Yield of Peptides

Equation 1 indicates that the yield of a peptide depends not only on the lability of the bonds involved in its terminal residues but also on the stability of the bonds within the molecules. It is also evident that in 

Various workers (Kuhn, 1930 Montroll and Simha, 1940 Warner, 1942b Myrback, 1949) have considered the mathematical treatment of the breakdown of high molecular chains assuming that all bonds were broken at the same rate, i.e., that ai = 2 = etc. Equation 1 then becomes  

Assuming the chain to be of infinite length or cyclic, the fraction of the original chain appearing as peptides containing n residues is given by  

It is difficult to know how far this type of treatment can really be applied to proteins where there is such great variation in the susceptibility of various bonds. Presumably in a large molecule where the intrinsic rates of hydrolysis are fairly evenly distributed about the mean, equation (3) would apply. 

It is interesting to note that for any value of a in equation (2), E is a maximum if n = 1 and decreases as n increases, in other words the molar yield of smaller peptides is always greater than the yield of larger peptides. On the average this is also true for equation (1), which expresses more closely the situation present in a protein, since F is never greater than aia +i however, as each a is different it will not apply to every case and the yield of certain higher peptides may be greater than the yield of certain smaller ones. 

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Two disadvantages are associated with the use of S-acetyl or 5-benzoyl derivatives in peptide syntheses (a) base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcys-teine occurs with /S-elimination to give olefinic side products, CH2=C-(NHPG)CO—(b) the yields of peptides formed by coupling an unprotected amino group in an 5-acylcysteine are low because of prior S-N acyl migration. ... 

The reaction conditions for this peptide synthesis are equimolar amounts of the reagents, five to eight hours, and 75-85 °C.[44J Use of a-isothiocyanatocarboxylic acids caused the yields of peptides to be lower. Instead of A-( 1 -imidazolylcarbonyl)amino acid esters, the corresponding triazolides were also utilized in the peptide synthesis. 

O-acylisourea generates peptide, the theoretical yield of peptide is one equivalent and one equivalent of A,/V -dialkylurea is liberated. However, a fourth and undesirable course of action is possible because of the nature the (9-acylisourea. The latter contains a basic nitrogen atom (C=NR3) in proximity to the activated carbonyl. This atom can act as a nucleophile, giving rise to a rearrangement (path J) that produces the. V-acylurea (see Section 1.12) that is a stable inert form of the acid. This reaction is irreversible and consumes starting acid without generating peptide. The exact fate of the O-acylisourea in any synthesis depends on a multitude of factors this is addressed in Section 2.3. 

The technology for chemical peptide synthesis is now automated. As in the sequencing reactions already considered, the most important limitation of the process is the efficiency of each chemical cycle, as can be seen by calculating the overall yields of peptides of various... 

Symmetrical anhydrides are stable to washing by dilute add or aqueous sodium hy-drogencarbonate at room temperature and, consequently, can readily be obtained pure using water-soluble carbodiimide.b l Furthermore, when pure, most of the crystaUine products are stable at room temperature. The symmetrical anhydride method was reported by Chen et al.f to give superior yields of peptides as compared to the normal carbodiimide coupling procedure on a sohd phase and to be superior to the mixed anhydride method in solution. 

A diverse collection of chemical reagents and endoproteases with broad specificity has become available for cleaving proteins (see Table 1). An ideal cleaving agent is the one that provides the highest yields of peptides of... 

The dipeptide synthesis with free thermolysin (2 mg/ml) was carried out in water 10 mM of CLCa at pH 6,8 and 20 °C. The yield of peptide was below 10%, due to some reaction byproducts coming from the hydrolysis of methyl ester group of the peptide and consequent product hydrolysis as is shown in the following reactions ... 

Reference 2. bReference 5a. Total yield of peptide after cleavage from resin. Reference 5a. dTolal yield of guanoxan HCI after guanidinylation and Boc removal. Reference 5b. 

DCC for peptide synthesis. Yields of peptides are high with only negligible racemization. The luteinizing hormone-releasing hormone, a decapeptide amide, was synthesized by this new active ester method. 

A simple compartment type of apparatus may be used to separate a peptide mixture into basic, neutral and acidic fractions (Gordon et al., 1941, 1943 Sanger and Tuppy, 1951a). Since the simplification of the mixture is usually more important than the yield of peptides, it is often advisable to repeat the ionophoresis on each fraction, and in this way clear cut fractionations may be obtained with very little overlapping. This method is especially useful for separating the basic peptides. By... 

Carbodiimide peptide synthesis. Racemization often presents a problem in the carbodiimide method (1, 233). Several additives have been recommended, such as N-hydroxysuccinimide and 1-hydroxybenzotriazole (3, 156 5, 342). Some Levris acids have also been found to suppress racemization SbCls is very effective, but yields of peptides are low. The most useful additive appears to be ZnClj, and this salt also increases the yield. ... 

Overall yields of peptides cleaved from polymer II were 70%-90% depending upon the type of polymer II used. Higher yields were obtained when using the pam resin.(8)... 

Racemization Suppression in Peptide Couplings. A mixture of copper(It) chloride and triethylamine catalyzes the formation of peptide bonds. Furthermore, when used as an additive, CuCl2 suppresses racemization in both the carbodiimide and mixed anhydride peptide coupling methods. Recently it was shown that a combination of 1-hydroxybenzotriazole and CuCl2 gives improved yields of peptides while eliminating racemization. ... 

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