resulting solution was 0.54 M with respect to the coupling reagent. Peptide concentrations were respectively 3l µM and 3l0 µM (2 and 20 nmole). The sample was vacuum dried and then dissolved in 50 µl of l2 N hydrochloric acid. After approximately thirty (30) minutes at room temperature, the sample was again dried and then dissolved in water for high performance liquid chromatography (HPLC) analysis. Table 3 reports data from experiments utilizing bradykinin as a model peptide and the materials specified above. Preferably approximately two (2) nanomols of a peptide are included in ten (l0) microliters of a solution containing the isothiocyanate to obtain a reaction which produces a high yield of peptidyl thiohydantoin derivative. This constitutes a distinct advantage over the prior art since an amount of peptides in the micromolar range has had to be used in the prior art to obtain any satisfactory results. The peptide may be included in the solution in either a liquid phase or a solid phase. When the peptide is included in the solution in the solid phase, the solid phase support may precipitate in the solution, but the peptide attached to the solid phase support may be considered as being effectively dissolved in the solution because it extends into the solution from the solid phase support in such a manner as to be positioned for reaction with the isothiocyanate in the solution. The use of a solid phase support for a peptide is well known in the art. The reaction of the peptide and the isothiocyanate preferably occurs in a temperature range of approximately 50°C to approximately 90°C. For example, when the reaction occurs at a temperature of approximately 90°C, a complete conversion of the peptide to a peptidyl thiohydantoin occurs in a period of time as short as thirty (30) minutes. The reaction of the peptide and the thiocyanate radical in the presence of acetic anhydride to obtain the peptidyl thiohydantoin derivative is shown in Equation l. Complete conversion of the peptide to a peptidyl thiohydantoin derivative is also obtained at temperatures of approximately 70°C and 50°C. However, the amount of time required to complete the conversion increases as the temperature of the solution is decreased. For example, a rate constant as shown in the table below has been obtained for the temperatures specified below: The above are in comparison to a rate constant of 4 × l0⁻³ when the reaction occurs at a temperature of approximately 90°C in the presence of a catalyst. The rate constant may be defined as the reciprocal of the rate at which the peptide is converted to the peptidyl thiohydantoin in the solution discussed above. After a complete conversion of the peptide to the peptidyl thiohydantoin derivative has been obtained as discussed above, excess reagents (such as trimethylsilyl isothiocyanate) may be removed from the solution in a conventional manner as by extraction with a suitable solvent. This removal is facilitated if the peptide is initially immobilized by fixation to a solid support or entrapment within a matrix or retentive material, such as polybrene. A suitable material such as hydrochloric acid is then added to the solution containing the peptidyl thiohydantoin derivative to cleave such derivative. The reaction may occur at ambient temperatures when the hydrochloric acid has a concentration such as approximately l2N. The chemical reaction producing the cleavage is shown schematically in Equation 2. As will be seen, this chemical reaction causes the thiohydantoin derivative of the amino acid at the carboxyl terminal of the peptidyl thiohydantoin derivative to be liberated. A peptide residue constituting the peptide minus the separated amino acid is also produced. The amino acid from which the thiohydantoin is derived can then be identified by conventional methods. The method steps discussed above can be repeated to cleave and identify the amino acids at successive positions from the carboxyl terminal of the peptide or the peptidyl residue. The number of amino acids cleaved and identified in this manner may be as high as twenty five (25) or more, depending on the nature of the amino acids constituting the peptide. When it is desired to cleave and identify the amino acid in a peptide containing more than fifty (50) amino acids, an additional material such as hexafluoroacetone may be included in the solution containing the trimethylsilyl isothiocyanate to facilitate the sequencing of such peptides with such extended sequences of amino acids. The methods described above have certain important advantages. One distinct advantage is that a complete conversion of the peptide to the peptidyl thiohydantoin derivative is obtained. This in turn provides for a complete cleavage of the peptidyl thiohydantoin derivative to obtain the thiohydantoin derivative of the amino acid initially at the carboxyl terminal of the peptide and to obtain the peptidyl residue. This facilitates the identification of the amino acid in the thiohydantoin derivative. It also maintains substantially the same relative molar concentration of the peptidyl residue as in the original peptide. As a result, the ease of identification of amino acids at subsequent positions of the peptidyl residue is enhanced. This is in contrast to methods of the prior art where portions of the peptidyl residue are removed for analysis to identify the deleted amino acid. Such methods progressively reduce, with each sequential removal of the amino acid from the carboxyl terminal, the quantity of the residual peptide available for the subsequent step. Furthermore, by maintaining the molar concentration of the peptidyl residue substantially constant with progressive sequencing of the amino acids at the carboxyl terminal of the peptidyl residue, the number of amino acids capable of being sequenced becomes considerably increased in comparison to the number of amino acids capable of being sequenced in the methods of the prior art. The methods disclosed above also have other important advantages. The methods provide a complete conversion of the peptide to the peptidyl thiohydantoin derivative by using chemicals (and specifically trimethylsilyl isothiocyanate) which are quite reactive with peptides, but which are considerably more stable under ambient conditions than the materials now being used. However, it is desirable that the trimethylsilyl isothiocyanate should be isolated from water and water vapor since the trimethylsilyl isothiocyanate is reactive with water. The methods constituting this invention are also advantageous because they minimize the times and temperatures needed to convert the peptide substantially completely to the peptidyl thiohydantoin.

Claims: A method for sequencing a peptide by carboxyl terminal degradation which includes the step of coupling the peptide with a thiocyanate, characterized in that a silyl isothiocyanate is utilized as the coupling reagent. The method of claim l in which the silyl isothiocyanate has the formula R (x) Si(SCN) y where x is 0 to 3, y is l to 4 and R is an alkyl or an aryl substituent. The method of claim 2 in which the silyl isothiocyanate is trimethylsilyl isothiocyanate. The method of claim 2 in which a catalyst is included. The method of claim 4 in which the catalyst is a tertiary amine. The method of claim 4 in which the catalyst is pyridine. The method of claim 2 in which a weak organic acid anhydride is included in the coupling reaction mixture. The method of claim 7 in which the weak organic acid anhydride is acetic anhydride.

Inventors: HAWKE, DAVID HARRY
Applicants: BECKMAN RESEARCH INSTITUTE OF THE CITY OF HOPE
Application year: 1986
Main classification: C07K1
IPC codes: G01N33, C07K14, C07K1, C07F7