Add note about Cys S-S bond frequency in proteome

doc/development/open-questions/index.mdwn

@@ -69,5 +69,14 @@ Discussion of metacognitive strategies that could have computer aides.
         + algorithm: etymology?
             + http://etym.org/
 - Discuss phenomenon in terms of statistical distributions:
-    * Make note of how frequent a phenonmenon occurs. A particular
-      phenomenon could be important in terms of its effects, but rare.
+    * Make note of how frequent a phenomenon occurs. A particular
+      phenomenon could be important in terms of its effects, but rare. For
+      example, a [[disulfide bridge (SS bond)|https://en.wikipedia.org/wiki/Disulfide]]
+      between cysteine (Cys) amino acids is important for the stability of proteins,
+      but per analysis of a *H. sapiens* proteome DB in
+      [[Cysteines and Disulfide Bonds as Structure-Forming Units: Insights From Different Domains of Life and the Potential for Characterization by NMR|https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7191308/]],
+       Cys is found in 97% of proteins, but disulfide bonds are only marked in
+       18% of proteins, but there is also another effect of protein length that is
+       correlated with the presence of a disulfide bond. Further information on this:
+        + [[Frequency of cysteine residue in proteins - Human Homo sapiens - BNID 103538|https://bionumbers.hms.harvard.edu/bionumber.aspx?s=n&v=2&id=103538]]
+        + [[Occurrence of protein disulfide bonds in different domains of life: a comparison of proteins from the Protein Data Bank - Protein Engineering, Design and Selection - Oxford Academic|https://academic.oup.com/peds/article/27/3/65/1518881]]