Calculate structure is based on Defined Secondary Structure of Protein (DSSP), a program written in Pascal.[2] The secondary structure recognition algorithms used in DSSP are based mainly on hydrogen-bonding patterns along with geometric structures , such as bends. There are two different hydrogen-bonding patterns which are recognized. The one determines the value of n in the expression i + n (i is a residue that forms a hydrogen bond with a residue n residues removed from residue i.) where n = 3, 4 or 5. These values define three types of turns. A peptide segment that has repeating turns of the same type are called 310-helix, α-helix, or п-helix, respectively. If the turn is isolate, it is simply called an n-turn. The other recognized pattern is a hydrogen bond which is between residues which are not close together in sequence. This type of hydrogen bond is called a bridge. Kabsch & Sanders define a ladder as a "set of one or more consecutive bridges of identical type" and a sheet as a "set of one or more ladders connected by shared residues"[2]. Bends are peptide segments with high curvature, and the determination of curvature involves angles of the Cα. Bends can overlap with helices and turns.
After Jmol completes the 'Calculate structure' computation the results of the computation is printed in the upper box of the console. One part of that output is a summary of the different types of secondary structure with each type having a one letter identifier. The key for the structural components is H: α-helix; B: β-bridge; E: β-strand; G: 310-helix; I: π-helix; T: 3-, 4-, 5-turn; S: bend. It is possible for a residue or a segment of residues to be assigned more than one structural type, for that reason the above list is rank ordered in decreasing priority of assignment. With bend having the lowest priority in assignment a structure is identified as a bend only if it is not assigned any other structural type. Below is a copy of that summary for myohemerytherin (2mhr):
SUMMARY:
G : A:12_A:14
H : A:19_A:37
H : A:41_A:64
T : A:65_A:66
T : A:68_A:69
H : A:70_A:85
T : A:86_A:86
H : A:93_A:109
T : A:110_A:110
G : A:111_A:114
T : A:115_A:117
Show structure of
SUMMARY:
B : A:486_A:486
T : A:488_A:488
I : A:489_A:494
T : A:495_A:495
H : A:497_A:507
G : A:510_A:513
G : A:515_A:524
T : A:525_A:526
H : A:528_A:551
E : A:562_A:567
G : A:572_A:574
H : A:576_A:592
T : A:594_A:595
E : A:601_A:606
T : A:611_A:612
H : A:614_A:632
T : A:635_A:638
E : A:640_A:644
H : A:650_A:659
E : A:662_A:665
T : A:669_A:670
T : A:676_A:677
H : A:678_A:682
T : A:683_A:685
E : A:687_A:691
T : A:694_A:695
H : A:696_A:703
G : A:705_A:707
E : A:709_A:711
H : A:715_A:724
T : A:728_A:728
H : A:729_A:734
H : A:736_A:746
T : A:747_A:750
T : A:752_A:753
G : A:755_A:758
H : A:759_A:768
T : A:773_A:773
G : A:774_A:776
T : A:777_A:777
H : A:778_A:791
H : A:794_A:806
T : A:807_A:807
G : A:808_A:811
B : A:812_A:812
H : A:813_A:821
T : A:822_A:825
