Peptide quantitation

S Hellberg, M Sjostrom, B Skagerberg, S Wold. Peptide quantitative structure-activity relationships, a multivariate approach. I Med Chem 30 1126-1135, 1987. 

Qualitative predictions concerning the degree of bitterness to be expected in peptides are possible with the aid of NEY s Q-value (12) (Table XI), which is based on TANFORD s studies (13). Using the Ft values of peptides, quantitative assessment of the expected range of threshold values is also possible on the basis of the amino acid composition (Table XI) (14). 

Hellberg, S., Sjostrom. M. and Wold. S. The Prediction of Bradykinin Potentiating Potency of Pentapeptides. An Example of a Peptide Quantitative Structure-Activity Relationship. Acta Chem. Scand. 1986, B40, 135-140. 

In cycle 1, all added peptide is shortened by one residue to give peptide-1. In cycle 2, this peptide quantitatively loses a second residue to become peptide-2 and the freshly added peptide becomes peptide-1. This process is repeated for the required number of cycles. The main practical requirement is that the starting peptide is dissolved in a defined volume of coupling buffer, dependent only on the number of cycles required and the volume applied per cycle. As all excess reagent, buffer and volatile reaction by-products are removed under vacuum, there are no extractive or transfer losses. 

D. A. Paterson, R. A. Conradi, A. R. HUgers, et al. A nonaqueous partitioning system for predicting the oral absorption potential of peptides. Quantitative Structure-Action Relationships, 13, 4-10 (1994)... 

Liang, G.-Z., Zhou, P., Zhou, Y., Zhang, Q.-L. and Li, Z. (2006) New descriptors of amino acids and their applications to peptide quantitative structure-activity relationship. Acto Chim. Sin., 64, 393-396. 

Figure 3. Circular dichroism spectra of Co-Pn-V maquettes, where n = 12,16, and 20 are represented by dotted, solid, and dotted-dashed curves, respectively. As the number of residues increases, the helicity of the bundles is enhanced, as shown by the increased negative elliptidty at 222 nm. All ellipticity measurements are expressed as mean residue ellipticity. The spectra were obtained in 100 mM formate and 50 mM phosphate, pH = 7.0, at 25 V. Peptide quantitation was by amino acid analysis in all cases.

The above facts do not favour protein or peptide quantitation using MALDI-MS. Some problems are associated with MALDI-MS quantification i) low shot-to-shot reproducibility, ii) various signal suppression effects, and iii) strong influence of sample preparation and matrix crystallisation. Nevertheless, it is possible to use MALDI-MS to obtain absolute or relative quantitation. In most cases, the idea is to use an internal standard for an absolute quantitation, but this standard must have the same physico-chemical characteristics as the quantified peptide. The use of a different peptide in terms of sequence may result in different desorption and ionisation properties. Usually, the internal standard is the same peptide labelled with a stable isotope to modify slightly the mass. 

Hellberg S, Sjostrom M, Skagerberg B, et al. Peptide quantitative structure-activity relationships A multivariate approach. J Med Chem 1987 30 1126-1135. 

Kerr, T. J. McLean, J. A., Peptide quantitation using primary amine selective metal chelation labels for mass spectrometry . Chemical Communications 2010, 46, 5479-5481. 

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