82 



Martynas Ycas 



isolation of various dipeptides varies, making it possible that the frequencies 

 of some peptides have been systematically over- or underestimated. 



Figure 2 can now be treated as a contingency table with 761 degrees of 



Fig. 2. Frequencies of occurrence of dipeptide sequences in proteins, plotted as 

 in Fig. 1. The sequences of clupein, collagen, salmine, silk fibroin and wool have 

 not been used. Sequences of less than three residues, as well as those where the 

 acid and amide forms of glx and asx are not differentiated, were also not used. On 

 the basis of the study of Ohno (68), glx and asx in lysozyme are assigned to glun 

 and aspn, respectively. The seven-residue sequence common to ACTH and MEH 

 was counted only once, a — marginal totals of rows ; b — marginal totals of columns 

 c — marginal totals of rows and columns. 



freedom, and the null hypothesis, that there is no correlation between adjacent 

 residues, tested. The deviation X from the expected distribution in Fig. 2 is 

 calculated as: 



{a^_ -f- a,^{Oj, + ^.;) 1^ 



(1) 



{a,^ -I a.i){aj. + a.j) 

 An 



where n is the sum of the marginal totals (330), a^j the value of a cell in column / 

 and row j, Oj and a,, the marginal totals in column and row respectively of 

 the residue defining the column, <7._, and a_,. the analogous values for the residue 

 defining the row. For computational purposes (1) reduces to: 



^="(2'K+«i. +«.)"') 



From Fig. 2, A = 392. The value of /, which is calculated from 

 is 0.414, which is less than 1.645, the 5 per cent confidence limit. 



(2) 



(3) 



