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/* Copyright (C) 2001 Damir Zucic */
/*=============================================================================
chi3_from_cbcgcdoe1.c
Purpose:
Calculate dihedral angle chi3, using CB, CG, CD and OE1 coordinates.
Input:
(1) Pointer to AtomS structure, pointing to the first atom of the
current macromolecular complex.
(2) Index of the first atom of the current residue.
(3) Index of the last atom of the currrent residue.
Output:
Return value.
Return value:
(1) Dihedral angle chi3, on success.
(2) BADDIHEDANGLE on failure.
========includes:============================================================*/
#include <stdio.h>
#include <math.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>
#include <X11/Xatom.h>
#include "defines.h"
#include "typedefs.h"
/*======function prototypes:=================================================*/
int ExtractFourAtoms_ (VectorS *, VectorS *, VectorS *, VectorS *,
char *, char *, char *, char *,
AtomS *, size_t, size_t);
void VectorProduct_ (VectorS *, VectorS *, VectorS *);
double AbsoluteValue_ (VectorS *);
double ScalarProduct_ (VectorS *, VectorS *);
/*======calculate chi3 from CB, CG, CD and OE1:===============================*/
double Chi3FromCBCGCDOE1_ (AtomS *atomSP, size_t atom_startI, size_t atom_endI)
{
static VectorS CB_vectorS, CG_vectorS, CD_vectorS, OE1_vectorS;
int n;
VectorS CG_CB_vectorS, CG_CD_vectorS;
VectorS CD_CG_vectorS, CD_OE1_vectorS;
VectorS u1S, u2S;
VectorS v1S, v2S;
double denom, ratio, alpha;
double chi3;
/* Extract CB, CG, CD and OE1 coordinates: */
n = ExtractFourAtoms_ (&CB_vectorS, &CG_vectorS, &CD_vectorS, &OE1_vectorS,
"CB", "CG", "CD", "OE1",
atomSP, atom_startI, atom_endI);
/* All four atoms are required to calculate the angle chi3: */
if (n < 4) return BADDIHEDANGLE;
/* The first pair of auxiliary vectors: */
CG_CB_vectorS.x = CB_vectorS.x - CG_vectorS.x;
CG_CB_vectorS.y = CB_vectorS.y - CG_vectorS.y;
CG_CB_vectorS.z = CB_vectorS.z - CG_vectorS.z;
CG_CD_vectorS.x = CD_vectorS.x - CG_vectorS.x;
CG_CD_vectorS.y = CD_vectorS.y - CG_vectorS.y;
CG_CD_vectorS.z = CD_vectorS.z - CG_vectorS.z;
/* The second pair of auxiliary vectors: */
CD_CG_vectorS.x = CG_vectorS.x - CD_vectorS.x;
CD_CG_vectorS.y = CG_vectorS.y - CD_vectorS.y;
CD_CG_vectorS.z = CG_vectorS.z - CD_vectorS.z;
CD_OE1_vectorS.x = OE1_vectorS.x - CD_vectorS.x;
CD_OE1_vectorS.y = OE1_vectorS.y - CD_vectorS.y;
CD_OE1_vectorS.z = OE1_vectorS.z - CD_vectorS.z;
/* Two vectors perpendicular to CG_CD_vectorS, mutually orthogonal, */
/* the second in the plane defined by CG_CB_vectorS and CG_CD_vectorS: */
VectorProduct_ (&u1S, &CG_CB_vectorS, &CG_CD_vectorS);
VectorProduct_ (&u2S, &u1S, &CG_CD_vectorS);
/* Two vectors perpendicular to CD_CG_vectorS, mutually orthogonal, */
/* the second in the plane defined by CD_CG_vectorS and CD_OE1_vectorS: */
VectorProduct_ (&v1S, &CD_CG_vectorS, &CD_OE1_vectorS);
VectorProduct_ (&v2S, &CD_CG_vectorS, &v1S);
/* Calculate the angle alpha, which will be used to calculate chi3: */
/* Avoid division by zero: */
denom = AbsoluteValue_ (&u1S) * AbsoluteValue_ (&v1S);
if (denom == 0.0) return BADDIHEDANGLE;
/* Use the scalar product to calculate the cosine of the angle: */
ratio = ScalarProduct_ (&u1S, &v1S) / denom;
/* Arc cosine is very sensitive to floating point errors: */
if (ratio <= -1.0) alpha = 3.1415927;
else if (ratio >= 1.0) alpha = 0.0;
else alpha = acos (ratio);
/* There are two possible solutions; the right one is resolved here: */
if (ScalarProduct_ (&v2S, &u1S) >= 0) chi3 = alpha;
else chi3 = -alpha;
/* Return the angle (in radians): */
return chi3;
}
/*===========================================================================*/
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