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/* Copyright (C) 2001 Damir Zucic */
/*=============================================================================
chi1_from_ncacbcg1.c
Purpose:
Calculate dihedral angle chi1, using N, CA, CB and CG1 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 chi1, 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 chi1 from N, CA, CB and CG1:===============================*/
double Chi1FromNCACBCG1_ (AtomS *atomSP, size_t atom_startI, size_t atom_endI)
{
static VectorS N_vectorS, CA_vectorS, CB_vectorS, CG1_vectorS;
int n;
VectorS CA_N_vectorS, CA_CB_vectorS;
VectorS CB_CA_vectorS, CB_CG1_vectorS;
VectorS u1S, u2S;
VectorS v1S, v2S;
double denom, ratio, alpha;
double chi1;
/* Extract N, CA, CB and CG1 coordinates: */
n = ExtractFourAtoms_ (&N_vectorS, &CA_vectorS, &CB_vectorS, &CG1_vectorS,
"N", "CA", "CB", "CG1",
atomSP, atom_startI, atom_endI);
/* All four atoms are required to calculate the angle chi1: */
if (n < 4) return BADDIHEDANGLE;
/* The first pair of auxiliary vectors: */
CA_N_vectorS.x = N_vectorS.x - CA_vectorS.x;
CA_N_vectorS.y = N_vectorS.y - CA_vectorS.y;
CA_N_vectorS.z = N_vectorS.z - CA_vectorS.z;
CA_CB_vectorS.x = CB_vectorS.x - CA_vectorS.x;
CA_CB_vectorS.y = CB_vectorS.y - CA_vectorS.y;
CA_CB_vectorS.z = CB_vectorS.z - CA_vectorS.z;
/* The second pair of auxiliary vectors: */
CB_CA_vectorS.x = CA_vectorS.x - CB_vectorS.x;
CB_CA_vectorS.y = CA_vectorS.y - CB_vectorS.y;
CB_CA_vectorS.z = CA_vectorS.z - CB_vectorS.z;
CB_CG1_vectorS.x = CG1_vectorS.x - CB_vectorS.x;
CB_CG1_vectorS.y = CG1_vectorS.y - CB_vectorS.y;
CB_CG1_vectorS.z = CG1_vectorS.z - CB_vectorS.z;
/* Two vectors perpendicular to CA_CB_vectorS, mutually orthogonal, */
/* the second in the plane defined by CA_N_vectorS and CA_CB_vectorS: */
VectorProduct_ (&u1S, &CA_N_vectorS, &CA_CB_vectorS);
VectorProduct_ (&u2S, &u1S, &CA_CB_vectorS);
/* Two vectors perpendicular to CB_CA_vectorS, mutually orthogonal, */
/* the second in the plane defined by CB_CA_vectorS and CB_CG1_vectorS: */
VectorProduct_ (&v1S, &CB_CA_vectorS, &CB_CG1_vectorS);
VectorProduct_ (&v2S, &CB_CA_vectorS, &v1S);
/* Calculate the angle alpha, which will be used to calculate chi1: */
/* 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) chi1 = alpha;
else chi1 = -alpha;
/* Return the angle (in radians): */
return chi1;
}
/*===========================================================================*/
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