## 3 Results and Discussion
Partial-F indicates the importance of a descriptor in the SAR prediction. The higher its value is, the higher the significance of the descriptor. The relative weights of the normalized coefficients are the measures of the impact of the specific property on the peptide structure. The negative values (bold in Table 2) of normalized coefficients suggest that the property improves the bioactivity of peptides, whereas the positive values mean that the property should rather be minimized (and/or eliminated) from the structure of a molecule in order to \"improve\" its biological function [37]. To recapitulate, the advantageous impact on dipeptide DPP IV-inhibitory activity was correlated with the descriptors defined as \"nitrogen count2\" and \"methyl count2\" (having been squared). Disadvantageous impact was correlated with the \"ring count 5\" (square value). In the case of tripeptides, the descriptors improving their bioactivity were: \"methyl count2\" (having been squared) and \"sqrt(molecular weight)\". Based on the normalized coefficient value, the importance of the first descriptor can be ignored as similar to that of the descriptor called \"Csp3 bonded to 3C\" (the higher its value, the lesser its impact). Some descriptors summarized in Table 2 are counts of simple structure features, possess only integer values, and may thus be understood as molecular quantum numbers according to a definition proposed by [PERSON] et al. [38]. Descriptors proposed here include, i.a., nitrogen count, methyl count, ring count 5 member, and Csp3 bonded to 3 C. Among them, the ring count 5 member, i.e., the number of rings containing five atoms, is one of the molecular quantum numbers proposed by [PERSON] and coworkers [38]. The above authors have divided nitrogen atoms into two classes: acyclic and cyclic nitrogens. In this work, it was enough to include the total count of nitrogen atoms in a molecule. The set of descriptors proposed by [PERSON] et al. [38] is designed to enable the classification of all classes of low-molecular compounds, whereas the set analyzed in this study is adapted to work with a limited number of short peptides, with limited variability of backbones and a limited number of possible side chains. Peptide fragments taken into account while constructing a set of descriptors, summarized in Table 2, are presented in Figure 5 using four peptides as examples. Focusing on the descriptors enhancing the DPP IV-inhibitory activity of dipeptides, it can be concluded that peptides rich in methyl groups as well as nitrogen might be greater enhancers of DPP IV inhibition. Data presented in Table 3 show 23 dipeptides out of the 46 in total whose predicted (ERM variant (see Section 2)) and observed values of logIC\({}_{50}\) were comparable. These peptides were as follows: GL, GP, KP, ML, MP, NH, RP, SL, TP, VL, VP, VR, WC, WI, WM, WN, WP, WQ, WR, WS, WT, WW, and WY. When looking at their sequences, some of them contained an amino acid with a non-polar side chain (e.g., G, V, L, I), which can be represented as the presence of methyl groups (the higher their \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Descriptor** & **Normalized** & **Partial-F** \\ \cline{2-4}  & & hydrogen donor partial surface area/total & \\  & & accessible surface area/MW & 0.7559 & 51.3355 \\  & & guanidine count/MW & 0.5086 & 14.5636 \\  & & **nitrogen count2** & **-1.0000** & **53.3368** \\  & & **methyl count2** & **-0.3414** & **9.8481** \\  & & ring count 5 member2 & 0.6070 & 28.3351 \\ \cline{2-4}  & & Csp3 bonded to 3 C & 0.0053 & 3.4655 \\  & & **methyl count2** & **-0.0091** & **11.0810** \\  & & ln(molecular weight) & 1.0000 & 20.7152 \\  & & 1.0/molecular weight & 0.3657 & 24.0262 \\  & & **sqrt(molecular weight)** & **-0.6413** & **19.3041** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The relative weight of each descriptor contributing to the structure–activity prediction of DPP IV-inhibitory di- and tripeptides (the statistical parameters affecting the improvement of bioactivity are in bold).